Avoid impls_fnonce to return true when the trait solving is ambiguous

crates/hir/src/code_model.rs

@@ -1398,7 +1398,7 @@ impl Type {
         };
 
         let canonical_ty = Canonical { value: self.ty.value.clone(), kinds: Arc::new([]) };
-        method_resolution::implements_trait(
+        method_resolution::implements_trait_unique(
             &canonical_ty,
             db,
             self.ty.environment.clone(),

crates/hir_ty/src/method_resolution.rs

@@ -740,6 +740,19 @@ pub fn implements_trait(
     solution.is_some()
 }
 
+pub fn implements_trait_unique(
+    ty: &Canonical<Ty>,
+    db: &dyn HirDatabase,
+    env: Arc<TraitEnvironment>,
+    krate: CrateId,
+    trait_: TraitId,
+) -> bool {
+    let goal = generic_implements_goal(db, env, trait_, ty.clone());
+    let solution = db.trait_solve(krate, goal);
+
+    matches!(solution, Some(crate::traits::Solution::Unique(_)))
+}
+
 /// This creates Substs for a trait with the given Self type and type variables
 /// for all other parameters, to query Chalk with it.
 fn generic_implements_goal(

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

@@ -115,6 +115,12 @@
     <span class="value_param callable">f</span><span class="punctuation">(</span><span class="punctuation">)</span>
 <span class="punctuation">}</span>
 
+<span class="keyword">fn</span> <span class="function declaration">foobar</span><span class="punctuation">(</span><span class="punctuation">)</span> <span class="operator">-&gt;</span> <span class="keyword">impl</span> <span class="unresolved_reference">Copy</span> <span class="punctuation">{</span><span class="punctuation">}</span>
+
+<span class="keyword">fn</span> <span class="function declaration">foo</span><span class="punctuation">(</span><span class="punctuation">)</span> <span class="punctuation">{</span>
+    <span class="keyword">let</span> <span class="variable declaration">bar</span> <span class="operator">=</span> <span class="function">foobar</span><span class="punctuation">(</span><span class="punctuation">)</span><span class="punctuation">;</span>
+<span class="punctuation">}</span>
+
 <span class="macro">macro_rules!</span> <span class="macro declaration">def_fn</span> <span class="punctuation">{</span>
     <span class="punctuation">(</span><span class="punctuation">$</span><span class="punctuation">(</span><span class="punctuation">$</span>tt<span class="punctuation">:</span>tt<span class="punctuation">)</span><span class="punctuation">*</span><span class="punctuation">)</span> <span class="operator">=</span><span class="punctuation">&gt;</span> <span class="punctuation">{</span><span class="punctuation">$</span><span class="punctuation">(</span><span class="punctuation">$</span>tt<span class="punctuation">)</span><span class="punctuation">*</span><span class="punctuation">}</span>
 <span class="punctuation">}</span>

crates/ide/src/syntax_highlighting/tests.rs

@@ -89,6 +89,12 @@ fn baz<F: Fn() -> ()>(f: F) {
     f()
 }
 
+fn foobar() -> impl Copy {}
+
+fn foo() {
+    let bar = foobar();
+}
+
 macro_rules! def_fn {
     ($($tt:tt)*) => {$($tt)*}
 }