Fix impl_trait_ty_to_ty substs

compiler/rustc_hir_analysis/src/astconv/mod.rs

@@ -3141,8 +3141,12 @@ impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
 
         debug!("impl_trait_ty_to_ty: generics={:?}", generics);
         let substs = InternalSubsts::for_item(tcx, def_id, |param, _| {
-            if let Some(i) = (param.index as usize).checked_sub(generics.parent_count) {
-                // Our own parameters are the resolved lifetimes.
+            // We use `generics.count() - lifetimes.len()` here instead of `generics.parent_count`
+            // since return-position impl trait in trait squashes all of the generics from its source fn
+            // into its own generics, so the opaque's "own" params isn't always just lifetimes.
+            if let Some(i) = (param.index as usize).checked_sub(generics.count() - lifetimes.len())
+            {
+                // Resolve our own lifetime parameters.
                 let GenericParamDefKind::Lifetime { .. } = param.kind else { bug!() };
                 let hir::GenericArg::Lifetime(lifetime) = &lifetimes[i] else { bug!() };
                 self.ast_region_to_region(lifetime, None).into()

tests/ui/async-await/in-trait/lifetime-mismatch.current.stderr

@@ -1,5 +1,5 @@
 warning: the feature `async_fn_in_trait` is incomplete and may not be safe to use and/or cause compiler crashes
-  --> $DIR/lifetime-mismatch.rs:3:12
+  --> $DIR/lifetime-mismatch.rs:5:12
    |
 LL | #![feature(async_fn_in_trait)]
    |            ^^^^^^^^^^^^^^^^^
@@ -8,7 +8,7 @@ LL | #![feature(async_fn_in_trait)]
    = note: `#[warn(incomplete_features)]` on by default
 
 error[E0195]: lifetime parameters or bounds on method `foo` do not match the trait declaration
-  --> $DIR/lifetime-mismatch.rs:12:17
+  --> $DIR/lifetime-mismatch.rs:14:17
    |
 LL |     async fn foo<'a>(&self);
    |                 ---- lifetimes in impl do not match this method in trait

tests/ui/async-await/in-trait/lifetime-mismatch.next.stderr

@@ -0,0 +1,21 @@
+warning: the feature `async_fn_in_trait` is incomplete and may not be safe to use and/or cause compiler crashes
+  --> $DIR/lifetime-mismatch.rs:5:12
+   |
+LL | #![feature(async_fn_in_trait)]
+   |            ^^^^^^^^^^^^^^^^^
+   |
+   = note: see issue #91611 <https://github.com/rust-lang/rust/issues/91611> for more information
+   = note: `#[warn(incomplete_features)]` on by default
+
+error[E0195]: lifetime parameters or bounds on method `foo` do not match the trait declaration
+  --> $DIR/lifetime-mismatch.rs:14:17
+   |
+LL |     async fn foo<'a>(&self);
+   |                 ---- lifetimes in impl do not match this method in trait
+...
+LL |     async fn foo(&self) {}
+   |                 ^ lifetimes do not match method in trait
+
+error: aborting due to previous error; 1 warning emitted
+
+For more information about this error, try `rustc --explain E0195`.

tests/ui/async-await/in-trait/lifetime-mismatch.rs

@@ -1,4 +1,6 @@
 // edition:2021
+// [next] compile-flags: -Zlower-impl-trait-in-trait-to-assoc-ty
+// revisions: current next
 
 #![feature(async_fn_in_trait)]
 //~^ WARN the feature `async_fn_in_trait` is incomplete and may not be safe to use and/or cause compiler crashes

tests/ui/impl-trait/in-trait/trait-more-generics-than-impl.current.stderr

@@ -1,5 +1,5 @@
 error[E0049]: method `bar` has 0 type parameters but its trait declaration has 1 type parameter
-  --> $DIR/trait-more-generics-than-impl.rs:11:11
+  --> $DIR/trait-more-generics-than-impl.rs:14:11
    |
 LL |     fn bar<T>() -> impl Sized;
    |            - expected 1 type parameter

tests/ui/impl-trait/in-trait/trait-more-generics-than-impl.next.stderr

@@ -0,0 +1,12 @@
+error[E0049]: method `bar` has 0 type parameters but its trait declaration has 1 type parameter
+  --> $DIR/trait-more-generics-than-impl.rs:14:11
+   |
+LL |     fn bar<T>() -> impl Sized;
+   |            - expected 1 type parameter
+...
+LL |     fn bar() -> impl Sized {}
+   |           ^ found 0 type parameters
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0049`.

tests/ui/impl-trait/in-trait/trait-more-generics-than-impl.rs

@@ -1,3 +1,6 @@
+// [next] compile-flags: -Zlower-impl-trait-in-trait-to-assoc-ty
+// revisions: current next
+
 #![feature(return_position_impl_trait_in_trait)]
 #![allow(incomplete_features)]