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Clarify that free constants are always evaluated at compile time #1328

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Merged
merged 2 commits into from
Mar 14, 2023
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Clarify that free constants are always evaluated at compile time #1328

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86691a1
Clarify that unnamed constants are always evaluated at compile time
XrXr Feb 3, 2023
021889f
Adjust language to also cover named constants
XrXr Feb 14, 2023
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16 changes: 16 additions & 0 deletions src/items/constant-items.md
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Expand Up @@ -89,6 +89,22 @@ m!(const _: () = (););
// const _: () = ();
```

## Evaluation

[Free][free] constants are always [evaluated][const_eval] at compile-time to surface
panics. This happens even within an unused function:

```rust,compile_fail
// Compile-time panic
const PANIC: () = std::unimplemented!();

fn unused_generic_function<T>() {
// A failing compile-time assertion
const _: () = assert!(usize::BITS == 0);
}
```

[const_eval]: ../const_eval.md
[associated constant]: ../items/associated-items.md#associated-constants
[constant value]: ../const_eval.md#constant-expressions
[free]: ../glossary.md#free-item
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