Display generic arguments for associated types

3 files changed, 78 insertions, 17 deletions

diff --git a/crates/hir-ty/src/display.rs b/crates/hir-ty/src/display.rs
index 0221f922feb..5ad66132635 100644
--- a/crates/hir-ty/src/display.rs
+++ b/crates/hir-ty/src/display.rs
@@ -289,16 +289,18 @@ impl HirDisplay for ProjectionTy {
             return write!(f, "{}", TYPE_HINT_TRUNCATION);
         }
 
-        let trait_ = f.db.trait_data(self.trait_(f.db));
+        let trait_ref = self.trait_ref(f.db);
         write!(f, "<")?;
-        self.self_type_parameter(f.db).hir_fmt(f)?;
-        write!(f, " as {}", trait_.name)?;
-        if self.substitution.len(Interner) > 1 {
+        fmt_trait_ref(&trait_ref, f, true)?;
+        write!(f, ">::{}", f.db.type_alias_data(from_assoc_type_id(self.associated_ty_id)).name)?;
+        let proj_params_count =
+            self.substitution.len(Interner) - trait_ref.substitution.len(Interner);
+        let proj_params = &self.substitution.as_slice(Interner)[..proj_params_count];
+        if !proj_params.is_empty() {
             write!(f, "<")?;
-            f.write_joined(&self.substitution.as_slice(Interner)[1..], ", ")?;
+            f.write_joined(proj_params, ", ")?;
             write!(f, ">")?;
         }
-        write!(f, ">::{}", f.db.type_alias_data(from_assoc_type_id(self.associated_ty_id)).name)?;
         Ok(())
     }
 }
@@ -641,9 +643,12 @@ impl HirDisplay for Ty {
                 // Use placeholder associated types when the target is test (https://rust-lang.github.io/chalk/book/clauses/type_equality.html#placeholder-associated-types)
                 if f.display_target.is_test() {
                     write!(f, "{}::{}", trait_.name, type_alias_data.name)?;
+                    // Note that the generic args for the associated type come before those for the
+                    // trait (including the self type).
+                    // FIXME: reconsider the generic args order upon formatting?
                     if parameters.len(Interner) > 0 {
                         write!(f, "<")?;
-                        f.write_joined(&*parameters.as_slice(Interner), ", ")?;
+                        f.write_joined(parameters.as_slice(Interner), ", ")?;
                         write!(f, ">")?;
                     }
                 } else {
@@ -972,9 +977,20 @@ fn write_bounds_like_dyn_trait(
                     angle_open = true;
                 }
                 if let AliasTy::Projection(proj) = alias {
-                    let type_alias =
-                        f.db.type_alias_data(from_assoc_type_id(proj.associated_ty_id));
-                    write!(f, "{} = ", type_alias.name)?;
+                    let assoc_ty_id = from_assoc_type_id(proj.associated_ty_id);
+                    let type_alias = f.db.type_alias_data(assoc_ty_id);
+                    write!(f, "{}", type_alias.name)?;
+
+                    let proj_arg_count = generics(f.db.upcast(), assoc_ty_id.into()).len_self();
+                    if proj_arg_count > 0 {
+                        write!(f, "<")?;
+                        f.write_joined(
+                            &proj.substitution.as_slice(Interner)[..proj_arg_count],
+                            ", ",
+                        )?;
+                        write!(f, ">")?;
+                    }
+                    write!(f, " = ")?;
                 }
                 ty.hir_fmt(f)?;
             }
diff --git a/crates/hir-ty/src/tests/display_source_code.rs b/crates/hir-ty/src/tests/display_source_code.rs
index 8a8ff08cfe8..425432479e8 100644
--- a/crates/hir-ty/src/tests/display_source_code.rs
+++ b/crates/hir-ty/src/tests/display_source_code.rs
@@ -196,3 +196,34 @@ fn test(
 "#,
     );
 }
+
+#[test]
+fn projection_type_correct_arguments_order() {
+    check_types_source_code(
+        r#"
+trait Foo<T> {
+    type Assoc<U>;
+}
+fn f<T: Foo<i32>>(a: T::Assoc<usize>) {
+    a;
+  //^ <T as Foo<i32>>::Assoc<usize>
+}
+"#,
+    );
+}
+
+#[test]
+fn generic_associated_type_binding_in_impl_trait() {
+    check_types_source_code(
+        r#"
+//- minicore: sized
+trait Foo<T> {
+    type Assoc<U>;
+}
+fn f(a: impl Foo<i8, Assoc<i16> = i32>) {
+    a;
+  //^ impl Foo<i8, Assoc<i16> = i32>
+}
+        "#,
+    );
+}
diff --git a/crates/hir-ty/src/tls.rs b/crates/hir-ty/src/tls.rs
index 547850b021c..92711a24fe3 100644
--- a/crates/hir-ty/src/tls.rs
+++ b/crates/hir-ty/src/tls.rs
@@ -5,7 +5,7 @@ use itertools::Itertools;
 
 use crate::{
     chalk_db, db::HirDatabase, from_assoc_type_id, from_chalk_trait_id, mapping::from_chalk,
-    CallableDefId, Interner,
+    CallableDefId, Interner, ProjectionTyExt,
 };
 use hir_def::{AdtId, ItemContainerId, Lookup, TypeAliasId};
 
@@ -63,17 +63,31 @@ impl DebugContext<'_> {
             ItemContainerId::TraitId(t) => t,
             _ => panic!("associated type not in trait"),
         };
-        let trait_data = self.0.trait_data(trait_);
-        let params = projection_ty.substitution.as_slice(Interner);
-        write!(fmt, "<{:?} as {}", &params[0], trait_data.name,)?;
-        if params.len() > 1 {
+        let trait_name = &self.0.trait_data(trait_).name;
+        let trait_ref = projection_ty.trait_ref(self.0);
+        let trait_params = trait_ref.substitution.as_slice(Interner);
+        let self_ty = trait_ref.self_type_parameter(Interner);
+        write!(fmt, "<{:?} as {}", self_ty, trait_name)?;
+        if trait_params.len() > 1 {
+            write!(
+                fmt,
+                "<{}>",
+                trait_params[1..].iter().format_with(", ", |x, f| f(&format_args!("{:?}", x))),
+            )?;
+        }
+        write!(fmt, ">::{}", type_alias_data.name)?;
+
+        let proj_params_count = projection_ty.substitution.len(Interner) - trait_params.len();
+        let proj_params = &projection_ty.substitution.as_slice(Interner)[..proj_params_count];
+        if !proj_params.is_empty() {
             write!(
                 fmt,
                 "<{}>",
-                &params[1..].iter().format_with(", ", |x, f| f(&format_args!("{:?}", x))),
+                proj_params.iter().format_with(", ", |x, f| f(&format_args!("{:?}", x))),
             )?;
         }
-        write!(fmt, ">::{}", type_alias_data.name)
+
+        Ok(())
     }
 
     pub(crate) fn debug_fn_def_id(