Rollup merge of #109392 - cbeuw:composite-ret, r=JakobDegen

Custom MIR: Allow optional RET type annotation

This currently doesn't compile because the type of `RET` is inferred, which fails if RET is a composite type and fields are initialised separately.
```rust
#![feature(custom_mir, core_intrinsics)]
extern crate core;
use core::intrinsics::mir::*;
#[custom_mir(dialect = "runtime", phase = "optimized")]
fn fn0() -> (i32, bool) {
    mir! ({
        RET.0 = 0;
        RET.1 = true;
        Return()
    })
}
```
```
error[E0282]: type annotations needed
 --> src/lib.rs:8:9
  |
8 |         RET.0 = 0;
  |         ^^^ cannot infer type

For more information about this error, try `rustc --explain E0282`.
```

This PR allows the user to manually specify the return type with `type RET = ...;` if required:

```rust
#[custom_mir(dialect = "runtime", phase = "optimized")]
fn fn0() -> (i32, bool) {
    mir! (
        type RET = (i32, bool);
        {
            RET.0 = 0;
            RET.1 = true;
            Return()
        }
    )
}
```

The syntax is not optimal, I'm happy to see other suggestions. Ideally I wanted it to be a normal type annotation like `let RET: ...;`, but this runs into the multiple parsing options error during macro expansion, as it can be parsed as a normal `let` declaration as well.

r? ```@oli-obk``` or ```@tmiasko``` or ```@JakobDegen```

0 files changed, 0 insertions, 0 deletions