- Fixes rust-lang#91628.
 - Fixes emscripten-core/emscripten#15722.

See discussion in both issues.

The TL;DR is that weak linkage causes LLVM to produce broken Wasm, presumably due to pointer mismatch. The code is casting a void pointer to a function pointer with specific signature, but Wasm is very strict about function pointer compatibility, so the resulting code is invalid.

Ideally LLVM should catch this earlier in the process rather than emit invalid Wasm, but it currently doesn't and this is an easy and valid fix, given that Emcripten doesn't have `__cxa_thread_atexit_impl` these days anyway.

Unfortunately, I can't add a regression test as even after looking into this issue for a long time, I couldn't reproduce it with any minimal Rust example, only with extracted LLVM IR or on a large project involving Rust   C  .

r? @alexcrichton

…extension.

gdb et al. expect to find the dwp file at <binary>.dwp, even if <binary> already
has an extension (e.g. libfoo.so's dwp is expected to be at libfoo.so.dwp).

This way we can properly deal with the types.

This works correctly with inference variables.

- add note and suggestion for casting both expected and found fn items
  to fn pointers
- add note for casting expected fn item to fn pointer

…tolnay

impl DispatchFromDyn for Cell and UnsafeCell

After some fruitful discussion on [Internals](https://internals.rust-lang.org/t/impl-dispatchfromdyn-for-cell-2/16520) here's my first PR to rust-lang/rust 🎉

Please let me know if there's something I missed.

This adds `DispatchFromDyn` impls for `Cell`, `UnsafeCell` and `SyncUnsafeCell`.
An existing test is also expanded to test the `Cell` impl (which requires the `UnsafeCell` impl)

The different `RefCell` types can not implement `DispatchFromDyn` since they have more than one (non ZST) field.

 

**Edit:**
### What:
These changes allow one to make types like `MyRc`(code below), to be object safe method receivers after implementing `DispatchFromDyn` and `Deref` for them.

This allows for code like this:
```rust
struct MyRc<T: ?Sized>(Cell<NonNull<RcBox<T>>>);

/* impls for DispatchFromDyn, CoerceUnsized and Deref for MyRc*/

trait Trait {
    fn foo(self: MyRc<Self>);
}

let impls_trait = ...;
let rc = MyRc::new(impls_trait) as MyRc<dyn Trait>;
rc.foo();
```

Note: `Cell` and `UnsafeCell` won't directly become valid method receivers since they don't implement `Deref`. Making use of these changes requires a wrapper type and nightly features.

### Why:
A custom pointer type with interior mutability allows one to store extra information in the pointer itself.
These changes allow for such a type to be a method receiver.

### Examples:
My use case is a cycle aware custom `Rc` implementation that when dropping a cycle marks some references dangling.

On the [forum](https://internals.rust-lang.org/t/impl-dispatchfromdyn-for-cell/14762/8) andersk mentioned that they track if a `Gc` reference is rooted with an extra bit in the reference itself.

Remove backwards compat for LLVM 12 coverage format

The minimum external LLVM was updated to 13 recently in rust-lang#100611, so this PR removes backwards compat with older coverage formats.

I kept the version check and error message there, in accordance with this comment: rust-lang#91207 (comment)

Avoid __cxa_thread_atexit_impl on Emscripten

 - Fixes rust-lang#91628.
 - Fixes emscripten-core/emscripten#15722.

See discussion in both issues.

The TL;DR is that weak linkage causes LLVM to produce broken Wasm, presumably due to pointer mismatch. The code is casting a void pointer to a function pointer with specific signature, but Wasm is very strict about function pointer compatibility, so the resulting code is invalid.

Ideally LLVM should catch this earlier in the process rather than emit invalid Wasm, but it currently doesn't and this is an easy and valid fix, given that Emcripten doesn't have `__cxa_thread_atexit_impl` these days anyway.

Unfortunately, I can't add a regression test as even after looking into this issue for a long time, I couldn't reproduce it with any minimal Rust example, only with extracted LLVM IR or on a large project involving Rust   C  .

Append .dwp to the binary filename instead of replacing the existing extension.

gdb et al. expect to find the dwp file at `<binary>`.dwp, even if <binary> already has an extension (e.g. libfoo.so's dwp is expected to be at libfoo.so.dwp).

…anieu

Remove optimistic spinning from `mpsc::SyncSender`

Per rust-lang#106701 (comment).
Closes rust-lang#106804

r? `@Amanieu`

implement Hash for proc_macro::LineColumn

For use in `HashMap<LineColumn, TokenTree>` or `HashMap<LineColumn, Comment>`, for example.

[Here is an example of one case complicated by the absence of this impl.](https://github.com/andrewbaxter/genemichaels/blob/71bc45e417c3f9dae09f890f1ec4630e758e5c70/src/comments.rs#L25-L34)

Tracking issue: rust-lang#54725

…compiler-errors

remove unnecessary check for opaque types

this isn't needed and may hide some errors.

after analysis there are no opaque types so it's a noop anyways

before analysis there are opaque types but due to `Reveal::UserFacing` we don't reveal them. `is_subtype` simply discards the opaque type constraints as these will get checked again during mir borrowck.

r? types

want to land this after the beta-cutoff as mir validator changes are apparently pretty scary

…s, r=compiler-errors

Improve fn pointer notes

continuation of rust-lang#105552

r? `@compiler-errors`

…cause what else are we gonna use in rustc_middle, r=compiler-errors

Use `can_eq` to compare types for default assoc type error

This correctly handles inference variables like `{integer}`. I had to move all of this `note_and_explain` code to `rustc_infer`, it made no sense for it to be in `rustc_middle` anyways.

The commits are reviewed separately.

Fixes rust-lang#106968