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const-eval: explain the final-value-byte-provenance restriction #2138

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const-eval: explain the final-value-byte-provenance restriction #2138

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const-eval: explain the final-value-byte-provenance restriction
RalfJung Jan 19, 2026
a5410c9
Revise text on initializer provenance restriction
traviscross Jan 20, 2026
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10 changes: 10 additions & 0 deletions src/const_eval.md
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Expand Up @@ -232,6 +232,14 @@ r[const-eval.const-expr.loop]
r[const-eval.const-expr.if-match]
* [if] and [match] expressions.

## Const initializers

r[const-eval.const-expr.final-value-provenance]
The representation of the final value of a [constant][constant initializer] or [static initializer] must only contain bytes with provenance in whole-pointer groups. If a byte has provenance but is not part of an adjacent group of bytes that form an entire pointer, compilation will fail.

> [!NOTE]
> If a byte in the representation of the final value is uninitialized, then it *may* end up having provenance, which can cause compilation to fail. `rustc` tries (but does not guarantee) to only actually fail if the initializer copies or overwrites parts of a pointer and those bytes appear in the final value.

r[const-eval.const-context]
## Const context
[const context]: #const-context
Expand Down Expand Up @@ -307,6 +315,7 @@ The types of a const function's parameters and return type are restricted to tho
[const generic argument]: items/generics.md#const-generics
[const generic parameters]: items/generics.md#const-generics
[constant expressions]: #constant-expressions
[constant initializer]: items.const
[constants]: items/constant-items.md
[Const parameters]: items/generics.md
[dereference expression]: expr.deref
Expand Down Expand Up @@ -336,6 +345,7 @@ The types of a const function's parameters and return type are restricted to tho
[promoted]: destructors.md#constant-promotion
[range expressions]: expressions/range-expr.md
[slice]: types/slice.md
[static initializer]: items.static.init
[statics]: items/static-items.md
[Struct expressions]: expressions/struct-expr.md
[temporary lifetime extension]: destructors.scope.lifetime-extension
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