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Rollup merge of #131285 - RalfJung:mir-projection-sem, r=cjgillot

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clarify semantics of ConstantIndex MIR projection

This documents what Miri does:
c4ce8c114b/compiler/rustc_const_eval/src/interpret/projection.rs (L272-L275)

I am not sure what exactly the purpose of this `min_length` field is, TBH... but this seems like the most obvious meaning it could have?
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Matthias Krüger 2024-10-05 19:07:55 +02:00 • committed by GitHub
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2 changed files with 8 additions and 4 deletions
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6
compiler/rustc_middle/src/mir/syntax.rs
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@ -1135,8 +1135,10 @@ pub enum ProjectionElem<V, T> {
ConstantIndex {
/// index or -index (in Python terms), depending on from_end
offset: u64,
/// The thing being indexed must be at least this long. For arrays this
/// is always the exact length.
/// The thing being indexed must be at least this long -- otherwise, the
/// projection is UB.
///
/// For arrays this is always the exact length.
min_length: u64,
/// Counting backwards from end? This is always false when indexing an
/// array.

6
compiler/stable_mir/src/mir/body.rs
View file

@ -768,8 +768,10 @@ pub enum ProjectionElem {
ConstantIndex {
/// index or -index (in Python terms), depending on from_end
offset: u64,
/// The thing being indexed must be at least this long. For arrays this
/// is always the exact length.
/// The thing being indexed must be at least this long -- otherwise, the
/// projection is UB.
///
/// For arrays this is always the exact length.
min_length: u64,
/// Counting backwards from end? This is always false when indexing an
/// array.