Auto merge of rust-lang#128985 - GrigorenkoPV:instantly-dangling-pointer, r=Urgau

Lint against getting pointers from immediately dropped temporaries

Fixes rust-lang#123613

## Changes:
1. New lint: `dangling_pointers_from_temporaries`. Is a generalization of `temporary_cstring_as_ptr` for more types and more ways to get a temporary.
2. `temporary_cstring_as_ptr` is removed and marked as renamed to `dangling_pointers_from_temporaries`.
3. `clippy::temporary_cstring_as_ptr` is marked as renamed to `dangling_pointers_from_temporaries`.
4. Fixed a false positive[^fp] for when the pointer is not actually dangling because of lifetime extension for function/method call arguments.
5. `core::cell::Cell` is now `rustc_diagnostic_item = "Cell"`

## Questions:
- [ ]  Instead of manually checking for a list of known methods and diagnostic items, maybe add some sort of annotation to those methods in library and check for the presence of that annotation? rust-lang#128985 (comment)

## Known limitations:

### False negatives[^fn]:

See the comments in `compiler/rustc_lint/src/dangling.rs`

1. Method calls that are not checked for:
   - `temporary_unsafe_cell.get()`
   - `temporary_sync_unsafe_cell.get()`
2. Ways to get a temporary that are not recognized:
   - `owning_temporary.field`
   - `owning_temporary[index]`
3. No checks for ref-to-ptr conversions:
   - `&raw [mut] temporary`
   - `&temporary as *(const|mut) _`
    - `ptr::from_ref(&temporary)` and friends

[^fn]: lint **should** be emitted, but **is not**

[^fp]: lint **should not** be emitted, but **is**

Author: bors

compiler/rustc_lint/messages.ftl

@@ -203,14 +203,14 @@ lint_confusable_identifier_pair = found both `{$existing_sym}` and `{$sym}` as i
     .current_use = this identifier can be confused with `{$existing_sym}`
     .other_use = other identifier used here
 
-lint_cstring_ptr = getting the inner pointer of a temporary `CString`
-    .as_ptr_label = this pointer will be invalid
-    .unwrap_label = this `CString` is deallocated at the end of the statement, bind it to a variable to extend its lifetime
-    .note = pointers do not have a lifetime; when calling `as_ptr` the `CString` will be deallocated at the end of the statement because nothing is referencing it as far as the type system is concerned
-    .help = for more information, see https://doc.rust-lang.org/reference/destructors.html
-
 lint_custom_inner_attribute_unstable = custom inner attributes are unstable
 
+lint_dangling_pointers_from_temporaries = a dangling pointer will be produced because the temporary `{$ty}` will be dropped
+    .label_ptr = this pointer will immediately be invalid
+    .label_temporary = this `{$ty}` is deallocated at the end of the statement, bind it to a variable to extend its lifetime
+    .note = pointers do not have a lifetime; when calling `{$callee}` the `{$ty}` will be deallocated at the end of the statement because nothing is referencing it as far as the type system is concerned
+    .help = for more information, see <https://doc.rust-lang.org/reference/destructors.html>
+
 lint_default_hash_types = prefer `{$preferred}` over `{$used}`, it has better performance
     .note = a `use rustc_data_structures::fx::{$preferred}` may be necessary
 

compiler/rustc_lint/src/dangling.rs

@@ -0,0 +1,223 @@
+use rustc_ast::visit::{visit_opt, walk_list};
+use rustc_hir::def_id::LocalDefId;
+use rustc_hir::intravisit::{FnKind, Visitor, walk_expr};
+use rustc_hir::{Block, Body, Expr, ExprKind, FnDecl, LangItem};
+use rustc_middle::ty::{Ty, TyCtxt};
+use rustc_session::{declare_lint, impl_lint_pass};
+use rustc_span::Span;
+use rustc_span::symbol::sym;
+
+use crate::lints::DanglingPointersFromTemporaries;
+use crate::{LateContext, LateLintPass};
+
+declare_lint! {
+    /// The `dangling_pointers_from_temporaries` lint detects getting a pointer to data
+    /// of a temporary that will immediately get dropped.
+    ///
+    /// ### Example
+    ///
+    /// ```rust
+    /// # #![allow(unused)]
+    /// # unsafe fn use_data(ptr: *const u8) { }
+    /// fn gather_and_use(bytes: impl Iterator<Item = u8>) {
+    ///     let x: *const u8 = bytes.collect::<Vec<u8>>().as_ptr();
+    ///     unsafe { use_data(x) }
+    /// }
+    /// ```
+    ///
+    /// {{produces}}
+    ///
+    /// ### Explanation
+    ///
+    /// Getting a pointer from a temporary value will not prolong its lifetime,
+    /// which means that the value can be dropped and the allocation freed
+    /// while the pointer still exists, making the pointer dangling.
+    /// This is not an error (as far as the type system is concerned)
+    /// but probably is not what the user intended either.
+    ///
+    /// If you need stronger guarantees, consider using references instead,
+    /// as they are statically verified by the borrow-checker to never dangle.
+    pub DANGLING_POINTERS_FROM_TEMPORARIES,
+    Warn,
+    "detects getting a pointer from a temporary"
+}
+
+/// FIXME: false negatives (i.e. the lint is not emitted when it should be)
+/// 1. Method calls that are not checked for:
+///    - [`temporary_unsafe_cell.get()`][`core::cell::UnsafeCell::get()`]
+///    - [`temporary_sync_unsafe_cell.get()`][`core::cell::SyncUnsafeCell::get()`]
+/// 2. Ways to get a temporary that are not recognized:
+///    - `owning_temporary.field`
+///    - `owning_temporary[index]`
+/// 3. No checks for ref-to-ptr conversions:
+///    - `&raw [mut] temporary`
+///    - `&temporary as *(const|mut) _`
+///    - `ptr::from_ref(&temporary)` and friends
+#[derive(Clone, Copy, Default)]
+pub(crate) struct DanglingPointers;
+
+impl_lint_pass!(DanglingPointers => [DANGLING_POINTERS_FROM_TEMPORARIES]);
+
+// This skips over const blocks, but they cannot use or return a dangling pointer anyways.
+impl<'tcx> LateLintPass<'tcx> for DanglingPointers {
+    fn check_fn(
+        &mut self,
+        cx: &LateContext<'tcx>,
+        _: FnKind<'tcx>,
+        _: &'tcx FnDecl<'tcx>,
+        body: &'tcx Body<'tcx>,
+        _: Span,
+        _: LocalDefId,
+    ) {
+        DanglingPointerSearcher { cx, inside_call_args: false }.visit_body(body)
+    }
+}
+
+/// This produces a dangling pointer:
+/// ```ignore (example)
+/// let ptr = CString::new("hello").unwrap().as_ptr();
+/// foo(ptr)
+/// ```
+///
+/// But this does not:
+/// ```ignore (example)
+/// foo(CString::new("hello").unwrap().as_ptr())
+/// ```
+///
+/// But this does:
+/// ```ignore (example)
+/// foo({ let ptr = CString::new("hello").unwrap().as_ptr(); ptr })
+/// ```
+///
+/// So we have to keep track of when we are inside of a function/method call argument.
+struct DanglingPointerSearcher<'lcx, 'tcx> {
+    cx: &'lcx LateContext<'tcx>,
+    /// Keeps track of whether we are inside of function/method call arguments,
+    /// where this lint should not be emitted.
+    ///
+    /// See [the main doc][`Self`] for examples.
+    inside_call_args: bool,
+}
+
+impl Visitor<'_> for DanglingPointerSearcher<'_, '_> {
+    fn visit_expr(&mut self, expr: &Expr<'_>) -> Self::Result {
+        if !self.inside_call_args {
+            lint_expr(self.cx, expr)
+        }
+        match expr.kind {
+            ExprKind::Call(lhs, args) | ExprKind::MethodCall(_, lhs, args, _) => {
+                self.visit_expr(lhs);
+                self.with_inside_call_args(true, |this| walk_list!(this, visit_expr, args))
+            }
+            ExprKind::Block(&Block { stmts, expr, .. }, _) => {
+                self.with_inside_call_args(false, |this| walk_list!(this, visit_stmt, stmts));
+                visit_opt!(self, visit_expr, expr)
+            }
+            _ => walk_expr(self, expr),
+        }
+    }
+}
+
+impl DanglingPointerSearcher<'_, '_> {
+    fn with_inside_call_args<R>(
+        &mut self,
+        inside_call_args: bool,
+        callback: impl FnOnce(&mut Self) -> R,
+    ) -> R {
+        let old = core::mem::replace(&mut self.inside_call_args, inside_call_args);
+        let result = callback(self);
+        self.inside_call_args = old;
+        result
+    }
+}
+
+fn lint_expr(cx: &LateContext<'_>, expr: &Expr<'_>) {
+    if let ExprKind::MethodCall(method, receiver, _args, _span) = expr.kind
+        && matches!(method.ident.name, sym::as_ptr | sym::as_mut_ptr)
+        && is_temporary_rvalue(receiver)
+        && let ty = cx.typeck_results().expr_ty(receiver)
+        && is_interesting(cx.tcx, ty)
+    {
+        // FIXME: use `emit_node_lint` when `#[primary_span]` is added.
+        cx.tcx.emit_node_span_lint(
+            DANGLING_POINTERS_FROM_TEMPORARIES,
+            expr.hir_id,
+            method.ident.span,
+            DanglingPointersFromTemporaries {
+                callee: method.ident.name,
+                ty,
+                ptr_span: method.ident.span,
+                temporary_span: receiver.span,
+            },
+        )
+    }
+}
+
+fn is_temporary_rvalue(expr: &Expr<'_>) -> bool {
+    match expr.kind {
+        // Const is not temporary.
+        ExprKind::ConstBlock(..) | ExprKind::Repeat(..) | ExprKind::Lit(..) => false,
+
+        // This is literally lvalue.
+        ExprKind::Path(..) => false,
+
+        // Calls return rvalues.
+        ExprKind::Call(..) | ExprKind::MethodCall(..) | ExprKind::Binary(..) => true,
+
+        // Inner blocks are rvalues.
+        ExprKind::If(..) | ExprKind::Loop(..) | ExprKind::Match(..) | ExprKind::Block(..) => true,
+
+        // FIXME: these should probably recurse and typecheck along the way.
+        //        Some false negatives are possible for now.
+        ExprKind::Index(..) | ExprKind::Field(..) | ExprKind::Unary(..) => false,
+
+        ExprKind::Struct(..) => true,
+
+        // FIXME: this has false negatives, but I do not want to deal with 'static/const promotion just yet.
+        ExprKind::Array(..) => false,
+
+        // These typecheck to `!`
+        ExprKind::Break(..) | ExprKind::Continue(..) | ExprKind::Ret(..) | ExprKind::Become(..) => {
+            false
+        }
+
+        // These typecheck to `()`
+        ExprKind::Assign(..) | ExprKind::AssignOp(..) | ExprKind::Yield(..) => false,
+
+        // Compiler-magic macros
+        ExprKind::AddrOf(..) | ExprKind::OffsetOf(..) | ExprKind::InlineAsm(..) => false,
+
+        // We are not interested in these
+        ExprKind::Cast(..)
+        | ExprKind::Closure(..)
+        | ExprKind::Tup(..)
+        | ExprKind::DropTemps(..)
+        | ExprKind::Let(..) => false,
+
+        // Not applicable
+        ExprKind::Type(..) | ExprKind::Err(..) => false,
+    }
+}
+
+// Array, Vec, String, CString, MaybeUninit, Cell, Box<[_]>, Box<str>, Box<CStr>,
+// or any of the above in arbitrary many nested Box'es.
+fn is_interesting(tcx: TyCtxt<'_>, ty: Ty<'_>) -> bool {
+    if ty.is_array() {
+        true
+    } else if let Some(inner) = ty.boxed_ty() {
+        inner.is_slice()
+            || inner.is_str()
+            || inner.ty_adt_def().is_some_and(|def| tcx.is_lang_item(def.did(), LangItem::CStr))
+            || is_interesting(tcx, inner)
+    } else if let Some(def) = ty.ty_adt_def() {
+        for lang_item in [LangItem::String, LangItem::MaybeUninit] {
+            if tcx.is_lang_item(def.did(), lang_item) {
+                return true;
+            }
+        }
+        tcx.get_diagnostic_name(def.did())
+            .is_some_and(|name| matches!(name, sym::cstring_type | sym::Vec | sym::Cell))
+    } else {
+        false
+    }
+}

