Merge #10896

10896: hir: resolve assoc trait type in path r=jhgg a=jhgg

fixes #9802

- [ ] write tests, maybe, if this is even a good fix...

Co-authored-by: Jake Heinz <jh@discordapp.com>

crates/hir/src/source_analyzer.rs

@@ -16,7 +16,7 @@ use hir_def::{
     expr::{ExprId, Pat, PatId},
     path::{ModPath, Path, PathKind},
     resolver::{resolver_for_scope, Resolver, TypeNs, ValueNs},
-    AsMacroCall, DefWithBodyId, FieldId, FunctionId, LocalFieldId, VariantId,
+    AsMacroCall, DefWithBodyId, FieldId, FunctionId, LocalFieldId, ModuleDefId, VariantId,
 };
 use hir_expand::{hygiene::Hygiene, name::AsName, HirFileId, InFile};
 use hir_ty::{
@@ -544,6 +544,17 @@ fn resolve_hir_path_(
                 }
             }
         }?;
+
+        // If we are in a TypeNs for a Trait, and we have an unresolved name, try to resolve it as a type
+        // within the trait's associated types.
+        if let (Some(unresolved), &TypeNs::TraitId(trait_id)) = (&unresolved, &ty) {
+            if let Some(type_alias_id) =
+                db.trait_data(trait_id).associated_type_by_name(&unresolved.name)
+            {
+                return Some(PathResolution::Def(ModuleDefId::from(type_alias_id).into()));
+            }
+        }
+
         let res = match ty {
             TypeNs::SelfType(it) => PathResolution::SelfType(it.into()),
             TypeNs::GenericParam(id) => PathResolution::TypeParam(TypeParam { id }),

crates/ide/src/syntax_highlighting/test_data/highlighting.html

@@ -265,4 +265,13 @@ proc_macros::<span class="macro">mirror!</span> <span class="brace">{</span>
 <span class="brace">}</span>
 <span class="keyword">const</span> <span class="constant declaration">USAGE_OF_BOOL</span><span class="colon">:</span><span class="builtin_type">bool</span> <span class="operator">=</span> <span class="enum public">Bool</span><span class="operator">::</span><span class="enum_variant public">True</span><span class="operator">.</span><span class="function associated consuming public">to_primitive</span><span class="parenthesis">(</span><span class="parenthesis">)</span><span class="semicolon">;</span>
 
+<span class="keyword">trait</span> <span class="trait declaration">Baz</span> <span class="brace">{</span>
+    <span class="keyword">type</span> <span class="type_alias associated declaration trait">Qux</span><span class="semicolon">;</span>
+<span class="brace">}</span>
+
+<span class="keyword">fn</span> <span class="function declaration">baz</span><span class="angle">&lt;</span><span class="type_param declaration">T</span><span class="angle">&gt;</span><span class="parenthesis">(</span><span class="value_param declaration">t</span><span class="colon">:</span> <span class="type_param">T</span><span class="parenthesis">)</span>
+<span class="keyword">where</span>
+    <span class="type_param">T</span><span class="colon">:</span> <span class="trait">Baz</span><span class="comma">,</span>
+    <span class="angle">&lt;</span><span class="type_param">T</span> <span class="keyword">as</span> <span class="trait">Baz</span><span class="angle">&gt;</span><span class="operator">::</span><span class="type_alias associated trait">Qux</span><span class="colon">:</span> <span class="trait">Bar</span> <span class="brace">{</span><span class="brace">}</span>
+
 </code></pre>

crates/ide/src/syntax_highlighting/tests.rs

@@ -238,6 +238,15 @@ impl Bool {
 }
 const USAGE_OF_BOOL:bool = Bool::True.to_primitive();
 
+trait Baz {
+    type Qux;
+}
+
+fn baz<T>(t: T)
+where
+    T: Baz,
+    <T as Baz>::Qux: Bar {}
+
 //- /foo.rs crate:foo
 pub struct Person {
     pub name: &'static str,