Clarify which kinds of MIR are allowed during which phases.

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.

compiler/rustc_mir_transform/src/deaggregator.rs

@@ -6,6 +6,10 @@ use rustc_middle::ty::TyCtxt;
 pub struct Deaggregator;
 
 impl<'tcx> MirPass<'tcx> for Deaggregator {
+    fn phase_change(&self) -> Option<MirPhase> {
+        Some(MirPhase::Deaggregated)
+    }
+
     fn run_pass(&self, tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
         let (basic_blocks, local_decls) = body.basic_blocks_and_local_decls_mut();
         let local_decls = &*local_decls;

compiler/rustc_mir_transform/src/elaborate_drops.rs

@@ -20,7 +20,7 @@ pub struct ElaborateDrops;
 
 impl<'tcx> MirPass<'tcx> for ElaborateDrops {
     fn phase_change(&self) -> Option<MirPhase> {
-        Some(MirPhase::DropLowering)
+        Some(MirPhase::DropsLowered)
     }
 
     fn run_pass(&self, tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {

compiler/rustc_mir_transform/src/generator.rs

@@ -1235,7 +1235,7 @@ fn create_cases<'tcx>(
 
 impl<'tcx> MirPass<'tcx> for StateTransform {
     fn phase_change(&self) -> Option<MirPhase> {
-        Some(MirPhase::GeneratorLowering)
+        Some(MirPhase::GeneratorsLowered)
     }
 
     fn run_pass(&self, tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {

compiler/rustc_mir_transform/src/lib.rs

@@ -341,7 +341,7 @@ fn inner_mir_for_ctfe(tcx: TyCtxt<'_>, def: ty::WithOptConstParam<LocalDefId>) -
             pm::run_passes(
                 tcx,
                 &mut body,
-                &[&const_prop::ConstProp, &marker::PhaseChange(MirPhase::Optimization)],
+                &[&const_prop::ConstProp, &marker::PhaseChange(MirPhase::Optimized)],
             );
         }
     }
@@ -398,7 +398,7 @@ fn mir_drops_elaborated_and_const_checked<'tcx>(
     }
 
     run_post_borrowck_cleanup_passes(tcx, &mut body);
-    assert!(body.phase == MirPhase::DropLowering);
+    assert!(body.phase == MirPhase::Deaggregated);
     tcx.alloc_steal_mir(body)
 }
 
@@ -458,7 +458,7 @@ fn run_optimization_passes<'tcx>(tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
         ],
     );
 
-    assert!(body.phase == MirPhase::GeneratorLowering);
+    assert!(body.phase == MirPhase::GeneratorsLowered);
 
     // The main optimizations that we do on MIR.
     pm::run_passes(
@@ -495,7 +495,7 @@ fn run_optimization_passes<'tcx>(tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
             &deduplicate_blocks::DeduplicateBlocks,
             // Some cleanup necessary at least for LLVM and potentially other codegen backends.
             &add_call_guards::CriticalCallEdges,
-            &marker::PhaseChange(MirPhase::Optimization),
+            &marker::PhaseChange(MirPhase::Optimized),
             // Dump the end result for testing and debugging purposes.
             &dump_mir::Marker("PreCodegen"),
         ],

compiler/rustc_mir_transform/src/pass_manager.rs

@@ -114,7 +114,7 @@ pub fn run_passes<'tcx>(tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>, passes: &[&dyn
         }
     }
 
-    if validate || body.phase == MirPhase::Optimization {
+    if validate || body.phase == MirPhase::Optimized {
         validate_body(tcx, body, format!("end of phase transition to {:?}", body.phase));
     }
 }