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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.

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We were wondering if it would make sense to change the wording here instead of removing it. Perhaps something like:

"The final value of a const item cannot contain a mutable reference."

Most of us seemed to think this is correct. Perhaps there is some other rule forbidding this already (implicitly or explicitly), but it seems like it wouldn't hurt to be clear about it here?

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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