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Rollup merge of rust-lang#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`
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