compiler/rustc_lint/src/lib.rs

@@ -40,6 +40,7 @@ mod async_closures;
 mod async_fn_in_trait;
 pub mod builtin;
 mod context;
+mod dangling;
 mod deref_into_dyn_supertrait;
 mod drop_forget_useless;
 mod early;
@@ -59,7 +60,6 @@ mod levels;
 mod lints;
 mod macro_expr_fragment_specifier_2024_migration;
 mod map_unit_fn;
-mod methods;
 mod multiple_supertrait_upcastable;
 mod non_ascii_idents;
 mod non_fmt_panic;
@@ -85,6 +85,7 @@ mod unused;
 use async_closures::AsyncClosureUsage;
 use async_fn_in_trait::AsyncFnInTrait;
 use builtin::*;
+use dangling::*;
 use deref_into_dyn_supertrait::*;
 use drop_forget_useless::*;
 use enum_intrinsics_non_enums::EnumIntrinsicsNonEnums;
@@ -97,7 +98,6 @@ use invalid_from_utf8::*;
 use let_underscore::*;
 use macro_expr_fragment_specifier_2024_migration::*;
 use map_unit_fn::*;
-use methods::*;
 use multiple_supertrait_upcastable::*;
 use non_ascii_idents::*;
 use non_fmt_panic::NonPanicFmt;
@@ -225,7 +225,7 @@ late_lint_methods!(
             UngatedAsyncFnTrackCaller: UngatedAsyncFnTrackCaller,
             ShadowedIntoIter: ShadowedIntoIter,
             DropTraitConstraints: DropTraitConstraints,
-            TemporaryCStringAsPtr: TemporaryCStringAsPtr,
+            DanglingPointers: DanglingPointers,
             NonPanicFmt: NonPanicFmt,
             NoopMethodCall: NoopMethodCall,
             EnumIntrinsicsNonEnums: EnumIntrinsicsNonEnums,
@@ -350,6 +350,7 @@ fn register_builtins(store: &mut LintStore) {
     store.register_renamed("non_fmt_panic", "non_fmt_panics");
     store.register_renamed("unused_tuple_struct_fields", "dead_code");
     store.register_renamed("static_mut_ref", "static_mut_refs");
+    store.register_renamed("temporary_cstring_as_ptr", "dangling_pointers_from_temporaries");
 
     // These were moved to tool lints, but rustc still sees them when compiling normally, before
     // tool lints are registered, so `check_tool_name_for_backwards_compat` doesn't work. Use

compiler/rustc_lint/src/lints.rs

@@ -1137,16 +1137,19 @@ pub(crate) struct IgnoredUnlessCrateSpecified<'a> {
     pub name: Symbol,
 }
 
-// methods.rs
+// dangling.rs
 #[derive(LintDiagnostic)]
-#[diag(lint_cstring_ptr)]
+#[diag(lint_dangling_pointers_from_temporaries)]
 #[note]
 #[help]
-pub(crate) struct CStringPtr {
-    #[label(lint_as_ptr_label)]
-    pub as_ptr: Span,
-    #[label(lint_unwrap_label)]
-    pub unwrap: Span,
+// FIXME: put #[primary_span] on `ptr_span` once it does not cause conflicts
+pub(crate) struct DanglingPointersFromTemporaries<'tcx> {
+    pub callee: Symbol,
+    pub ty: Ty<'tcx>,
+    #[label(lint_label_ptr)]
+    pub ptr_span: Span,
+    #[label(lint_label_temporary)]
+    pub temporary_span: Span,
 }
 
 // multiple_supertrait_upcastable.rs

compiler/rustc_lint/src/methods.rs

@@ -171,6 +171,7 @@ symbols! {
         CallOnceFuture,
         CallRefFuture,
         Capture,
+        Cell,
         Center,
         Cleanup,
         Clone,
@@ -409,6 +410,7 @@ symbols! {
         arm,
         arm_target_feature,
         array,
+        as_mut_ptr,
         as_ptr,
         as_ref,
         as_str,

compiler/rustc_span/src/symbol.rs

@@ -4,6 +4,7 @@ use core::mem::MaybeUninit;
 use core::ptr::NonNull;
 
