Rollup merge of #106541 - fee1-dead-contrib:no-const-check-no, r=thomcc

implement const iterator using `rustc_do_not_const_check`

Previous experiment: #102225.

Explanation: rather than making all default methods work under `const` all at once, this uses `rustc_do_not_const_check` as a workaround to "trick" the compiler to not run any checks on those other default methods. Any const implementations are only required to implement the `next` method. Any actual calls to the trait methods other than `next` will either error in compile time (at CTFE runs), or run the methods correctly if they do not have any non-const operations. This is extremely easy to maintain, remove, or improve.

Author: Dylan-DPC

compiler/rustc_hir_typeck/src/lib.rs

@@ -71,7 +71,7 @@ use rustc_middle::ty::{self, Ty, TyCtxt};
 use rustc_session::config;
 use rustc_session::Session;
 use rustc_span::def_id::{DefId, LocalDefId};
-use rustc_span::Span;
+use rustc_span::{sym, Span};
 
 fluent_messages! { "../locales/en-US.ftl" }
 
@@ -207,6 +207,11 @@ fn typeck_with_fallback<'tcx>(
 
     let typeck_results = Inherited::build(tcx, def_id).enter(|inh| {
         let param_env = tcx.param_env(def_id);
+        let param_env = if tcx.has_attr(def_id.to_def_id(), sym::rustc_do_not_const_check) {
+            param_env.without_const()
+        } else {
+            param_env
+        };
         let mut fcx = FnCtxt::new(&inh, param_env, def_id);
 
         if let Some(hir::FnSig { header, decl, .. }) = fn_sig {

library/core/src/iter/traits/iterator.rs

@@ -194,6 +194,7 @@
 #![feature(cfg_target_has_atomic_equal_alignment)]
 #![feature(const_closures)]
 #![feature(const_fn_floating_point_arithmetic)]
+#![feature(const_for)]
 #![feature(const_mut_refs)]
 #![feature(const_precise_live_drops)]
 #![feature(const_refs_to_cell)]

library/core/src/lib.rs

@@ -0,0 +1,5 @@
+#[rustc_do_not_const_check]
+//~^ ERROR this is an internal attribute that will never be stable
+const fn foo() {}
+
+fn main() {}

tests/ui/consts/gate-do-not-const-check.rs

@@ -0,0 +1,11 @@
+error[E0658]: this is an internal attribute that will never be stable
+  --> $DIR/gate-do-not-const-check.rs:1:1
+   |
+LL | #[rustc_do_not_const_check]
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = help: add `#![feature(rustc_attrs)]` to the crate attributes to enable
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0658`.

tests/ui/consts/gate-do-not-const-check.stderr

@@ -0,0 +1,19 @@
+// check-pass
+#![feature(const_trait_impl, rustc_attrs)]
+
+#[const_trait]
+trait Foo {
+    #[rustc_do_not_const_check]
+    fn into_iter(&self) { println!("FEAR ME!") }
+}
+
+
+impl const Foo for () {
+    fn into_iter(&self) {
+        // ^_^
+    }
+}
+
+const _: () = Foo::into_iter(&());
+
+fn main() {}

tests/ui/rfc-2632-const-trait-impl/do-not-const-check-override.rs

@@ -0,0 +1,18 @@
+// check-pass
+#![feature(const_trait_impl, rustc_attrs)]
+
+#[const_trait]
+trait IntoIter {
+    fn into_iter(self);
+}
+
+#[const_trait]
+trait Hmm: Sized {
+    #[rustc_do_not_const_check]
+    fn chain<U>(self, other: U) where U: IntoIter,
+    {
+        other.into_iter()
+    }
+}
+
+fn main() {}

tests/ui/rfc-2632-const-trait-impl/do-not-const-check.rs

@@ -227,4 +227,6 @@ fn evens_squared(n: usize) -> _ {
 }
 
 const _: _ = (1..10).filter(|x| x % 2 == 0).map(|x| x * x);
-//~^ ERROR the placeholder `_` is not allowed within types on item signatures for constants
+//~^ ERROR the trait bound
+//~| ERROR the trait bound
+//~| ERROR the placeholder

tests/ui/typeck/typeck_type_placeholder_item.rs

@@ -437,17 +437,37 @@ LL | fn evens_squared(n: usize) -> _ {
    |                               not allowed in type signatures
    |                               help: replace with an appropriate return type: `impl Iterator<Item = usize>`
 
-error[E0121]: the placeholder `_` is not allowed within types on item signatures for constants
-  --> $DIR/typeck_type_placeholder_item.rs:229:10
+error[E0277]: the trait bound `std::ops::Range<{integer}>: Iterator` is not satisfied
+  --> $DIR/typeck_type_placeholder_item.rs:229:22
    |
 LL | const _: _ = (1..10).filter(|x| x % 2 == 0).map(|x| x * x);
-   |          ^ not allowed in type signatures
+   |                      ^^^^^^ `std::ops::Range<{integer}>` is not an iterator
+   |
+   = help: the trait `~const Iterator` is not implemented for `std::ops::Range<{integer}>`
+note: the trait `Iterator` is implemented for `std::ops::Range<{integer}>`, but that implementation is not `const`
+  --> $DIR/typeck_type_placeholder_item.rs:229:14
+   |
+LL | const _: _ = (1..10).filter(|x| x % 2 == 0).map(|x| x * x);
+   |              ^^^^^^^
+
+error[E0277]: the trait bound `Filter<std::ops::Range<{integer}>, [closure@$DIR/typeck_type_placeholder_item.rs:229:29: 229:32]>: Iterator` is not satisfied
+  --> $DIR/typeck_type_placeholder_item.rs:229:45
    |
-note: however, the inferred type `Map<Filter<Range<i32>, [closure@typeck_type_placeholder_item.rs:229:29]>, [closure@typeck_type_placeholder_item.rs:229:49]>` cannot be named
+LL | const _: _ = (1..10).filter(|x| x % 2 == 0).map(|x| x * x);
+   |                                             ^^^ `Filter<std::ops::Range<{integer}>, [closure@$DIR/typeck_type_placeholder_item.rs:229:29: 229:32]>` is not an iterator
+   |
+   = help: the trait `~const Iterator` is not implemented for `Filter<std::ops::Range<{integer}>, [closure@$DIR/typeck_type_placeholder_item.rs:229:29: 229:32]>`
+note: the trait `Iterator` is implemented for `Filter<std::ops::Range<{integer}>, [closure@$DIR/typeck_type_placeholder_item.rs:229:29: 229:32]>`, but that implementation is not `const`
   --> $DIR/typeck_type_placeholder_item.rs:229:14
    |
 LL | const _: _ = (1..10).filter(|x| x % 2 == 0).map(|x| x * x);
-   |              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error[E0121]: the placeholder `_` is not allowed within types on item signatures for constants
+  --> $DIR/typeck_type_placeholder_item.rs:229:10
+   |
+LL | const _: _ = (1..10).filter(|x| x % 2 == 0).map(|x| x * x);
+   |          ^ not allowed in type signatures
 
 error[E0121]: the placeholder `_` is not allowed within types on item signatures for functions
   --> $DIR/typeck_type_placeholder_item.rs:140:31
@@ -657,7 +677,7 @@ LL |     const D: _ = 42;
    |              not allowed in type signatures
    |              help: replace with the correct type: `i32`
 
-error: aborting due to 71 previous errors
+error: aborting due to 73 previous errors
 
-Some errors have detailed explanations: E0121, E0282, E0403.
+Some errors have detailed explanations: E0121, E0277, E0282, E0403.
 For more information about an error, try `rustc --explain E0121`.