Auto merge of #108872 - cjgillot:simp-const-prop, r=oli-obk

Strengthen state tracking in const-prop

Some/many of the changes are replicated between both the const-prop lint and the const-prop optimization.

Behaviour changes:
- const-prop opt does not give a span to propagated values. This was useless as that span's primary purpose is to diagnose evaluation failure in codegen.
- we remove the `OnlyPropagateInto` mode. It was only used for function arguments, which are better modeled by a write before entry.
- the tracking of assignments and discriminants make clearer that we do nothing in `NoPropagation` mode or on indirect places.

1 files changed, 13 insertions, 17 deletions

diff --git a/compiler/rustc_const_eval/src/interpret/eval_context.rs b/compiler/rustc_const_eval/src/interpret/eval_context.rs
index 39c74191258..0918ffcd982 100644
--- a/compiler/rustc_const_eval/src/interpret/eval_context.rs
+++ b/compiler/rustc_const_eval/src/interpret/eval_context.rs
@@ -536,24 +536,20 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
         local: mir::Local,
         layout: Option<TyAndLayout<'tcx>>,
     ) -> InterpResult<'tcx, TyAndLayout<'tcx>> {
-        // `const_prop` runs into this with an invalid (empty) frame, so we
-        // have to support that case (mostly by skipping all caching).
-        match frame.locals.get(local).and_then(|state| state.layout.get()) {
-            None => {
-                let layout = from_known_layout(self.tcx, self.param_env, layout, || {
-                    let local_ty = frame.body.local_decls[local].ty;
-                    let local_ty =
-                        self.subst_from_frame_and_normalize_erasing_regions(frame, local_ty)?;
-                    self.layout_of(local_ty)
-                })?;
-                if let Some(state) = frame.locals.get(local) {
-                    // Layouts of locals are requested a lot, so we cache them.
-                    state.layout.set(Some(layout));
-                }
-                Ok(layout)
-            }
-            Some(layout) => Ok(layout),
+        let state = &frame.locals[local];
+        if let Some(layout) = state.layout.get() {
+            return Ok(layout);
         }
+
+        let layout = from_known_layout(self.tcx, self.param_env, layout, || {
+            let local_ty = frame.body.local_decls[local].ty;
+            let local_ty = self.subst_from_frame_and_normalize_erasing_regions(frame, local_ty)?;
+            self.layout_of(local_ty)
+        })?;
+
+        // Layouts of locals are requested a lot, so we cache them.
+        state.layout.set(Some(layout));
+        Ok(layout)
     }
 
     /// Returns the actual dynamic size and alignment of the place at the given type.