 #[test]
+#[cfg_attr(not(bootstrap), expect(dangling_pointers_from_temporaries))]
 fn uninitialized_zero_size_box() {
     assert_eq!(
         &*Box::<()>::new_uninit() as *const _,

library/alloc/tests/boxed.rs

@@ -304,6 +304,7 @@ pub use once::OnceCell;
 /// ```
 ///
 /// See the [module-level documentation](self) for more.
+#[cfg_attr(not(test), rustc_diagnostic_item = "Cell")]
 #[stable(feature = "rust1", since = "1.0.0")]
 #[repr(transparent)]
 #[rustc_pub_transparent]

library/core/src/cell.rs

@@ -464,7 +464,9 @@ impl CStr {
     /// behavior when `ptr` is used inside the `unsafe` block:
     ///
     /// ```no_run
-    /// # #![allow(unused_must_use)] #![allow(temporary_cstring_as_ptr)]
+    /// # #![allow(unused_must_use)]
+    /// # #![cfg_attr(bootstrap, expect(temporary_cstring_as_ptr))]
+    /// # #![cfg_attr(not(bootstrap), expect(dangling_pointers_from_temporaries))]
     /// use std::ffi::CString;
     ///
     /// // Do not do this:

library/core/src/ffi/c_str.rs

@@ -166,7 +166,7 @@ declare_with_version! { RENAMED(RENAMED_VERSION): &[(&str, &str)] = &[
     #[clippy::version = ""]
     ("clippy::positional_named_format_parameters", "named_arguments_used_positionally"),
     #[clippy::version = ""]
-    ("clippy::temporary_cstring_as_ptr", "temporary_cstring_as_ptr"),
+    ("clippy::temporary_cstring_as_ptr", "dangling_pointers_from_temporaries"),
     #[clippy::version = ""]
     ("clippy::undropped_manually_drops", "undropped_manually_drops"),
     #[clippy::version = ""]

src/tools/clippy/clippy_lints/src/deprecated_lints.rs

@@ -54,7 +54,7 @@
 #![allow(enum_intrinsics_non_enums)]
 #![allow(non_fmt_panics)]
 #![allow(named_arguments_used_positionally)]
-#![allow(temporary_cstring_as_ptr)]
+#![allow(dangling_pointers_from_temporaries)]
 #![allow(undropped_manually_drops)]
 #![allow(unknown_lints)]
 #![allow(unused_labels)]
@@ -120,7 +120,7 @@
 #![warn(unexpected_cfgs)] //~ ERROR: lint `clippy::mismatched_target_os`
 #![warn(non_fmt_panics)] //~ ERROR: lint `clippy::panic_params`
 #![warn(named_arguments_used_positionally)] //~ ERROR: lint `clippy::positional_named_format_parameters`
-#![warn(temporary_cstring_as_ptr)] //~ ERROR: lint `clippy::temporary_cstring_as_ptr`
+#![warn(dangling_pointers_from_temporaries)] //~ ERROR: lint `clippy::temporary_cstring_as_ptr`
 #![warn(undropped_manually_drops)] //~ ERROR: lint `clippy::undropped_manually_drops`
 #![warn(unknown_lints)] //~ ERROR: lint `clippy::unknown_clippy_lints`
 #![warn(unused_labels)] //~ ERROR: lint `clippy::unused_label`

src/tools/clippy/tests/ui/rename.fixed

@@ -1,11 +1,17 @@
+error: lint `temporary_cstring_as_ptr` has been renamed to `dangling_pointers_from_temporaries`
+  --> tests/ui/rename.rs:57:10
+   |
+LL | #![allow(temporary_cstring_as_ptr)]
+   |          ^^^^^^^^^^^^^^^^^^^^^^^^ help: use the new name: `dangling_pointers_from_temporaries`
+   |
+   = note: `-D renamed-and-removed-lints` implied by `-D warnings`
+   = help: to override `-D warnings` add `#[allow(renamed_and_removed_lints)]`
+
 error: lint `clippy::almost_complete_letter_range` has been renamed to `clippy::almost_complete_range`
   --> tests/ui/rename.rs:63:9
    |
 LL | #![warn(clippy::almost_complete_letter_range)]
    |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: use the new name: `clippy::almost_complete_range`
-   |
-   = note: `-D renamed-and-removed-lints` implied by `-D warnings`
-   = help: to override `-D warnings` add `#[allow(renamed_and_removed_lints)]`
 
 error: lint `clippy::blacklisted_name` has been renamed to `clippy::disallowed_names`
   --> tests/ui/rename.rs:64:9
@@ -361,11 +367,11 @@ error: lint `clippy::positional_named_format_parameters` has been renamed to `na
 LL | #![warn(clippy::positional_named_format_parameters)]
    |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: use the new name: `named_arguments_used_positionally`
 
-error: lint `clippy::temporary_cstring_as_ptr` has been renamed to `temporary_cstring_as_ptr`
+error: lint `clippy::temporary_cstring_as_ptr` has been renamed to `dangling_pointers_from_temporaries`
   --> tests/ui/rename.rs:123:9
    |
 LL | #![warn(clippy::temporary_cstring_as_ptr)]
-   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: use the new name: `temporary_cstring_as_ptr`
+   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: use the new name: `dangling_pointers_from_temporaries`
 
 error: lint `clippy::undropped_manually_drops` has been renamed to `undropped_manually_drops`
   --> tests/ui/rename.rs:124:9
@@ -397,5 +403,5 @@ error: lint `clippy::reverse_range_loop` has been renamed to `clippy::reversed_e
 LL | #![warn(clippy::reverse_range_loop)]
    |         ^^^^^^^^^^^^^^^^^^^^^^^^^^ help: use the new name: `clippy::reversed_empty_ranges`
 
-error: aborting due to 66 previous errors
+error: aborting due to 67 previous errors
 

src/tools/clippy/tests/ui/rename.stderr

@@ -17,8 +17,11 @@ fn main() {
 
     // The exact addresses returned by these library functions are not necessarily stable guarantees
     // but for now we assert that we're still matching.
-    assert_eq!(<Vec<i32>>::new().as_ptr(), <&[i32]>::default().as_ptr());
-    assert_eq!(<Box<[i32]>>::default().as_ptr(), (&[]).as_ptr());
+    #[allow(dangling_pointers_from_temporaries)]
+    {
+        assert_eq!(<Vec<i32>>::new().as_ptr(), <&[i32]>::default().as_ptr());
+        assert_eq!(<Box<[i32]>>::default().as_ptr(), (&[]).as_ptr());
+    };
 
     // statics must have a unique address (see https://github.com/rust-lang/rust/issues/18297, not
     // clear whether this is a stable guarantee)

tests/ui/consts/zst_no_llvm_alloc.rs

@@ -0,0 +1,23 @@
+#![allow(dangling_pointers_from_temporaries)]
+
+fn main() {
+    dbg!(String::new().as_ptr());
+    // ^ no error
+
+    #[deny(dangling_pointers_from_temporaries)]
+    {
+        dbg!(String::new().as_ptr());
+        //~^ ERROR a dangling pointer will be produced because the temporary `String` will be dropped
+    }
+    S.foo()
+}
+
+struct S;
+
+impl S {
+    #[warn(dangling_pointers_from_temporaries)]
+    fn foo(self) {
+        dbg!(String::new().as_ptr());
+        //~^ WARNING a dangling pointer will be produced because the temporary `String` will be dropped
+    }
+}

tests/ui/lint/dangling-pointers-from-temporaries/allow.rs

@@ -0,0 +1,34 @@
+error: a dangling pointer will be produced because the temporary `String` will be dropped
+  --> $DIR/allow.rs:9:28
+   |
+LL |         dbg!(String::new().as_ptr());
+   |              ------------- ^^^^^^ this pointer will immediately be invalid
+   |              |
+   |              this `String` is deallocated at the end of the statement, bind it to a variable to extend its lifetime
+   |
+   = note: pointers do not have a lifetime; when calling `as_ptr` the `String` will be deallocated at the end of the statement because nothing is referencing it as far as the type system is concerned
+   = help: for more information, see <https://doc.rust-lang.org/reference/destructors.html>
+note: the lint level is defined here
+  --> $DIR/allow.rs:7:12
+   |
+LL |     #[deny(dangling_pointers_from_temporaries)]
+   |            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+warning: a dangling pointer will be produced because the temporary `String` will be dropped
+  --> $DIR/allow.rs:20:28
+   |
+LL |         dbg!(String::new().as_ptr());
+   |              ------------- ^^^^^^ this pointer will immediately be invalid
+   |              |
+   |              this `String` is deallocated at the end of the statement, bind it to a variable to extend its lifetime
+   |
+   = note: pointers do not have a lifetime; when calling `as_ptr` the `String` will be deallocated at the end of the statement because nothing is referencing it as far as the type system is concerned
+   = help: for more information, see <https://doc.rust-lang.org/reference/destructors.html>
+note: the lint level is defined here
+  --> $DIR/allow.rs:18:12
+   |
+LL |     #[warn(dangling_pointers_from_temporaries)]
+   |            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: aborting due to 1 previous error; 1 warning emitted
+

