Rollup merge of #128171 - compiler-errors:arg-compat, r=oli-obk

Make sure that args are compatible in `resolve_associated_item`

Implements a similar check to the one that we have in projection for GATs (#102488, #123240), where we check that the args of an impl item are compatible before returning it. This is done in `resolve_assoc_item`, which is backing `Instance::resolve`, so this is conceptually generalizing the check from GATs to methods/assoc consts. This is important to make sure that the inliner will only visit and substitute MIR bodies that are compatible w/ their trait definitions.

This shouldn't happen in codegen, but there are a few ways to get the inliner to be invoked (via calls to `optimized_mir`) before codegen, namely polymorphization and CTFE.

Fixes #121957
Fixes #120792
Fixes #120793
Fixes #121063

1 files changed, 13 insertions, 2 deletions

diff --git a/compiler/rustc_ty_utils/src/instance.rs b/compiler/rustc_ty_utils/src/instance.rs
index 7b6d86d22a5..a2bed61a7ae 100644
--- a/compiler/rustc_ty_utils/src/instance.rs
+++ b/compiler/rustc_ty_utils/src/instance.rs
@@ -191,11 +191,22 @@ fn resolve_associated_item<'tcx>(
 
             // Any final impl is required to define all associated items.
             if !leaf_def.item.defaultness(tcx).has_value() {
-                let guard = tcx.dcx().span_delayed_bug(
+                let guar = tcx.dcx().span_delayed_bug(
                     tcx.def_span(leaf_def.item.def_id),
                     "missing value for assoc item in impl",
                 );
-                return Err(guard);
+                return Err(guar);
+            }
+
+            // Make sure that we're projecting to an item that has compatible args.
+            // This may happen if we are resolving an instance before codegen, such
+            // as during inlining. This check is also done in projection.
+            if !tcx.check_args_compatible(leaf_def.item.def_id, args) {
+                let guar = tcx.dcx().span_delayed_bug(
+                    tcx.def_span(leaf_def.item.def_id),
+                    "missing value for assoc item in impl",
+                );
+                return Err(guar);
             }
 
             let args = tcx.erase_regions(args);