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

clarify semantics of ConstantIndex MIR projection

This documents what Miri does:
https://github.com/rust-lang/rust/blob/c4ce8c114b06840c3521a189ee44958b713fb33a/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?

compiler/rustc_middle/src/mir/syntax.rs

@@ -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.

compiler/stable_mir/src/mir/body.rs

@@ -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.