tests/ui/lint/dangling-pointers-from-temporaries/allow.stderr

@@ -0,0 +1,52 @@
+#![deny(dangling_pointers_from_temporaries)]
+
+use std::ffi::{c_char, CString};
+
+fn cstring() -> CString {
+    CString::new("hello").unwrap()
+}
+
+fn consume(ptr: *const c_char) {
+    let c = unsafe { ptr.read() };
+    dbg!(c);
+}
+
+// None of these should trigger the lint.
+fn ok() {
+    consume(cstring().as_ptr());
+    consume({ cstring() }.as_ptr());
+    consume({ cstring().as_ptr() });
+    consume(cstring().as_ptr().cast());
+    consume({ cstring() }.as_ptr().cast());
+    consume({ cstring().as_ptr() }.cast());
+}
+
+// All of these should trigger the lint.
+fn not_ok() {
+    {
+        let ptr = cstring().as_ptr();
+        //~^ ERROR a dangling pointer will be produced because the temporary `CString` will be dropped
+        consume(ptr);
+    }
+    consume({
+        let ptr = cstring().as_ptr();
+        //~^ ERROR a dangling pointer will be produced because the temporary `CString` will be dropped
+        ptr
+    });
+    consume({
+        let s = cstring();
+        s.as_ptr()
+        //^ FIXME: should error
+    });
+    let _ptr: *const u8 = cstring().as_ptr().cast();
+    //~^ ERROR a dangling pointer will be produced because the temporary `CString` will be dropped
+    let _ptr: *const u8 = { cstring() }.as_ptr().cast();
+    //~^ ERROR a dangling pointer will be produced because the temporary `CString` will be dropped
+    let _ptr: *const u8 = { cstring().as_ptr() }.cast();
+    //~^ ERROR a dangling pointer will be produced because the temporary `CString` will be dropped
+}
+
+fn main() {
+    ok();
+    not_ok();
+}

tests/ui/lint/dangling-pointers-from-temporaries/calls.rs

@@ -0,0 +1,62 @@
+error: a dangling pointer will be produced because the temporary `CString` will be dropped
+  --> $DIR/calls.rs:27:29
+   |
+LL |         let ptr = cstring().as_ptr();
+   |                   --------- ^^^^^^ this pointer will immediately be invalid
+   |                   |
+   |                   this `CString` is deallocated at the end of the statement, bind it to a variable to extend its lifetime
+   |
+   = note: pointers do not have a lifetime; when calling `as_ptr` the `CString` will be deallocated at the end of the statement because nothing is referencing it as far as the type system is concerned
+   = help: for more information, see <https://doc.rust-lang.org/reference/destructors.html>
+note: the lint level is defined here
+  --> $DIR/calls.rs:1:9
+   |
+LL | #![deny(dangling_pointers_from_temporaries)]
+   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: a dangling pointer will be produced because the temporary `CString` will be dropped
+  --> $DIR/calls.rs:32:29
+   |
+LL |         let ptr = cstring().as_ptr();
+   |                   --------- ^^^^^^ this pointer will immediately be invalid
+   |                   |
+   |                   this `CString` is deallocated at the end of the statement, bind it to a variable to extend its lifetime
+   |
+   = note: pointers do not have a lifetime; when calling `as_ptr` the `CString` will be deallocated at the end of the statement because nothing is referencing it as far as the type system is concerned
+   = help: for more information, see <https://doc.rust-lang.org/reference/destructors.html>
+
+error: a dangling pointer will be produced because the temporary `CString` will be dropped
+  --> $DIR/calls.rs:41:37
+   |
+LL |     let _ptr: *const u8 = cstring().as_ptr().cast();
+   |                           --------- ^^^^^^ this pointer will immediately be invalid
+   |                           |
+   |                           this `CString` is deallocated at the end of the statement, bind it to a variable to extend its lifetime
+   |
+   = note: pointers do not have a lifetime; when calling `as_ptr` the `CString` will be deallocated at the end of the statement because nothing is referencing it as far as the type system is concerned
+   = help: for more information, see <https://doc.rust-lang.org/reference/destructors.html>
+
+error: a dangling pointer will be produced because the temporary `CString` will be dropped
+  --> $DIR/calls.rs:43:41
+   |
+LL |     let _ptr: *const u8 = { cstring() }.as_ptr().cast();
+   |                           ------------- ^^^^^^ this pointer will immediately be invalid
+   |                           |
+   |                           this `CString` is deallocated at the end of the statement, bind it to a variable to extend its lifetime
+   |
+   = note: pointers do not have a lifetime; when calling `as_ptr` the `CString` will be deallocated at the end of the statement because nothing is referencing it as far as the type system is concerned
+   = help: for more information, see <https://doc.rust-lang.org/reference/destructors.html>
+
+error: a dangling pointer will be produced because the temporary `CString` will be dropped
+  --> $DIR/calls.rs:45:39
+   |
+LL |     let _ptr: *const u8 = { cstring().as_ptr() }.cast();
+   |                             --------- ^^^^^^ this pointer will immediately be invalid
+   |                             |
+   |                             this `CString` is deallocated at the end of the statement, bind it to a variable to extend its lifetime
+   |
+   = note: pointers do not have a lifetime; when calling `as_ptr` the `CString` will be deallocated at the end of the statement because nothing is referencing it as far as the type system is concerned
+   = help: for more information, see <https://doc.rust-lang.org/reference/destructors.html>
+
+error: aborting due to 5 previous errors
+

tests/ui/lint/dangling-pointers-from-temporaries/calls.stderr

@@ -1,4 +1,7 @@
+//@ check-pass
+
 #![deny(temporary_cstring_as_ptr)]
+//~^ WARNING lint `temporary_cstring_as_ptr` has been renamed to `dangling_pointers_from_temporaries`
 
 use std::ffi::CString;
 use std::os::raw::c_char;
@@ -7,5 +10,4 @@ fn some_function(data: *const c_char) {}
 
 fn main() {
     some_function(CString::new("").unwrap().as_ptr());
-    //~^ ERROR getting the inner pointer of a temporary `CString`
 }

tests/ui/lint/lint-temporary-cstring-as-param.rs renamed to tests/ui/lint/dangling-pointers-from-temporaries/cstring-as-param.rs

@@ -0,0 +1,10 @@
+warning: lint `temporary_cstring_as_ptr` has been renamed to `dangling_pointers_from_temporaries`
+  --> $DIR/cstring-as-param.rs:3:9
+   |
+LL | #![deny(temporary_cstring_as_ptr)]
+   |         ^^^^^^^^^^^^^^^^^^^^^^^^ help: use the new name: `dangling_pointers_from_temporaries`
+   |
+   = note: `#[warn(renamed_and_removed_lints)]` on by default
+
+warning: 1 warning emitted
+

tests/ui/lint/dangling-pointers-from-temporaries/cstring-as-param.stderr

@@ -1,17 +1,18 @@
 // this program is not technically incorrect, but is an obscure enough style to be worth linting
 #![deny(temporary_cstring_as_ptr)]
+//~^ WARNING lint `temporary_cstring_as_ptr` has been renamed to `dangling_pointers_from_temporaries`
 
 use std::ffi::CString;
 
 macro_rules! mymacro {
     () => {
         let s = CString::new("some text").unwrap().as_ptr();
-        //~^ ERROR getting the inner pointer of a temporary `CString`
+        //~^ ERROR a dangling pointer will be produced because the temporary `CString` will be dropped
     }
 }
 
 fn main() {
     let s = CString::new("some text").unwrap().as_ptr();
-    //~^ ERROR getting the inner pointer of a temporary `CString`
+    //~^ ERROR a dangling pointer will be produced because the temporary `CString` will be dropped
     mymacro!();
 }

tests/ui/lint/lint-temporary-cstring-as-ptr.rs renamed to tests/ui/lint/dangling-pointers-from-temporaries/cstring-as-ptr.rs

