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Rollup merge of #79087 - ThePuzzlemaker:issue-79083-docfix, r=RalfJung

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Update E0744 about control flow in `const` contexts to accurately describe when the error is triggered and why

This PR fixes #79083. `const fn` currently supports `if`, `match`, `loop`, and `while` in terms of control flow. The error relating to control flow in `const` contexts currently states that those control flow constructs are not allowed in `const` contexts. That is no longer true, as RFC 2342 and 2344 were [stabilized](https://github.com/rust-lang/rust/pull/72437). `for` loops, however, as well as `?` and `.await` are still not allowed, so I changed the error message to be more descriptive of the error as it is not just control flow that could trigger this error. I also added links to tracking issues that mark things that are blocking the usage of these expressions.
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Mara Bos 2020-11-16 17:26:38 +01:00 committed by GitHub
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compiler/rustc_error_codes/src/error_codes/E0744.md
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@ -1,4 +1,4 @@
A control-flow expression was used inside a const context. An unsupported expression was used inside a const context.
Erroneous code example: Erroneous code example:
@ -12,12 +12,15 @@ const _: i32 = {
}; };
``` ```
At the moment, `if` and `match`, as well as the looping constructs `for`, At the moment, `for` loops, `.await`, and the `Try` operator (`?`) are forbidden
`while`, and `loop`, are forbidden inside a `const`, `static`, or `const fn`. inside a `const`, `static`, or `const fn`.
This will be allowed at some point in the future, but the implementation is not This may be allowed at some point in the future, but the implementation is not
yet complete. See the tracking issue for [conditionals] or [loops] in a const yet complete. See the tracking issues for [`async`] and [`?`] in `const fn`, and
context for the current status. (to support `for` loops in `const fn`) the tracking issues for [`impl const
Trait for Ty`] and [`&mut T`] in `const fn`.
[conditionals]: https://github.com/rust-lang/rust/issues/49146 [`async`]: https://github.com/rust-lang/rust/issues/69431
[loops]: https://github.com/rust-lang/rust/issues/52000 [`?`]: https://github.com/rust-lang/rust/issues/74935
[`impl const Trait for Ty`]: https://github.com/rust-lang/rust/issues/67792
[`&mut T`]: https://github.com/rust-lang/rust/issues/57349