Auto merge of rust-lang#118482 - RalfJung:interpret-local-type, r=WaffleLapkin

bors · bors · commit a1c65db95cf7 · 2023-12-01T04:08:22.000Z
explain a good reason for why LocalValue does not store the type of the local As found out by `@lcnr` in rust-lang#112307, storing the type here can lead to subtle bugs when it gets out of sync with the MIR body. That's not the reason why the interpreter does it this way I think, but good thing we dodged that bullet. :)
diff --git a/compiler/rustc_const_eval/src/interpret/eval_context.rs b/compiler/rustc_const_eval/src/interpret/eval_context.rs
@@ -173,6 +173,9 @@ impl<Prov: Provenance> std::fmt::Debug for LocalState<'_, Prov> {
 }
 
 /// Current value of a local variable
+///
+/// This does not store the type of the local; the type is given by `body.local_decls` and can never
+/// change, so by not storing here we avoid having to maintain that as an invariant.
 #[derive(Copy, Clone, Debug)] // Miri debug-prints these
 pub(super) enum LocalValue<Prov: Provenance = AllocId> {
     /// This local is not currently alive, and cannot be used at all.

Original file line number	Diff line number	Diff line change
`@@ -173,6 +173,9 @@ impl<Prov: Provenance> std::fmt::Debug for LocalState<'_, Prov> {`
`173`	`173`	`}`
`174`	`174`
`175`	`175`	`/// Current value of a local variable`
	`176`	`+///`
	`177`	+/// This does not store the type of the local; the type is given by `body.local_decls` and can never
	`178`	`+/// change, so by not storing here we avoid having to maintain that as an invariant.`
`176`	`179`	`#[derive(Copy, Clone, Debug)] // Miri debug-prints these`
`177`	`180`	`pub(super) enum LocalValue<Prov: Provenance = AllocId> {`
`178`	`181`	`/// This local is not currently alive, and cannot be used at all.`