Auto merge of #86155 - alexcrichton:abort-on-unwind, r=nikomatsakis

rustc: Fill out remaining parts of C-unwind ABI

This commit intends to fill out some of the remaining pieces of the
C-unwind ABI. This has a number of other changes with it though to move
this design space forward a bit. Notably contained within here is:

* On `panic=unwind`, the `extern "C"` ABI is now considered as "may
  unwind". This fixes a longstanding soundness issue where if you
  `panic!()` in an `extern "C"` function defined in Rust that's actually
  UB because the LLVM representation for the function has the `nounwind`
  attribute, but then you unwind.

* Whether or not a function unwinds now mainly considers the ABI of the
  function instead of first checking the panic strategy. This fixes a
  miscompile of `extern "C-unwind"` with `panic=abort` because that ABI
  can still unwind.

* The aborting stub for non-unwinding ABIs with `panic=unwind` has been
  reimplemented. Previously this was done as a small tweak during MIR
  generation, but this has been moved to a separate and dedicated MIR
  pass. This new pass will, for appropriate functions and function
  calls, insert a `cleanup` landing pad for any function call that may
  unwind within a function that is itself not allowed to unwind. Note
  that this subtly changes some behavior from before where previously on
  an unwind which was caught-to-abort it would run active destructors in
  the function, and now it simply immediately aborts the process.

* The `#[unwind]` attribute has been removed and all users in tests and
  such are now using `C-unwind` and `#![feature(c_unwind)]`.

I think this is largely the last piece of the RFC to implement.
Unfortunately I believe this is still not stabilizable as-is because
activating the feature gate changes the behavior of the existing `extern
"C"` ABI in a way that has no replacement. My thinking for how to enable
this is that we add support for the `C-unwind` ABI on stable Rust first,
and then after it hits stable we change the behavior of the `C` ABI.
That way anyone straddling stable/beta/nightly can switch to `C-unwind`
safely.

Author: bors

compiler/rustc_attr/src/builtin.rs

@@ -87,50 +87,6 @@ pub enum OptimizeAttr {
     Size,
 }
 
-#[derive(Copy, Clone, PartialEq)]
-pub enum UnwindAttr {
-    Allowed,
-    Aborts,
-}
-
-/// Determine what `#[unwind]` attribute is present in `attrs`, if any.
-pub fn find_unwind_attr(sess: &Session, attrs: &[Attribute]) -> Option<UnwindAttr> {
-    attrs.iter().fold(None, |ia, attr| {
-        if sess.check_name(attr, sym::unwind) {
-            if let Some(meta) = attr.meta() {
-                if let MetaItemKind::List(items) = meta.kind {
-                    if items.len() == 1 {
-                        if items[0].has_name(sym::allowed) {
-                            return Some(UnwindAttr::Allowed);
-                        } else if items[0].has_name(sym::aborts) {
-                            return Some(UnwindAttr::Aborts);
-                        }
-                    }
-
-                    struct_span_err!(
-                        sess.diagnostic(),
-                        attr.span,
-                        E0633,
-                        "malformed `unwind` attribute input"
-                    )
-                    .span_label(attr.span, "invalid argument")
-                    .span_suggestions(
-                        attr.span,
-                        "the allowed arguments are `allowed` and `aborts`",
-                        (vec!["allowed", "aborts"])
-                            .into_iter()
-                            .map(|s| format!("#[unwind({})]", s)),
-                        Applicability::MachineApplicable,
-                    )
-                    .emit();
-                }
-            }
-        }
-
-        ia
-    })
-}
-
 /// Represents the following attributes:
 ///
 /// - `#[stable]`

compiler/rustc_error_codes/src/error_codes/E0633.md

@@ -1,8 +1,10 @@
+#### Note: this error code is no longer emitted by the compiler.
+
 The `unwind` attribute was malformed.
 
 Erroneous code example:
 
-```compile_fail,E0633
+```compile_fail
 #![feature(unwind_attributes)]
 
 #[unwind()] // error: expected one argument