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Update E0744 about control flow in const contexts to accurately describe when the error is triggered and why #79087

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Nov 17, 2020
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Update E0744 about control flow in const contexts to accurately describe when the error is triggered and why #79087

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19 changes: 11 additions & 8 deletions compiler/rustc_error_codes/src/error_codes/E0744.md
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Original file line number Diff line number Diff line change
@@ -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:

Expand All @@ -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