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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:
@ -12,12 +12,15 @@ const _: i32 = {
};
```
At the moment, `if` and `match`, as well as the looping constructs `for`,
`while`, and `loop`, are forbidden inside a `const`, `static`, or `const fn`.
At the moment, `for` loops, `.await`, and the `Try` operator (`?`) are forbidden
inside a `const`, `static`, or `const fn`.
This will 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
context for the current status.
This may be allowed at some point in the future, but the implementation is not
yet complete. See the tracking issues for [`async`] and [`?`] in `const fn`, and
(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
[loops]: https://github.com/rust-lang/rust/issues/52000
[`async`]: https://github.com/rust-lang/rust/issues/69431
[`?`]: 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