update movepath docs

Author: beepster4096

compiler/rustc_mir_dataflow/src/move_paths/mod.rs

@@ -1,14 +1,4 @@
-//! The move-analysis portion of borrowck needs to work in an abstract domain of lifted `Place`s.
-//! Most of the `Place` variants fall into a one-to-one mapping between the concrete and abstract
-//! (e.g., a field projection on a local variable, `x.field`, has the same meaning in both
-//! domains). In other words, all field projections for the same field on the same local do not
-//! have meaningfully different types if ever. Indexed projections are the exception: `a[x]` needs
-//! to be treated as mapping to the same move path as `a[y]` as well as `a[13]`, etc. So we map
-//! these `x`/`y` values to `()`.
-//!
-//! (In theory, the analysis could be extended to work with sets of paths, so that `a[0]` and
-//! `a[13]` could be kept distinct, while `a[x]` would still overlap them both. But that is not
-//! what this representation does today.)
+//! [`MovePath`]s track the initialization state of places and their sub-paths.
 
 use std::fmt;
 use std::ops::{Index, IndexMut};
@@ -399,6 +389,7 @@ impl<'tcx> MoveData<'tcx> {
     }
 }
 
+/// A projection into a move path producing a child path
 #[derive(Copy, Clone, Debug, PartialEq, Eq, Hash)]
 pub enum MoveSubPath {
     Deref,

src/doc/rustc-dev-guide/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