From 5954edec19c0cb35315059d91a381d24afd1d28f Mon Sep 17 00:00:00 2001
From: Aditya-PS-05 <adityapratapsjnhh7654@gmail.com>
Date: Thu, 20 Nov 2025 02:06:34 +0530
Subject: [PATCH 1/4] feat: add regression test for #19957

---
 .../hir-ty/src/tests/regression/new_solver.rs | 50 +++++++++++++++++++
 1 file changed, 50 insertions(+)

diff --git a/crates/hir-ty/src/tests/regression/new_solver.rs b/crates/hir-ty/src/tests/regression/new_solver.rs
index 933a9a5c3519..ef0c03803cf2 100644
--- a/crates/hir-ty/src/tests/regression/new_solver.rs
+++ b/crates/hir-ty/src/tests/regression/new_solver.rs
@@ -552,3 +552,53 @@ where
         "#]],
     );
 }
+
+#[test]
+fn regression_19957() {
+    // async-trait patterns should not produce false type mismatches between
+    // Pin<Box<dyn Future>> and Pin<Box<impl Future>>
+    check_no_mismatches(
+        r#"
+//- minicore: future, pin, result, error, send
+use core::{future::Future, pin::Pin};
+
+pub enum SimpleAsyncTraitResult {
+    Ok,
+    Error,
+}
+
+pub struct ExampleData {
+    pub id: i32,
+    pub name: String,
+}
+// As we can't directly use #[async_trait] directly in tests
+// This simulates what #[async_trait] expands to
+pub trait SimpleAsyncTraitModel {
+    fn save<'life0, 'async_trait>(
+        &'life0 self,
+    ) -> Pin<Box<dyn Future<Output = Result<SimpleAsyncTraitResult, Box<dyn core::error::Error + Send + Sync>>> + Send + 'async_trait>>
+    where
+        'life0: 'async_trait,
+        Self: 'async_trait;
+}
+
+impl SimpleAsyncTraitModel for ExampleData {
+    fn save<'life0, 'async_trait>(
+        &'life0 self,
+    ) -> Pin<Box<dyn Future<Output = Result<SimpleAsyncTraitResult, Box<dyn core::error::Error + Send + Sync>>> + Send + 'async_trait>>
+    where
+        'life0: 'async_trait,
+        Self: 'async_trait,
+    {
+        Box::pin(async move {
+            if self.id > 0 {
+                Ok(SimpleAsyncTraitResult::Ok)
+            } else {
+                Ok(SimpleAsyncTraitResult::Error)
+            }
+        })
+    }
+}
+"#,
+    );
+}

From 772e15e8e36c4b9676abbd23d18e9f652188405a Mon Sep 17 00:00:00 2001
From: Aditya-PS-05 <adityapratapsjnhh7654@gmail.com>
Date: Thu, 20 Nov 2025 14:21:39 +0530
Subject: [PATCH 2/4] feat: update test

---
 .../hir-ty/src/tests/regression/new_solver.rs | 49 +++++++++++--------
 1 file changed, 29 insertions(+), 20 deletions(-)

diff --git a/crates/hir-ty/src/tests/regression/new_solver.rs b/crates/hir-ty/src/tests/regression/new_solver.rs
index ef0c03803cf2..4866a038d64a 100644
--- a/crates/hir-ty/src/tests/regression/new_solver.rs
+++ b/crates/hir-ty/src/tests/regression/new_solver.rs
@@ -554,49 +554,58 @@ where
 }
 
 #[test]
+// #[should_panic(expected = "Unexpected type mismatches")]
 fn regression_19957() {
-    // async-trait patterns should not produce false type mismatches between
-    // Pin<Box<dyn Future>> and Pin<Box<impl Future>>
+    // This test documents issue #19957: async-trait patterns incorrectly produce
+    // type mismatches between Pin<Box<dyn Future>> and Pin<Box<impl Future>>.
+    //
+    // The test currently FAILS (as expected) because the bug is not yet fixed.
+    // When the bug is fixed, remove the #[should_panic] attribute.
     check_no_mismatches(
         r#"
-//- minicore: future, pin, result, error, send
+//- minicore: future, pin, result, error, send, coerce_unsized, dispatch_from_dyn
 use core::{future::Future, pin::Pin};
 
-pub enum SimpleAsyncTraitResult {
-    Ok,
-    Error,
+#[lang = "owned_box"]
+pub struct Box<T: ?Sized> {
+    inner: *mut T,
 }
 
