From 6f0575f08adca0c8a8f38a301913a1cfe636eb94 Mon Sep 17 00:00:00 2001
From: Lukas Wirth <lukastw97@gmail.com>
Date: Tue, 30 Mar 2021 21:43:23 +0200
Subject: Fix generic arguments being incorrectly offset in qualified trait
 casts

---
 crates/hir_def/src/path/lower.rs  |  4 +++-
 crates/hir_ty/src/tests/traits.rs | 43 +++++++++++++++++++++++++++++++++++++++
 2 files changed, 46 insertions(+), 1 deletion(-)

diff --git a/crates/hir_def/src/path/lower.rs b/crates/hir_def/src/path/lower.rs
index 505493a74..4de951fd3 100644
--- a/crates/hir_def/src/path/lower.rs
+++ b/crates/hir_def/src/path/lower.rs
@@ -74,6 +74,7 @@ pub(super) fn lower_path(mut path: ast::Path, hygiene: &Hygiene) -> Option<Path>
                     // <T as Trait<A>>::Foo desugars to Trait<Self=T, A>::Foo
                     Some(trait_ref) => {
                         let path = Path::from_src(trait_ref.path()?, hygiene)?;
+                        let num_segments = path.mod_path.segments.len();
                         kind = path.mod_path.kind;
 
                         let mut prefix_segments = path.mod_path.segments;
@@ -85,7 +86,8 @@ pub(super) fn lower_path(mut path: ast::Path, hygiene: &Hygiene) -> Option<Path>
                         generic_args.extend(prefix_args);
 
                         // Insert the type reference (T in the above example) as Self parameter for the trait
-                        let last_segment = generic_args.last_mut()?;
+                        let last_segment =
+                            generic_args.iter_mut().rev().nth(num_segments.saturating_sub(1))?;
                         if last_segment.is_none() {
                             *last_segment = Some(Arc::new(GenericArgs::empty()));
                         };
diff --git a/crates/hir_ty/src/tests/traits.rs b/crates/hir_ty/src/tests/traits.rs
index 2ba97f814..65b71fdfa 100644
--- a/crates/hir_ty/src/tests/traits.rs
+++ b/crates/hir_ty/src/tests/traits.rs
@@ -3370,3 +3370,46 @@ fn test() {
         "#]],
     )
 }
+
+#[test]
+fn qualified_path_as_qualified_trait() {
+    check_infer(
+        r#"
+mod foo {
+
+    pub trait Foo {
+        type Target;
+    }
+    pub trait Bar {
+        type Output;
+        fn boo() -> Self::Output {
+            loop {}
+        }
+    }
+}
+
+struct F;
+impl foo::Foo for F {
+    type Target = ();
+}
+impl foo::Bar for F {
+    type Output = <F as foo::Foo>::Target;
+}
+
+fn foo() {
+    use foo::Bar;
+    let x = <F as Bar>::boo();
+}
+
+        "#,
+        expect![[r#"
+            132..163 '{     ...     }': Bar::Output<Self>
+            146..153 'loop {}': !
+            151..153 '{}': ()
+            306..358 '{     ...o(); }': ()
+            334..335 'x': ()
+            338..353 '<F as Bar>::boo': fn boo<F>() -> <F as Bar>::Output
+            338..355 '<F as ...:boo()': ()
+        "#]],
+    );
+}
-- 
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