Merge pull request #20238 from rust-lang/gat-infer-lifetimes

fix: Infer lifetimes for GATs in expression/pattern position

src/tools/rust-analyzer/crates/hir-ty/src/lower/path.rs

@@ -1018,8 +1018,12 @@ fn check_generic_args_len(
     }
 
     let lifetime_args_len = def_generics.len_lifetimes_self();
-    if provided_lifetimes_count == 0 && lifetime_args_len > 0 && !lowering_assoc_type_generics {
-        // In generic associated types, we never allow inferring the lifetimes.
+    if provided_lifetimes_count == 0
+        && lifetime_args_len > 0
+        && (!lowering_assoc_type_generics || infer_args)
+    {
+        // In generic associated types, we never allow inferring the lifetimes, but only in type context, that is
+        // when `infer_args == false`. In expression/pattern context we always allow inferring them, even for GATs.
         match lifetime_elision {
             &LifetimeElisionKind::AnonymousCreateParameter { report_in_path } => {
                 ctx.report_elided_lifetimes_in_path(def, lifetime_args_len as u32, report_in_path);

src/tools/rust-analyzer/crates/ide-diagnostics/src/handlers/incorrect_generics_len.rs

@@ -183,4 +183,28 @@ fn main() {
 "#,
         );
     }
+
+    #[test]
+    fn generic_assoc_type_infer_lifetime_in_expr_position() {
+        check_diagnostics(
+            r#"
+//- minicore: sized
+struct Player;
+
+struct Foo<'c, C> {
+    _v: &'c C,
+}
+trait WithSignals: Sized {
+    type SignalCollection<'c, C>;
+    fn __signals_from_external(&self) -> Self::SignalCollection<'_, Self>;
+}
+impl WithSignals for Player {
+    type SignalCollection<'c, C> = Foo<'c, C>;
+    fn __signals_from_external(&self) -> Self::SignalCollection<'_, Self> {
+        Self::SignalCollection { _v: self }
+    }
+}
+        "#,
+        );
+    }
 }