Rollup merge of #99050 - JakobDegen:storage-docs, r=tmiasko

Clarify MIR semantics of storage statements

Seems worthwhile to start closing out some of the less controversial open questions about MIR semantics. Hopefully this is fairly non-controversial - it's what we implement already, and I see no reason to do anything more restrictive. cc ``@tmiasko`` who commented on this when it was discussed in the original PR that added these docs.

compiler/rustc_const_eval/src/transform/validate.rs

@@ -206,7 +206,13 @@ impl<'a, 'tcx> Visitor<'tcx> for TypeChecker<'a, 'tcx> {
         }
 
         if self.reachable_blocks.contains(location.block) && context.is_use() {
-            // Uses of locals must occur while the local's storage is allocated.
+            // We check that the local is live whenever it is used. Technically, violating this
+            // restriction is only UB and not actually indicative of not well-formed MIR. This means
+            // that an optimization which turns MIR that already has UB into MIR that fails this
+            // check is not necessarily wrong. However, we have no such optimizations at the moment,
+            // and so we include this check anyway to help us catch bugs. If you happen to write an
+            // optimization that might cause this to incorrectly fire, feel free to remove this
+            // check.
             self.storage_liveness.seek_after_primary_effect(location);
             let locals_with_storage = self.storage_liveness.get();
             if !locals_with_storage.contains(local) {