From f8c10ff8b7da40868ceb085e7d85d8adcce2c764 Mon Sep 17 00:00:00 2001
From: Fabian Wolff <fabian.wolff@alumni.ethz.ch>
Date: Thu, 17 Jun 2021 14:48:19 +0200
Subject: [PATCH] Remove invalid suggestion involving Fn trait bound

---
 .../src/traits/error_reporting/mod.rs            |  1 +
 src/test/ui/traits/issue-85735.rs                | 13 +++++++++++++
 src/test/ui/traits/issue-85735.stderr            | 16 ++++++++++++++++
 3 files changed, 30 insertions(+)
 create mode 100644 src/test/ui/traits/issue-85735.rs
 create mode 100644 src/test/ui/traits/issue-85735.stderr

diff --git a/compiler/rustc_trait_selection/src/traits/error_reporting/mod.rs b/compiler/rustc_trait_selection/src/traits/error_reporting/mod.rs
index f46fae1326c8f..3f713ce3c3914 100644
--- a/compiler/rustc_trait_selection/src/traits/error_reporting/mod.rs
+++ b/compiler/rustc_trait_selection/src/traits/error_reporting/mod.rs
@@ -1604,6 +1604,7 @@ impl<'a, 'tcx> InferCtxtPrivExt<'tcx> for InferCtxt<'a, 'tcx> {
                     if generics.params.iter().any(|p| p.name != kw::SelfUpper)
                         && !snippet.ends_with('>')
                         && !generics.has_impl_trait()
+                        && !self.tcx.fn_trait_kind_from_lang_item(*def_id).is_some()
                     {
                         // FIXME: To avoid spurious suggestions in functions where type arguments
                         // where already supplied, we check the snippet to make sure it doesn't
diff --git a/src/test/ui/traits/issue-85735.rs b/src/test/ui/traits/issue-85735.rs
new file mode 100644
index 0000000000000..16e874ee21e62
--- /dev/null
+++ b/src/test/ui/traits/issue-85735.rs
@@ -0,0 +1,13 @@
+// Regression test for the invalid suggestion in #85735 (the
+// underlying issue #21974 still exists here).
+
+trait Foo {}
+impl<'a, 'b, T> Foo for T
+where
+    T: FnMut(&'a ()),
+    //~^ ERROR: type annotations needed [E0283]
+    T: FnMut(&'b ()),
+{
+}
+
+fn main() {}
diff --git a/src/test/ui/traits/issue-85735.stderr b/src/test/ui/traits/issue-85735.stderr
new file mode 100644
index 0000000000000..8c958e3844cbe
--- /dev/null
+++ b/src/test/ui/traits/issue-85735.stderr
@@ -0,0 +1,16 @@
+error[E0283]: type annotations needed
+  --> $DIR/issue-85735.rs:7:8
+   |
+LL |     T: FnMut(&'a ()),
+   |        ^^^^^^^^^^^^^ cannot infer type for type parameter `T`
+   | 
+  ::: $SRC_DIR/core/src/ops/function.rs:LL:COL
+   |
+LL | pub trait FnMut<Args>: FnOnce<Args> {
+   | ----------------------------------- required by this bound in `FnMut`
+   |
+   = note: cannot satisfy `T: FnMut<(&'a (),)>`
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0283`.