Previously, a Drop terminator was considered a move in MIR.
This commit changes the behavior to only treat Drop as a mutable
access to the dropped place.
In order for this change to be correct, we need to guarantee that
a) A dropped value won't be used again
b) Places that appear in a drop won't be used again before a
subsequent initialization.
We can ensure this to be correct at MIR construction because Drop
will only be emitted when a variable goes out of scope,
thus having:
(a) as there is no way of reaching the old value. drop-elaboration
will also remove any uninitialized drop.
(b) as the place can't be named following the end of the scope.
However, the initialization status, previously tracked by moves,
should also be tied to the execution of a Drop, hence the
additional logic in the dataflow analyses.
This enhances documentation with these details and extends the validator to check these requirements
more thoroughly. As a part of this, we add a new `Deaggregated` phase, and rename other phases so
that their names more naturally correspond to what they represent.
This follows changes from #67967 and converts remaining `span_bug`s into
delayed bugs, since for const items drop elaboration might be executed
on a MIR which failed borrowck.
This one is a heavy `'tcx` user.
Two interesting ones:
This one had the `'tcx` declared on the function, despite the trait taking a `'tcx`:
```diff
-impl Visitor<'_> for UsedLocals {
+impl<'tcx> Visitor<'tcx> for UsedLocals {
fn visit_statement(&mut self, statement: &Statement<'tcx>, location: Location) {
```
This one use in-band for one, and underscore for the other:
```diff
-pub fn remove_dead_blocks(tcx: TyCtxt<'tcx>, body: &mut Body<'_>) {
+pub fn remove_dead_blocks<'tcx>(tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
```
