Rollup merge of #147575 - beepster4096:movesubpath, r=oli-obk

Refactor move analysis subpath representation

Follow up to rust-lang/rust#147055

This PR does two things:
1. Document/validate move analysis's assumptions about `Subslice` projections
2. Decouple move paths from `ProjectionElem`, using a new enum `MoveSubPath` instead
    - This would be needed eventually when `ProjectionElem::Deref` is removed

I wanted to do even more abstraction, making `MovePathLookup::find` return an iterator to remove the special handling of subslices in borrowck, but that regressed diagnostics and just wasn't worth the complexity.

Author: matthiaskrgr

src/borrow_check/moves_and_initialization/move_paths.md

@@ -67,8 +67,6 @@ We don't actually create a move-path for **every** [`Place`] that gets
 used.  In particular, if it is illegal to move from a [`Place`], then
 there is no need for a [`MovePathIndex`]. Some examples:
 
-- You cannot move from a static variable, so we do not create a [`MovePathIndex`]
-  for static variables.
 - You cannot move an individual element of an array, so if we have e.g. `foo: [String; 3]`,
   there would be no move-path for `foo[1]`.
 - You cannot move from inside of a borrowed reference, so if we have e.g. `foo: &String`,
@@ -82,6 +80,18 @@ initialized (which lowers overhead).
 
 [`move_path_for`]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_mir_dataflow/move_paths/builder/struct.MoveDataBuilder.html#method.move_path_for
 
+## Projections
+
+Instead of using [`PlaceElem`], projections in move paths are stored as [`MoveSubPath`]s.
+Projections that can't be moved out of and projections that can be skipped are not represented.
+
+Subslice projections of arrays (produced by slice patterns) are special; they're turned into
+multiple [`ConstantIndex`] subpaths, one for each element in the subslice.
+
+[`PlaceElem`]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/mir/type.PlaceElem.html
+[`MoveSubPath`]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_mir_dataflow/move_paths/enum.MoveSubPath.html
+[`ConstantIndex`]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_mir_dataflow/move_paths/enum.MoveSubPath.html#variant.ConstantIndex
+
 ## Looking up a move-path
 
 If you have a [`Place`] and you would like to convert it to a [`MovePathIndex`], you