@@ -1,33 +1,41 @@
-error: getting the inner pointer of a temporary `CString`
-  --> $DIR/lint-temporary-cstring-as-ptr.rs:14:48
+warning: lint `temporary_cstring_as_ptr` has been renamed to `dangling_pointers_from_temporaries`
+  --> $DIR/cstring-as-ptr.rs:2:9
+   |
+LL | #![deny(temporary_cstring_as_ptr)]
+   |         ^^^^^^^^^^^^^^^^^^^^^^^^ help: use the new name: `dangling_pointers_from_temporaries`
+   |
+   = note: `#[warn(renamed_and_removed_lints)]` on by default
+
+error: a dangling pointer will be produced because the temporary `CString` will be dropped
+  --> $DIR/cstring-as-ptr.rs:15:48
    |
 LL |     let s = CString::new("some text").unwrap().as_ptr();
-   |             ---------------------------------- ^^^^^^ this pointer will be invalid
+   |             ---------------------------------- ^^^^^^ this pointer will immediately be invalid
    |             |
    |             this `CString` is deallocated at the end of the statement, bind it to a variable to extend its lifetime
    |
    = note: pointers do not have a lifetime; when calling `as_ptr` the `CString` will be deallocated at the end of the statement because nothing is referencing it as far as the type system is concerned
-   = help: for more information, see https://doc.rust-lang.org/reference/destructors.html
+   = help: for more information, see <https://doc.rust-lang.org/reference/destructors.html>
 note: the lint level is defined here
-  --> $DIR/lint-temporary-cstring-as-ptr.rs:2:9
+  --> $DIR/cstring-as-ptr.rs:2:9
    |
 LL | #![deny(temporary_cstring_as_ptr)]
    |         ^^^^^^^^^^^^^^^^^^^^^^^^
 
-error: getting the inner pointer of a temporary `CString`
-  --> $DIR/lint-temporary-cstring-as-ptr.rs:8:52
+error: a dangling pointer will be produced because the temporary `CString` will be dropped
+  --> $DIR/cstring-as-ptr.rs:9:52
    |
 LL |         let s = CString::new("some text").unwrap().as_ptr();
-   |                 ---------------------------------- ^^^^^^ this pointer will be invalid
+   |                 ---------------------------------- ^^^^^^ this pointer will immediately be invalid
    |                 |
    |                 this `CString` is deallocated at the end of the statement, bind it to a variable to extend its lifetime
 ...
 LL |     mymacro!();
    |     ---------- in this macro invocation
    |
    = note: pointers do not have a lifetime; when calling `as_ptr` the `CString` will be deallocated at the end of the statement because nothing is referencing it as far as the type system is concerned
-   = help: for more information, see https://doc.rust-lang.org/reference/destructors.html
+   = help: for more information, see <https://doc.rust-lang.org/reference/destructors.html>
    = note: this error originates in the macro `mymacro` (in Nightly builds, run with -Z macro-backtrace for more info)
 
-error: aborting due to 2 previous errors
+error: aborting due to 2 previous errors; 1 warning emitted
 

tests/ui/lint/lint-temporary-cstring-as-ptr.stderr renamed to tests/ui/lint/dangling-pointers-from-temporaries/cstring-as-ptr.stderr

@@ -0,0 +1,19 @@
+//@ check-pass
+
+#![deny(dangling_pointers_from_temporaries)]
+
+// The original code example comes from bindgen-produced code for emacs.
+// Hence the name of the test.
+// https://github.com/rust-lang/rust/pull/128985#issuecomment-2338951363
+
+use std::ffi::{c_char, CString};
+
+fn read(ptr: *const c_char) -> c_char {
+    unsafe { ptr.read() }
+}
+
+fn main() {
+    let fnptr: Option<fn(ptr: *const c_char) -> c_char> = Some(read);
+    let x = fnptr.unwrap()(CString::new("foo").unwrap().as_ptr());
+    assert_eq!(x as u8, b'f');
+}

tests/ui/lint/dangling-pointers-from-temporaries/emacs.rs

@@ -0,0 +1,13 @@
+#![deny(dangling_pointers_from_temporaries)]
+
+const MAX_PATH: usize = 260;
+fn main() {
+    let str1 = String::with_capacity(MAX_PATH).as_mut_ptr();
+    //~^ ERROR a dangling pointer will be produced because the temporary `String` will be dropped
+    let str2 = String::from("TotototototototototototototototototoT").as_ptr();
+    //~^ ERROR a dangling pointer will be produced because the temporary `String` will be dropped
+    unsafe {
+        std::ptr::copy_nonoverlapping(str2, str1, 30);
+        println!("{:?}", String::from_raw_parts(str1, 30, 30));
+    }
+}

tests/ui/lint/dangling-pointers-from-temporaries/example-from-issue123613.rs

@@ -0,0 +1,29 @@
+error: a dangling pointer will be produced because the temporary `String` will be dropped
+  --> $DIR/example-from-issue123613.rs:5:48
+   |
+LL |     let str1 = String::with_capacity(MAX_PATH).as_mut_ptr();
+   |                ------------------------------- ^^^^^^^^^^ this pointer will immediately be invalid
+   |                |
+   |                this `String` is deallocated at the end of the statement, bind it to a variable to extend its lifetime
+   |
+   = note: pointers do not have a lifetime; when calling `as_mut_ptr` the `String` will be deallocated at the end of the statement because nothing is referencing it as far as the type system is concerned
+   = help: for more information, see <https://doc.rust-lang.org/reference/destructors.html>
+note: the lint level is defined here
+  --> $DIR/example-from-issue123613.rs:1:9
+   |
+LL | #![deny(dangling_pointers_from_temporaries)]
+   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: a dangling pointer will be produced because the temporary `String` will be dropped
+  --> $DIR/example-from-issue123613.rs:7:70
+   |
+LL |     let str2 = String::from("TotototototototototototototototototoT").as_ptr();
+   |                ----------------------------------------------------- ^^^^^^ this pointer will immediately be invalid
+   |                |
+   |                this `String` is deallocated at the end of the statement, bind it to a variable to extend its lifetime
+   |
+   = note: pointers do not have a lifetime; when calling `as_ptr` the `String` will be deallocated at the end of the statement because nothing is referencing it as far as the type system is concerned
+   = help: for more information, see <https://doc.rust-lang.org/reference/destructors.html>
+
+error: aborting due to 2 previous errors
+

tests/ui/lint/dangling-pointers-from-temporaries/example-from-issue123613.stderr

@@ -0,0 +1,32 @@
+#![deny(dangling_pointers_from_temporaries)]
+
+use std::fmt::Debug;
+
+trait Ext1 {
+    fn dbg(self) -> Self
+    where
+        Self: Sized + Debug,
+    {
+        dbg!(&self);
+        self
+    }
+}
+
+impl<T> Ext1 for *const T {}
+
+trait Ext2 {
+    fn foo(self);
+}
+
+impl Ext2 for *const u32 {
+    fn foo(self) {
+        dbg!(unsafe { self.read() });
+    }
+}
+
+fn main() {
+    let _ptr1 = Vec::<u32>::new().as_ptr().dbg();
+    //~^ ERROR a dangling pointer will be produced because the temporary `Vec<u32>` will be dropped
+    let _ptr2 = vec![0].as_ptr().foo();
+    //~^ ERROR a dangling pointer will be produced because the temporary `Vec<u32>` will be dropped
+}

tests/ui/lint/dangling-pointers-from-temporaries/ext.rs

@@ -0,0 +1,29 @@
+error: a dangling pointer will be produced because the temporary `Vec<u32>` will be dropped
+  --> $DIR/ext.rs:28:35
+   |
+LL |     let _ptr1 = Vec::<u32>::new().as_ptr().dbg();
+   |                 ----------------- ^^^^^^ this pointer will immediately be invalid
+   |                 |
+   |                 this `Vec<u32>` is deallocated at the end of the statement, bind it to a variable to extend its lifetime
+   |
+   = note: pointers do not have a lifetime; when calling `as_ptr` the `Vec<u32>` will be deallocated at the end of the statement because nothing is referencing it as far as the type system is concerned
+   = help: for more information, see <https://doc.rust-lang.org/reference/destructors.html>
+note: the lint level is defined here
+  --> $DIR/ext.rs:1:9
+   |
+LL | #![deny(dangling_pointers_from_temporaries)]
+   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: a dangling pointer will be produced because the temporary `Vec<u32>` will be dropped
+  --> $DIR/ext.rs:30:25
+   |
+LL |     let _ptr2 = vec![0].as_ptr().foo();
+   |                 ------- ^^^^^^ this pointer will immediately be invalid
+   |                 |
+   |                 this `Vec<u32>` is deallocated at the end of the statement, bind it to a variable to extend its lifetime
+   |
+   = note: pointers do not have a lifetime; when calling `as_ptr` the `Vec<u32>` will be deallocated at the end of the statement because nothing is referencing it as far as the type system is concerned
+   = help: for more information, see <https://doc.rust-lang.org/reference/destructors.html>
+
+error: aborting due to 2 previous errors
+

