Rollup merge of #122656 - RalfJung:simplify-cfg, r=compiler-errors

simplify_cfg: rename some passes so that they make more sense

I was extremely confused by `SimplifyCfg::ElaborateDrops`, since it runs way later than drop elaboration. It is used e.g. in `mir-opt/retag.rs` even though that pass doesn't care about drop elaboration at all.

"Early opt" is also very confusing since that makes it sounds like it runs early during optimizations, i.e. on runtime MIR, but actually it runs way before that.

So I decided to rename
- early-opt -> post-analysis
- elaborate-drops -> pre-optimizations

I am open to other suggestions.

compiler/rustc_mir_transform/src/lib.rs

@@ -507,7 +507,7 @@ fn run_analysis_cleanup_passes<'tcx>(tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
     let passes: &[&dyn MirPass<'tcx>] = &[
         &cleanup_post_borrowck::CleanupPostBorrowck,
         &remove_noop_landing_pads::RemoveNoopLandingPads,
-        &simplify::SimplifyCfg::EarlyOpt,
+        &simplify::SimplifyCfg::PostAnalysis,
         &deref_separator::Derefer,
     ];
 
@@ -544,7 +544,7 @@ fn run_runtime_cleanup_passes<'tcx>(tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
     let passes: &[&dyn MirPass<'tcx>] = &[
         &lower_intrinsics::LowerIntrinsics,
         &remove_place_mention::RemovePlaceMention,
-        &simplify::SimplifyCfg::ElaborateDrops,
+        &simplify::SimplifyCfg::PreOptimizations,
     ];
 
     pm::run_passes(tcx, body, passes, Some(MirPhase::Runtime(RuntimePhase::PostCleanup)));

compiler/rustc_mir_transform/src/simplify.rs

@@ -37,8 +37,11 @@ pub enum SimplifyCfg {
     Initial,
     PromoteConsts,
     RemoveFalseEdges,
-    EarlyOpt,
-    ElaborateDrops,
+    /// Runs at the beginning of "analysis to runtime" lowering, *before* drop elaboration.
+    PostAnalysis,
+    /// Runs at the end of "analysis to runtime" lowering, *after* drop elaboration.
+    /// This is before the main optimization passes on runtime MIR kick in.
+    PreOptimizations,
     Final,
     MakeShim,
     AfterUninhabitedEnumBranching,
@@ -50,8 +53,8 @@ impl SimplifyCfg {
             SimplifyCfg::Initial => "SimplifyCfg-initial",
             SimplifyCfg::PromoteConsts => "SimplifyCfg-promote-consts",
             SimplifyCfg::RemoveFalseEdges => "SimplifyCfg-remove-false-edges",
-            SimplifyCfg::EarlyOpt => "SimplifyCfg-early-opt",
-            SimplifyCfg::ElaborateDrops => "SimplifyCfg-elaborate-drops",
+            SimplifyCfg::PostAnalysis => "SimplifyCfg-post-analysis",
+            SimplifyCfg::PreOptimizations => "SimplifyCfg-pre-optimizations",
             SimplifyCfg::Final => "SimplifyCfg-final",
             SimplifyCfg::MakeShim => "SimplifyCfg-make_shim",
             SimplifyCfg::AfterUninhabitedEnumBranching => {