diff --git a/compiler/rustc_hir_analysis/src/astconv/mod.rs b/compiler/rustc_hir_analysis/src/astconv/mod.rs
index f830269b45dae..333e7502041e7 100644
--- a/compiler/rustc_hir_analysis/src/astconv/mod.rs
+++ b/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()
diff --git a/compiler/rustc_ty_utils/src/assoc.rs b/compiler/rustc_ty_utils/src/assoc.rs
index 68b1086e8e3f5..49cb778118863 100644
--- a/compiler/rustc_ty_utils/src/assoc.rs
+++ b/compiler/rustc_ty_utils/src/assoc.rs
@@ -383,6 +383,8 @@ fn impl_associated_item_for_impl_trait_in_trait(
     impl_assoc_ty.impl_defaultness(tcx.impl_defaultness(impl_fn_def_id));
 
     // Copy generics_of the trait's associated item but the impl as the parent.
+    // FIXME(-Zlower-impl-trait-in-trait-to-assoc-ty) resolves to the trait instead of the impl
+    // generics.
     impl_assoc_ty.generics_of({
         let trait_assoc_generics = tcx.generics_of(trait_assoc_def_id);
         let trait_assoc_parent_count = trait_assoc_generics.parent_count;
@@ -391,16 +393,10 @@ fn impl_associated_item_for_impl_trait_in_trait(
         let parent_generics = tcx.generics_of(impl_def_id);
         let parent_count = parent_generics.parent_count + parent_generics.params.len();
 
-        let mut impl_fn_params = tcx.generics_of(impl_fn_def_id).params.clone();
-
         for param in &mut params {
-            param.index = param.index + parent_count as u32 + impl_fn_params.len() as u32
-                - trait_assoc_parent_count as u32;
+            param.index = param.index + parent_count as u32 - trait_assoc_parent_count as u32;
         }
 
-        impl_fn_params.extend(params);
-        params = impl_fn_params;
-
         let param_def_id_to_index =
             params.iter().map(|param| (param.def_id, param.index)).collect();
 
diff --git a/tests/ui/async-await/in-trait/issue-102310.rs b/tests/ui/async-await/in-trait/issue-102310.rs
index 49c3e9feeb4c4..8e5dbd08eb98b 100644
--- a/tests/ui/async-await/in-trait/issue-102310.rs
+++ b/tests/ui/async-await/in-trait/issue-102310.rs
@@ -1,5 +1,7 @@
 // check-pass
 // edition:2021
+// [next] compile-flags: -Zlower-impl-trait-in-trait-to-assoc-ty
+// revisions: current next
 
 #![feature(async_fn_in_trait)]
 #![allow(incomplete_features)]
diff --git a/tests/ui/async-await/in-trait/lifetime-mismatch.stderr b/tests/ui/async-await/in-trait/lifetime-mismatch.current.stderr
similarity index 91%
rename from tests/ui/async-await/in-trait/lifetime-mismatch.stderr
rename to tests/ui/async-await/in-trait/lifetime-mismatch.current.stderr
index d87adcc78b6c8..0e9477544a4a6 100644
--- a/tests/ui/async-await/in-trait/lifetime-mismatch.stderr
+++ b/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
diff --git a/tests/ui/async-await/in-trait/lifetime-mismatch.next.stderr b/tests/ui/async-await/in-trait/lifetime-mismatch.next.stderr
new file mode 100644
index 0000000000000..0e9477544a4a6
--- /dev/null
+++ b/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`.
diff --git a/tests/ui/async-await/in-trait/lifetime-mismatch.rs b/tests/ui/async-await/in-trait/lifetime-mismatch.rs
index 45ede193c0fc6..5ff5a01a1ee03 100644
--- a/tests/ui/async-await/in-trait/lifetime-mismatch.rs
+++ b/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
diff --git a/tests/ui/impl-trait/in-trait/early.rs b/tests/ui/impl-trait/in-trait/early.rs
index 9c1c2b5033904..831033a588061 100644
--- a/tests/ui/impl-trait/in-trait/early.rs
+++ b/tests/ui/impl-trait/in-trait/early.rs
@@ -1,5 +1,7 @@
 // check-pass
 // edition:2021
+// [next] compile-flags: -Zlower-impl-trait-in-trait-to-assoc-ty
+// revisions: current next
 
 #![feature(async_fn_in_trait, return_position_impl_trait_in_trait)]
 #![allow(incomplete_features)]
diff --git a/tests/ui/impl-trait/in-trait/issue-102301.rs b/tests/ui/impl-trait/in-trait/issue-102301.rs
index a93714a658ef0..1329ca29d06aa 100644
--- a/tests/ui/impl-trait/in-trait/issue-102301.rs
+++ b/tests/ui/impl-trait/in-trait/issue-102301.rs
@@ -1,4 +1,6 @@
 // check-pass
+// [next] compile-flags: -Zlower-impl-trait-in-trait-to-assoc-ty
+// revisions: current next
 
 #![feature(return_position_impl_trait_in_trait)]
 #![allow(incomplete_features)]
diff --git a/tests/ui/impl-trait/in-trait/opaque-in-impl.rs b/tests/ui/impl-trait/in-trait/opaque-in-impl.rs
index 2e06629699aad..f48d9fa26c003 100644
--- a/tests/ui/impl-trait/in-trait/opaque-in-impl.rs
+++ b/tests/ui/impl-trait/in-trait/opaque-in-impl.rs
@@ -1,4 +1,6 @@
 // check-pass
+// [next] compile-flags: -Zlower-impl-trait-in-trait-to-assoc-ty
+// revisions: current next
 
 #![feature(return_position_impl_trait_in_trait)]
 #![allow(incomplete_features)]
diff --git a/tests/ui/impl-trait/in-trait/trait-more-generics-than-impl.stderr b/tests/ui/impl-trait/in-trait/trait-more-generics-than-impl.current.stderr
similarity index 88%
rename from tests/ui/impl-trait/in-trait/trait-more-generics-than-impl.stderr
rename to tests/ui/impl-trait/in-trait/trait-more-generics-than-impl.current.stderr
index 8ff54cad95139..64c942705cf8e 100644
--- a/tests/ui/impl-trait/in-trait/trait-more-generics-than-impl.stderr
+++ b/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
diff --git a/tests/ui/impl-trait/in-trait/trait-more-generics-than-impl.next.stderr b/tests/ui/impl-trait/in-trait/trait-more-generics-than-impl.next.stderr
new file mode 100644
index 0000000000000..64c942705cf8e
--- /dev/null
+++ b/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`.
diff --git a/tests/ui/impl-trait/in-trait/trait-more-generics-than-impl.rs b/tests/ui/impl-trait/in-trait/trait-more-generics-than-impl.rs
index 0bbe50ea6fd37..c2e394a1f6645 100644
--- a/tests/ui/impl-trait/in-trait/trait-more-generics-than-impl.rs
+++ b/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)]
 
diff --git a/tests/ui/impl-trait/in-trait/where-clause.rs b/tests/ui/impl-trait/in-trait/where-clause.rs
index 87bac519cf304..88d86e2b54192 100644
--- a/tests/ui/impl-trait/in-trait/where-clause.rs
+++ b/tests/ui/impl-trait/in-trait/where-clause.rs
@@ -1,5 +1,7 @@
 // check-pass
 // edition: 2021
+// [next] compile-flags: -Zlower-impl-trait-in-trait-to-assoc-ty
+// revisions: current next
 
 #![feature(return_position_impl_trait_in_trait)]
 #![allow(incomplete_features)]