tests/ui/lint/dangling-pointers-from-temporaries/ext.stderr

@@ -0,0 +1,8 @@
+#![deny(dangling_pointers_from_temporaries)]
+
+fn main() {
+    vec![0u8].as_ptr();
+    //~^ ERROR a dangling pointer will be produced because the temporary `Vec<u8>` will be dropped
+    vec![0u8].as_mut_ptr();
+    //~^ ERROR a dangling pointer will be produced because the temporary `Vec<u8>` will be dropped
+}

tests/ui/lint/dangling-pointers-from-temporaries/methods.rs

@@ -0,0 +1,29 @@
+error: a dangling pointer will be produced because the temporary `Vec<u8>` will be dropped
+  --> $DIR/methods.rs:4:15
+   |
+LL |     vec![0u8].as_ptr();
+   |     --------- ^^^^^^ this pointer will immediately be invalid
+   |     |
+   |     this `Vec<u8>` is deallocated at the end of the statement, bind it to a variable to extend its lifetime
+   |
+   = note: pointers do not have a lifetime; when calling `as_ptr` the `Vec<u8>` will be deallocated at the end of the statement because nothing is referencing it as far as the type system is concerned
+   = help: for more information, see <https://doc.rust-lang.org/reference/destructors.html>
+note: the lint level is defined here
+  --> $DIR/methods.rs:1:9
+   |
+LL | #![deny(dangling_pointers_from_temporaries)]
+   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: a dangling pointer will be produced because the temporary `Vec<u8>` will be dropped
+  --> $DIR/methods.rs:6:15
+   |
+LL |     vec![0u8].as_mut_ptr();
+   |     --------- ^^^^^^^^^^ this pointer will immediately be invalid
+   |     |
+   |     this `Vec<u8>` is deallocated at the end of the statement, bind it to a variable to extend its lifetime
+   |
+   = note: pointers do not have a lifetime; when calling `as_mut_ptr` the `Vec<u8>` will be deallocated at the end of the statement because nothing is referencing it as far as the type system is concerned
+   = help: for more information, see <https://doc.rust-lang.org/reference/destructors.html>
+
+error: aborting due to 2 previous errors
+

tests/ui/lint/dangling-pointers-from-temporaries/methods.stderr

@@ -0,0 +1,136 @@
+#![allow(unused)]
+#![deny(dangling_pointers_from_temporaries)]
+
+fn string() -> String {
+    "hello".into()
+}
+
+struct Wrapper(String);
+
+fn main() {
+    // ConstBlock
+    const { String::new() }.as_ptr();
+
+    // Array
+    {
+        [string()].as_ptr(); // False negative
+        [true].as_ptr();
+    }
+
+    // Call
+    string().as_ptr();
+    //~^ ERROR a dangling pointer will be produced because the temporary `String` will be dropped
+
+    // MethodCall
+    "hello".to_string().as_ptr();
+    //~^ ERROR a dangling pointer will be produced because the temporary `String` will be dropped
+
+    // Tup
+    // impossible
+
+    // Binary
+    (string() + "hello").as_ptr();
+    //~^ ERROR a dangling pointer will be produced because the temporary `String` will be dropped
+
+    // Path
+    {
+        let x = string();
+        x.as_ptr();
+    }
+
+    // Unary
+    {
+        let x = string();
+        let x: &String = &x;
+        (*x).as_ptr();
+        (&[0u8]).as_ptr();
+        (&string()).as_ptr(); // False negative
+        (*&string()).as_ptr(); // False negative
+    }
+
+    // Lit
+    "hello".as_ptr();
+
+    // Cast
+    // impossible
+
+    // Type
+    // impossible
+
+    // DropTemps
+    // impossible
+
+    // Let
+    // impossible
+
+    // If
+    {
+        (if true { String::new() } else { "hello".into() }).as_ptr();
+        //~^ ERROR a dangling pointer will be produced because the temporary `String` will be dropped
+    }
+
+    // Loop
+    {
+        (loop {
+            break String::new();
+        })
+        .as_ptr();
+        //~^ ERROR a dangling pointer will be produced because the temporary `String` will be dropped
+    }
+
+    // Match
+    {
+        match string() {
+            s => s,
+        }
+        .as_ptr();
+        //~^ ERROR a dangling pointer will be produced because the temporary `String` will be dropped
+    }
+
+    // Closure
+    // impossible
+
+    // Block
+    { string() }.as_ptr();
+    //~^ ERROR a dangling pointer will be produced because the temporary `String` will be dropped
+
+    // Assign, AssignOp
+    // impossible
+
+    // Field
+    {
+        Wrapper(string()).0.as_ptr(); // False negative
+        let x = Wrapper(string());
+        x.0.as_ptr();
+    }
+
+    // Index
+    {
+        vec![string()][0].as_ptr(); // False negative
+        let x = vec![string()];
+        x[0].as_ptr();
+    }
+
+    // AddrOf, InlineAsm, OffsetOf
+    // impossible
+
+    // Break, Continue, Ret
+    // are !
+
+    // Become, Yield
+    // unstable, are !
+
+    // Repeat
+    [0u8; 100].as_ptr();
+    [const { String::new() }; 100].as_ptr();
+
+    // Struct
+    // Cannot test this without access to private fields of the linted types.
+
+    // Err
+    // impossible
+
+    // Macro
+    vec![0u8].as_ptr();
+    //~^ ERROR a dangling pointer will be produced because the temporary `Vec<u8>` will be dropped
+}

