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allow constants to refer to mutable/external memory, but reject such constants as patterns #1859

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allow constants to refer to mutable/external memory, but reject such constants as patterns #1859

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3 changes: 0 additions & 3 deletions src/items/constant-items.md
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Expand Up @@ -48,9 +48,6 @@ const BITS_N_STRINGS: BitsNStrings<'static> = BitsNStrings {
};
```

r[items.const.final-value-immutable]
The final value of a `const` item cannot contain references to anything mutable.

r[items.const.expr-omission]
The constant expression may only be omitted in a [trait definition].

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3 changes: 3 additions & 0 deletions src/patterns.md
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Expand Up @@ -975,6 +975,9 @@ r[patterns.const.generic]
In particular, the value of `C` must be known at pattern-building time (which is pre-monomorphization).
This means that associated consts that involve generic parameters cannot be used as patterns.

r[patterns.const.immutable]
Finally, the value of `C` must not contain any references to mutable statics (`static mut` or interior mutable) or `extern` statics.

r[patterns.const.translation]
After ensuring all conditions are met, the constant value is translated into a pattern, and now behaves exactly as-if that pattern had been written directly.
In particular, it fully participates in exhaustiveness checking.
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