Remove `SelectionContext::infcx()` in favor of field access
Encapsulation doesn't seem particularly important here, and having two choices is always more confusing than having one.
r? types
Avoid `GenFuture` shim when compiling async constructs
Previously, async constructs would be lowered to "normal" generators, with an additional `from_generator` / `GenFuture` shim in between to convert from `Generator` to `Future`.
The compiler will now special-case these generators internally so that async constructs will *directly* implement `Future` without the need to go through the `from_generator` / `GenFuture` shim.
The primary motivation for this change was hiding this implementation detail in stack traces and debuginfo, but it can in theory also help the optimizer as there is less abstractions to see through.
---
Given this demo code:
```rust
pub async fn a(arg: u32) -> Backtrace {
let bt = b().await;
let _arg = arg;
bt
}
pub async fn b() -> Backtrace {
Backtrace::force_capture()
}
```
I would get the following with the latest stable compiler (on Windows):
```
4: async_codegen:🅱️:async_fn$0
at .\src\lib.rs:10
5: core::future::from_generator::impl$1::poll<enum2$<async_codegen:🅱️:async_fn_env$0> >
at /rustc/897e37553bba8b42751c67658967889d11ecd120\library\core\src\future\mod.rs:91
6: async_codegen:🅰️:async_fn$0
at .\src\lib.rs:4
7: core::future::from_generator::impl$1::poll<enum2$<async_codegen:🅰️:async_fn_env$0> >
at /rustc/897e37553bba8b42751c67658967889d11ecd120\library\core\src\future\mod.rs:91
```
whereas now I get a much cleaner stack trace:
```
3: async_codegen:🅱️:async_fn$0
at .\src\lib.rs:10
4: async_codegen:🅰️:async_fn$0
at .\src\lib.rs:4
```
Previously, async constructs would be lowered to "normal" generators,
with an additional `from_generator` / `GenFuture` shim in between to
convert from `Generator` to `Future`.
The compiler will now special-case these generators internally so that
async constructs will *directly* implement `Future` without the need
to go through the `from_generator` / `GenFuture` shim.
The primary motivation for this change was hiding this implementation
detail in stack traces and debuginfo, but it can in theory also help
the optimizer as there is less abstractions to see through.
Don't allow `CoerceUnsized` into `dyn*` (except for trait upcasting)
This makes sure we don't accidentally allow coercions like `Box<T>` -> `Box<dyn* Trait>`, or in the case of this ICE, `&T` to `&dyn* Trait`. These coercions don't make sense, at least not via the `CoerceUnsized` trait.
Fixes#102172Fixes#102429
a fn pointer doesn't implement `Fn`/`FnMut`/`FnOnce` if its return type isn't sized
I stumbled upon #83915 which hasn't received much attention recently, and I wanted to revive it since this is one existing soundness hole that seems pretty easy to fix.
I'm not actually sure that the [alternative approach described here](https://github.com/rust-lang/rust/pull/83915#issuecomment-823643322) is sufficient, given the `src/test/ui/function-pointer/unsized-ret.rs` example I provided below. Rebasing the branch mentioned in that comment and testing that UI test, it seems that we actually end up only observing that `str: !Sized` during monomorphization, whereupon we ICE. Even if we were to fix that ICE, ideally we'd be raising an error that a fn pointer is being used badly during _typecheck_ instead of monomorphization, hence adapting the original approach in #83915.
I am happy to close this if people would prefer we rebase the original PR and land that -- I am partly opening to be annoying and get people thinking about this unsoundness again ❤️😸
cc: `@estebank` and `@nikomatsakis`
r? types
Here's a link to the thread: 235421351 for more context.