tests/ui/lint/dangling-pointers-from-temporaries/temporaries.rs

@@ -0,0 +1,99 @@
+error: a dangling pointer will be produced because the temporary `String` will be dropped
+  --> $DIR/temporaries.rs:21:14
+   |
+LL |     string().as_ptr();
+   |     -------- ^^^^^^ this pointer will immediately be invalid
+   |     |
+   |     this `String` is deallocated at the end of the statement, bind it to a variable to extend its lifetime
+   |
+   = note: pointers do not have a lifetime; when calling `as_ptr` the `String` will be deallocated at the end of the statement because nothing is referencing it as far as the type system is concerned
+   = help: for more information, see <https://doc.rust-lang.org/reference/destructors.html>
+note: the lint level is defined here
+  --> $DIR/temporaries.rs:2:9
+   |
+LL | #![deny(dangling_pointers_from_temporaries)]
+   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: a dangling pointer will be produced because the temporary `String` will be dropped
+  --> $DIR/temporaries.rs:25:25
+   |
+LL |     "hello".to_string().as_ptr();
+   |     ------------------- ^^^^^^ this pointer will immediately be invalid
+   |     |
+   |     this `String` is deallocated at the end of the statement, bind it to a variable to extend its lifetime
+   |
+   = note: pointers do not have a lifetime; when calling `as_ptr` the `String` will be deallocated at the end of the statement because nothing is referencing it as far as the type system is concerned
+   = help: for more information, see <https://doc.rust-lang.org/reference/destructors.html>
+
+error: a dangling pointer will be produced because the temporary `String` will be dropped
+  --> $DIR/temporaries.rs:32:26
+   |
+LL |     (string() + "hello").as_ptr();
+   |     -------------------- ^^^^^^ this pointer will immediately be invalid
+   |     |
+   |     this `String` is deallocated at the end of the statement, bind it to a variable to extend its lifetime
+   |
+   = note: pointers do not have a lifetime; when calling `as_ptr` the `String` will be deallocated at the end of the statement because nothing is referencing it as far as the type system is concerned
+   = help: for more information, see <https://doc.rust-lang.org/reference/destructors.html>
+
+error: a dangling pointer will be produced because the temporary `String` will be dropped
+  --> $DIR/temporaries.rs:68:61
+   |
+LL |         (if true { String::new() } else { "hello".into() }).as_ptr();
+   |         --------------------------------------------------- ^^^^^^ this pointer will immediately be invalid
+   |         |
+   |         this `String` is deallocated at the end of the statement, bind it to a variable to extend its lifetime
+   |
+   = note: pointers do not have a lifetime; when calling `as_ptr` the `String` will be deallocated at the end of the statement because nothing is referencing it as far as the type system is concerned
+   = help: for more information, see <https://doc.rust-lang.org/reference/destructors.html>
+
+error: a dangling pointer will be produced because the temporary `String` will be dropped
+  --> $DIR/temporaries.rs:77:10
+   |
+LL | /         (loop {
+LL | |             break String::new();
+LL | |         })
+   | |__________- this `String` is deallocated at the end of the statement, bind it to a variable to extend its lifetime
+LL |           .as_ptr();
+   |            ^^^^^^ this pointer will immediately be invalid
+   |
+   = note: pointers do not have a lifetime; when calling `as_ptr` the `String` will be deallocated at the end of the statement because nothing is referencing it as far as the type system is concerned
+   = help: for more information, see <https://doc.rust-lang.org/reference/destructors.html>
+
+error: a dangling pointer will be produced because the temporary `String` will be dropped
+  --> $DIR/temporaries.rs:86:10
+   |
+LL | /         match string() {
+LL | |             s => s,
+LL | |         }
+   | |_________- this `String` is deallocated at the end of the statement, bind it to a variable to extend its lifetime
+LL |           .as_ptr();
+   |            ^^^^^^ this pointer will immediately be invalid
+   |
+   = note: pointers do not have a lifetime; when calling `as_ptr` the `String` will be deallocated at the end of the statement because nothing is referencing it as far as the type system is concerned
+   = help: for more information, see <https://doc.rust-lang.org/reference/destructors.html>
+
+error: a dangling pointer will be produced because the temporary `String` will be dropped
+  --> $DIR/temporaries.rs:94:18
+   |
+LL |     { string() }.as_ptr();
+   |     ------------ ^^^^^^ this pointer will immediately be invalid
+   |     |
+   |     this `String` is deallocated at the end of the statement, bind it to a variable to extend its lifetime
+   |
+   = note: pointers do not have a lifetime; when calling `as_ptr` the `String` will be deallocated at the end of the statement because nothing is referencing it as far as the type system is concerned
+   = help: for more information, see <https://doc.rust-lang.org/reference/destructors.html>
+
+error: a dangling pointer will be produced because the temporary `Vec<u8>` will be dropped
+  --> $DIR/temporaries.rs:134:15
+   |
+LL |     vec![0u8].as_ptr();
+   |     --------- ^^^^^^ this pointer will immediately be invalid
+   |     |
+   |     this `Vec<u8>` is deallocated at the end of the statement, bind it to a variable to extend its lifetime
+   |
+   = note: pointers do not have a lifetime; when calling `as_ptr` the `Vec<u8>` will be deallocated at the end of the statement because nothing is referencing it as far as the type system is concerned
+   = help: for more information, see <https://doc.rust-lang.org/reference/destructors.html>
+
+error: aborting due to 8 previous errors
+

tests/ui/lint/dangling-pointers-from-temporaries/temporaries.stderr

@@ -0,0 +1,52 @@
+#![deny(dangling_pointers_from_temporaries)]
+
+use std::cell::Cell;
+use std::ffi::{CStr, CString};
+use std::mem::MaybeUninit;
+
+struct AsPtrFake;
+
+impl AsPtrFake {
+    fn as_ptr(&self) -> *const () {
+        std::ptr::null()
+    }
+}
+
+fn declval<T>() -> T {
+    loop {}
+}
+
+fn main() {
+    declval::<CString>().as_ptr();
+    //~^ ERROR a dangling pointer will be produced because the temporary `CString` will be dropped
+    declval::<String>().as_ptr();
+    //~^ ERROR a dangling pointer will be produced because the temporary `String` will be dropped
+    declval::<Vec<u8>>().as_ptr();
+    //~^ ERROR a dangling pointer will be produced because the temporary `Vec<u8>` will be dropped
+    declval::<Box<CString>>().as_ptr();
+    //~^ ERROR a dangling pointer will be produced because the temporary `Box<CString>` will be dropped
+    declval::<Box<[u8]>>().as_ptr();
+    //~^ ERROR a dangling pointer will be produced because the temporary `Box<[u8]>` will be dropped
+    declval::<Box<str>>().as_ptr();
+    //~^ ERROR a dangling pointer will be produced because the temporary `Box<str>` will be dropped
+    declval::<Box<CStr>>().as_ptr();
+    //~^ ERROR a dangling pointer will be produced because the temporary `Box<CStr>` will be dropped
+    declval::<[u8; 10]>().as_ptr();
+    //~^ ERROR a dangling pointer will be produced because the temporary `[u8; 10]` will be dropped
+    declval::<Box<[u8; 10]>>().as_ptr();
+    //~^ ERROR a dangling pointer will be produced because the temporary `Box<[u8; 10]>` will be dropped
+    declval::<Box<Vec<u8>>>().as_ptr();
+    //~^ ERROR a dangling pointer will be produced because the temporary `Box<Vec<u8>>` will be dropped
+    declval::<Box<String>>().as_ptr();
+    //~^ ERROR a dangling pointer will be produced because the temporary `Box<String>` will be dropped
+    declval::<Box<Box<Box<Box<[u8]>>>>>().as_ptr();
+    //~^ ERROR a dangling pointer will be produced because the temporary `Box<Box<Box<Box<[u8]>>>>` will be dropped
+    declval::<Cell<u8>>().as_ptr();
+    //~^ ERROR a dangling pointer will be produced because the temporary `Cell<u8>` will be dropped
+    declval::<MaybeUninit<u8>>().as_ptr();
+    //~^ ERROR a dangling pointer will be produced because the temporary `MaybeUninit<u8>` will be dropped
+    declval::<Vec<AsPtrFake>>().as_ptr();
+    //~^ ERROR a dangling pointer will be produced because the temporary `Vec<AsPtrFake>` will be dropped
+    declval::<Box<AsPtrFake>>().as_ptr();
+    declval::<AsPtrFake>().as_ptr();
+}

