Fix check with is_impl_trait_in_trait() before expecting local id

compiler/rustc_hir_analysis/src/check/compare_impl_item.rs

@@ -2141,7 +2141,16 @@ fn compare_const_generic_param_types<'tcx>(
         iter::zip(const_params_of(impl_item.def_id), const_params_of(trait_item.def_id));
 
     for (param_impl, param_trait) in param_iter {
-        let param_impl_ty_span = tcx.ty_span(param_impl.def_id.expect_local());
+        // If `impl_item` comes from RPITIT:
+        // <https://rustc-dev-guide.rust-lang.org/return-position-impl-trait-in-trait.html>
+        // then expecting a `LocalDefId` for `param_impl` is fraught: the synthetic
+        // associated type reuses the trait's generic parameters, which may be nonlocal.
+        // Use the impl item's span instead.
+        let param_impl_ty_span = if impl_item.is_impl_trait_in_trait() {
+            tcx.def_span(impl_item.def_id)
+        } else {
+            tcx.ty_span(param_impl.def_id.expect_local())
+        };
 
         let cause = ObligationCause::new(
             param_impl_ty_span,

tests/ui/const-generics/auxiliary/non_local.rs

@@ -1,3 +1,8 @@
 pub trait NonLocal {
     fn method<const N: i32>();
 }
+
+use std::future::Future;
+pub trait R {
+    fn read<const L: usize>(&mut self) -> impl Future<Output = [u8; L]>;
+}

tests/ui/const-generics/non-local-method-same-ok.rs

@@ -0,0 +1,15 @@
+// Adapted from; https://github.com/rust-lang/rust/issues/164180#issuecomment-6168887401
+//@ check-pass
+//@ compile-flags: --crate-type lib
+//@ aux-build:non_local.rs
+//@ edition: 2024
+
+extern crate non_local;
+
+struct Local;
+
+impl non_local::R for Local {
+    async fn read<const L: usize>(&mut self) -> [u8; L] {
+        [0; L]
+    }
+}

tests/ui/const-generics/non-local-method.rs

@@ -1,5 +1,6 @@
 //@ compile-flags: --crate-type lib
 //@ aux-build:non_local.rs
+//@ edition: 2024
 
 extern crate non_local;
 
@@ -13,3 +14,10 @@ impl NonLocal for Local {
         print!("hello\n");
     }
 }
+
+impl non_local::R for Local {
+    async fn read<const L: usize>(&mut self) -> [u16; L] {
+        //~^ ERROR: expected `impl Future<Output = [u16; L]>` to be a future that resolves to `[u8; L]`, but it resolves to `[u16; L]`
+        [0; L]
+    }
+}

tests/ui/const-generics/non-local-method.stderr

@@ -1,9 +1,25 @@
 error[E0053]: associated function `method` has an incompatible type for const generic parameter
-  --> $DIR/non-local-method.rs:11:24
+  --> $DIR/non-local-method.rs:12:24
    |
 LL |     fn method<const N: u16>() {
    |                        ^^^ expected `i32`, found `u16`
 
-error: aborting due to 1 previous error
+error[E0271]: expected `impl Future<Output = [u16; L]>` to be a future that resolves to `[u8; L]`, but it resolves to `[u16; L]`
+  --> $DIR/non-local-method.rs:19:49
+   |
+LL |     async fn read<const L: usize>(&mut self) -> [u16; L] {
+   |                                                 ^^^^^^^^ expected `[u8; L]`, found `[u16; L]`
+   |
+   = note: expected array `[u8; L]`
+              found array `[u16; L]`
+note: required by a bound in `R::read::{anon_assoc#0}`
+  --> $DIR/auxiliary/non_local.rs:7:55
+   |
+LL | pub trait R {
+LL |     fn read<const L: usize>(&mut self) -> impl Future<Output = [u8; L]>;
+   |                                                       ^^^^^^^^^^^^^^^^ required by this bound in `R::read::{anon_assoc#0}`
+
+error: aborting due to 2 previous errors
 
-For more information about this error, try `rustc --explain E0053`.
+Some errors have detailed explanations: E0053, E0271.
+For more information about an error, try `rustc --explain E0053`.