Check CoerceUnsized impl validity before coercing

compiler/rustc_hir_typeck/src/coercion.rs

@@ -46,7 +46,7 @@ use rustc_hir_analysis::hir_ty_lowering::HirTyLowerer;
 use rustc_infer::infer::relate::RelateResult;
 use rustc_infer::infer::{Coercion, DefineOpaqueTypes, InferOk, InferResult};
 use rustc_infer::traits::{
-    IfExpressionCause, MatchExpressionArmCause, Obligation, PredicateObligation,
+    IfExpressionCause, ImplSource, MatchExpressionArmCause, Obligation, PredicateObligation,
     PredicateObligations, SelectionError,
 };
 use rustc_middle::span_bug;
@@ -704,6 +704,19 @@ impl<'f, 'tcx> Coerce<'f, 'tcx> {
                     // be silent, as it causes a type mismatch later.
                 }
 
+                Ok(Some(ImplSource::UserDefined(impl_source))) => {
+                    queue.extend(impl_source.nested);
+                    // Certain incoherent `CoerceUnsized` implementations may cause ICEs,
+                    // so check the impl's validity. Taint the body so that we don't try
+                    // to evaluate these invalid coercions in CTFE. We only need to do this
+                    // for local impls, since upstream impls should be valid.
+                    if impl_source.impl_def_id.is_local()
+                        && let Err(guar) =
+                            self.tcx.ensure_ok().coerce_unsized_info(impl_source.impl_def_id)
+                    {
+                        self.fcx.set_tainted_by_errors(guar);
+                    }
+                }
                 Ok(Some(impl_source)) => queue.extend(impl_source.nested_obligations()),
             }
         }

tests/ui/coercion/invalid-blanket-coerce-unsized-impl.rs

@@ -0,0 +1,13 @@
+// Regression test minimized from #126982.
+// We used to apply a coerce_unsized coercion to literally every argument since
+// the blanket applied in literally all cases, even though it was incoherent.
+
+#![feature(coerce_unsized)]
+
+impl<A> std::ops::CoerceUnsized<A> for A {}
+//~^ ERROR type parameter `A` must be used as the type parameter for some local type
+//~| ERROR the trait `CoerceUnsized` may only be implemented for a coercion between structures
+
+const C: usize = 1;
+
+fn main() {}

tests/ui/coercion/invalid-blanket-coerce-unsized-impl.stderr

@@ -0,0 +1,19 @@
+error[E0210]: type parameter `A` must be used as the type parameter for some local type (e.g., `MyStruct<A>`)
+  --> $DIR/invalid-blanket-coerce-unsized-impl.rs:7:6
+   |
+LL | impl<A> std::ops::CoerceUnsized<A> for A {}
+   |      ^ type parameter `A` must be used as the type parameter for some local type
+   |
+   = note: implementing a foreign trait is only possible if at least one of the types for which it is implemented is local
+   = note: only traits defined in the current crate can be implemented for a type parameter
+
+error[E0377]: the trait `CoerceUnsized` may only be implemented for a coercion between structures
+  --> $DIR/invalid-blanket-coerce-unsized-impl.rs:7:1
+   |
+LL | impl<A> std::ops::CoerceUnsized<A> for A {}
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: aborting due to 2 previous errors
+
+Some errors have detailed explanations: E0210, E0377.
+For more information about an error, try `rustc --explain E0210`.