tests/ui/lint/dangling-pointers-from-temporaries/types.rs

@@ -0,0 +1,172 @@
+error: a dangling pointer will be produced because the temporary `CString` will be dropped
+  --> $DIR/types.rs:20:26
+   |
+LL |     declval::<CString>().as_ptr();
+   |     -------------------- ^^^^^^ this pointer will immediately be invalid
+   |     |
+   |     this `CString` is deallocated at the end of the statement, bind it to a variable to extend its lifetime
+   |
+   = note: pointers do not have a lifetime; when calling `as_ptr` the `CString` will be deallocated at the end of the statement because nothing is referencing it as far as the type system is concerned
+   = help: for more information, see <https://doc.rust-lang.org/reference/destructors.html>
+note: the lint level is defined here
+  --> $DIR/types.rs:1:9
+   |
+LL | #![deny(dangling_pointers_from_temporaries)]
+   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: a dangling pointer will be produced because the temporary `String` will be dropped
+  --> $DIR/types.rs:22:25
+   |
+LL |     declval::<String>().as_ptr();
+   |     ------------------- ^^^^^^ this pointer will immediately be invalid
+   |     |
+   |     this `String` is deallocated at the end of the statement, bind it to a variable to extend its lifetime
+   |
+   = note: pointers do not have a lifetime; when calling `as_ptr` the `String` will be deallocated at the end of the statement because nothing is referencing it as far as the type system is concerned
+   = help: for more information, see <https://doc.rust-lang.org/reference/destructors.html>
+
+error: a dangling pointer will be produced because the temporary `Vec<u8>` will be dropped
+  --> $DIR/types.rs:24:26
+   |
+LL |     declval::<Vec<u8>>().as_ptr();
+   |     -------------------- ^^^^^^ this pointer will immediately be invalid
+   |     |
+   |     this `Vec<u8>` is deallocated at the end of the statement, bind it to a variable to extend its lifetime
+   |
+   = note: pointers do not have a lifetime; when calling `as_ptr` the `Vec<u8>` will be deallocated at the end of the statement because nothing is referencing it as far as the type system is concerned
+   = help: for more information, see <https://doc.rust-lang.org/reference/destructors.html>
+
+error: a dangling pointer will be produced because the temporary `Box<CString>` will be dropped
+  --> $DIR/types.rs:26:31
+   |
+LL |     declval::<Box<CString>>().as_ptr();
+   |     ------------------------- ^^^^^^ this pointer will immediately be invalid
+   |     |
+   |     this `Box<CString>` is deallocated at the end of the statement, bind it to a variable to extend its lifetime
+   |
+   = note: pointers do not have a lifetime; when calling `as_ptr` the `Box<CString>` will be deallocated at the end of the statement because nothing is referencing it as far as the type system is concerned
+   = help: for more information, see <https://doc.rust-lang.org/reference/destructors.html>
+
+error: a dangling pointer will be produced because the temporary `Box<[u8]>` will be dropped
+  --> $DIR/types.rs:28:28
+   |
+LL |     declval::<Box<[u8]>>().as_ptr();
+   |     ---------------------- ^^^^^^ this pointer will immediately be invalid
+   |     |
+   |     this `Box<[u8]>` is deallocated at the end of the statement, bind it to a variable to extend its lifetime
+   |
+   = note: pointers do not have a lifetime; when calling `as_ptr` the `Box<[u8]>` will be deallocated at the end of the statement because nothing is referencing it as far as the type system is concerned
+   = help: for more information, see <https://doc.rust-lang.org/reference/destructors.html>
+
+error: a dangling pointer will be produced because the temporary `Box<str>` will be dropped
+  --> $DIR/types.rs:30:27
+   |
+LL |     declval::<Box<str>>().as_ptr();
+   |     --------------------- ^^^^^^ this pointer will immediately be invalid
+   |     |
+   |     this `Box<str>` is deallocated at the end of the statement, bind it to a variable to extend its lifetime
+   |
+   = note: pointers do not have a lifetime; when calling `as_ptr` the `Box<str>` will be deallocated at the end of the statement because nothing is referencing it as far as the type system is concerned
+   = help: for more information, see <https://doc.rust-lang.org/reference/destructors.html>
+
+error: a dangling pointer will be produced because the temporary `Box<CStr>` will be dropped
+  --> $DIR/types.rs:32:28
+   |
+LL |     declval::<Box<CStr>>().as_ptr();
+   |     ---------------------- ^^^^^^ this pointer will immediately be invalid
+   |     |
+   |     this `Box<CStr>` is deallocated at the end of the statement, bind it to a variable to extend its lifetime
+   |
+   = note: pointers do not have a lifetime; when calling `as_ptr` the `Box<CStr>` will be deallocated at the end of the statement because nothing is referencing it as far as the type system is concerned
+   = help: for more information, see <https://doc.rust-lang.org/reference/destructors.html>
+
+error: a dangling pointer will be produced because the temporary `[u8; 10]` will be dropped
+  --> $DIR/types.rs:34:27
+   |
+LL |     declval::<[u8; 10]>().as_ptr();
+   |     --------------------- ^^^^^^ this pointer will immediately be invalid
+   |     |
+   |     this `[u8; 10]` is deallocated at the end of the statement, bind it to a variable to extend its lifetime
+   |
+   = note: pointers do not have a lifetime; when calling `as_ptr` the `[u8; 10]` will be deallocated at the end of the statement because nothing is referencing it as far as the type system is concerned
+   = help: for more information, see <https://doc.rust-lang.org/reference/destructors.html>
+
+error: a dangling pointer will be produced because the temporary `Box<[u8; 10]>` will be dropped
+  --> $DIR/types.rs:36:32
+   |
+LL |     declval::<Box<[u8; 10]>>().as_ptr();
+   |     -------------------------- ^^^^^^ this pointer will immediately be invalid
+   |     |
+   |     this `Box<[u8; 10]>` is deallocated at the end of the statement, bind it to a variable to extend its lifetime
+   |
+   = note: pointers do not have a lifetime; when calling `as_ptr` the `Box<[u8; 10]>` will be deallocated at the end of the statement because nothing is referencing it as far as the type system is concerned
+   = help: for more information, see <https://doc.rust-lang.org/reference/destructors.html>
+
+error: a dangling pointer will be produced because the temporary `Box<Vec<u8>>` will be dropped
+  --> $DIR/types.rs:38:31
+   |
+LL |     declval::<Box<Vec<u8>>>().as_ptr();
+   |     ------------------------- ^^^^^^ this pointer will immediately be invalid
+   |     |
+   |     this `Box<Vec<u8>>` is deallocated at the end of the statement, bind it to a variable to extend its lifetime
+   |
+   = note: pointers do not have a lifetime; when calling `as_ptr` the `Box<Vec<u8>>` will be deallocated at the end of the statement because nothing is referencing it as far as the type system is concerned
+   = help: for more information, see <https://doc.rust-lang.org/reference/destructors.html>
+
+error: a dangling pointer will be produced because the temporary `Box<String>` will be dropped
+  --> $DIR/types.rs:40:30
+   |
+LL |     declval::<Box<String>>().as_ptr();
+   |     ------------------------ ^^^^^^ this pointer will immediately be invalid
+   |     |
+   |     this `Box<String>` is deallocated at the end of the statement, bind it to a variable to extend its lifetime
+   |
+   = note: pointers do not have a lifetime; when calling `as_ptr` the `Box<String>` will be deallocated at the end of the statement because nothing is referencing it as far as the type system is concerned
+   = help: for more information, see <https://doc.rust-lang.org/reference/destructors.html>
+
+error: a dangling pointer will be produced because the temporary `Box<Box<Box<Box<[u8]>>>>` will be dropped
+  --> $DIR/types.rs:42:43
+   |
+LL |     declval::<Box<Box<Box<Box<[u8]>>>>>().as_ptr();
+   |     ------------------------------------- ^^^^^^ this pointer will immediately be invalid
+   |     |
+   |     this `Box<Box<Box<Box<[u8]>>>>` is deallocated at the end of the statement, bind it to a variable to extend its lifetime
+   |
+   = note: pointers do not have a lifetime; when calling `as_ptr` the `Box<Box<Box<Box<[u8]>>>>` will be deallocated at the end of the statement because nothing is referencing it as far as the type system is concerned
+   = help: for more information, see <https://doc.rust-lang.org/reference/destructors.html>
+
+error: a dangling pointer will be produced because the temporary `Cell<u8>` will be dropped
+  --> $DIR/types.rs:44:27
+   |
+LL |     declval::<Cell<u8>>().as_ptr();
+   |     --------------------- ^^^^^^ this pointer will immediately be invalid
+   |     |
+   |     this `Cell<u8>` is deallocated at the end of the statement, bind it to a variable to extend its lifetime
+   |
+   = note: pointers do not have a lifetime; when calling `as_ptr` the `Cell<u8>` will be deallocated at the end of the statement because nothing is referencing it as far as the type system is concerned
+   = help: for more information, see <https://doc.rust-lang.org/reference/destructors.html>
+
+error: a dangling pointer will be produced because the temporary `MaybeUninit<u8>` will be dropped
+  --> $DIR/types.rs:46:34
+   |
+LL |     declval::<MaybeUninit<u8>>().as_ptr();
+   |     ---------------------------- ^^^^^^ this pointer will immediately be invalid
+   |     |
+   |     this `MaybeUninit<u8>` is deallocated at the end of the statement, bind it to a variable to extend its lifetime
+   |
+   = note: pointers do not have a lifetime; when calling `as_ptr` the `MaybeUninit<u8>` will be deallocated at the end of the statement because nothing is referencing it as far as the type system is concerned
+   = help: for more information, see <https://doc.rust-lang.org/reference/destructors.html>
+
+error: a dangling pointer will be produced because the temporary `Vec<AsPtrFake>` will be dropped
+  --> $DIR/types.rs:48:33
+   |
+LL |     declval::<Vec<AsPtrFake>>().as_ptr();
+   |     --------------------------- ^^^^^^ this pointer will immediately be invalid
+   |     |
+   |     this `Vec<AsPtrFake>` is deallocated at the end of the statement, bind it to a variable to extend its lifetime
+   |
+   = note: pointers do not have a lifetime; when calling `as_ptr` the `Vec<AsPtrFake>` will be deallocated at the end of the statement because nothing is referencing it as far as the type system is concerned
+   = help: for more information, see <https://doc.rust-lang.org/reference/destructors.html>
+
+error: aborting due to 15 previous errors
+