+impl<T> Box<T> {
+    fn pin(value: T) -> Pin<Box<T>> {
+        // Implementation details don't matter here for type checking
+        loop {}
+    }
+}
+
+impl<T: ?Sized + core::marker::Unsize<U>, U: ?Sized> core::ops::CoerceUnsized<Box<U>> for Box<T> {}
+
+impl<T: ?Sized + core::ops::DispatchFromDyn<U>, U: ?Sized> core::ops::DispatchFromDyn<Box<U>> for Box<T> {}
+
 pub struct ExampleData {
     pub id: i32,
-    pub name: String,
 }
-// As we can't directly use #[async_trait] directly in tests
-// This simulates what #[async_trait] expands to
-pub trait SimpleAsyncTraitModel {
+
+// Simulates what #[async_trait] expands to
+pub trait SimpleModel {
     fn save<'life0, 'async_trait>(
         &'life0 self,
-    ) -> Pin<Box<dyn Future<Output = Result<SimpleAsyncTraitResult, Box<dyn core::error::Error + Send + Sync>>> + Send + 'async_trait>>
+    ) -> Pin<Box<dyn Future<Output = i32> + Send + 'async_trait>>
     where
         'life0: 'async_trait,
         Self: 'async_trait;
 }
 
-impl SimpleAsyncTraitModel for ExampleData {
+impl SimpleModel for ExampleData {
     fn save<'life0, 'async_trait>(
         &'life0 self,
-    ) -> Pin<Box<dyn Future<Output = Result<SimpleAsyncTraitResult, Box<dyn core::error::Error + Send + Sync>>> + Send + 'async_trait>>
+    ) -> Pin<Box<dyn Future<Output = i32> + Send + 'async_trait>>
     where
         'life0: 'async_trait,
         Self: 'async_trait,
     {
-        Box::pin(async move {
-            if self.id > 0 {
-                Ok(SimpleAsyncTraitResult::Ok)
-            } else {
-                Ok(SimpleAsyncTraitResult::Error)
-            }
-        })
+        // Body creates Pin<Box<impl Future>>, which should coerce to Pin<Box<dyn Future>>
+        Box::pin(async move { self.id })
     }
 }
 "#,

From a63900837a3f393f382c52f6e5ced84e387bb5ce Mon Sep 17 00:00:00 2001
From: Aditya-PS-05 <adityapratapsjnhh7654@gmail.com>
Date: Fri, 21 Nov 2025 10:59:14 +0530
Subject: [PATCH 3/4] remove comments

---
 crates/hir-ty/src/tests/regression/new_solver.rs | 2 --
 1 file changed, 2 deletions(-)

diff --git a/crates/hir-ty/src/tests/regression/new_solver.rs b/crates/hir-ty/src/tests/regression/new_solver.rs
index 4866a038d64a..420a316009a3 100644
--- a/crates/hir-ty/src/tests/regression/new_solver.rs
+++ b/crates/hir-ty/src/tests/regression/new_solver.rs
@@ -554,13 +554,11 @@ where
 }
 
 #[test]
-// #[should_panic(expected = "Unexpected type mismatches")]
 fn regression_19957() {
     // This test documents issue #19957: async-trait patterns incorrectly produce
     // type mismatches between Pin<Box<dyn Future>> and Pin<Box<impl Future>>.
     //
     // The test currently FAILS (as expected) because the bug is not yet fixed.
-    // When the bug is fixed, remove the #[should_panic] attribute.
     check_no_mismatches(
         r#"
 //- minicore: future, pin, result, error, send, coerce_unsized, dispatch_from_dyn

From f39579c04e8ecafdc6843d1656c30fbc0430acd9 Mon Sep 17 00:00:00 2001
From: Aditya-PS-05 <adityapratapsjnhh7654@gmail.com>
Date: Fri, 21 Nov 2025 19:05:36 +0530
Subject: [PATCH 4/4] update minicore

---
 crates/hir-ty/src/tests/regression/new_solver.rs | 2 --
 crates/test-utils/src/minicore.rs                | 6 ++++++
 2 files changed, 6 insertions(+), 2 deletions(-)

diff --git a/crates/hir-ty/src/tests/regression/new_solver.rs b/crates/hir-ty/src/tests/regression/new_solver.rs
index 420a316009a3..18509c52847f 100644
--- a/crates/hir-ty/src/tests/regression/new_solver.rs
+++ b/crates/hir-ty/src/tests/regression/new_solver.rs
@@ -557,8 +557,6 @@ where
 fn regression_19957() {
     // This test documents issue #19957: async-trait patterns incorrectly produce
     // type mismatches between Pin<Box<dyn Future>> and Pin<Box<impl Future>>.
-    //
-    // The test currently FAILS (as expected) because the bug is not yet fixed.
     check_no_mismatches(
         r#"
 //- minicore: future, pin, result, error, send, coerce_unsized, dispatch_from_dyn
diff --git a/crates/test-utils/src/minicore.rs b/crates/test-utils/src/minicore.rs
index d5905afc3896..679fe420b05c 100644
--- a/crates/test-utils/src/minicore.rs
+++ b/crates/test-utils/src/minicore.rs
@@ -1518,6 +1518,12 @@ pub mod pin {
     {
     }
     // endregion:dispatch_from_dyn
+    // region:coerce_unsized
+    impl<Ptr, U> crate::ops::CoerceUnsized<Pin<U>> for Pin<Ptr> where
+        Ptr: crate::ops::CoerceUnsized<U>
+    {
+    }
+    // endregion:coerce_unsized
 }
 // endregion:pin