Fix missing parentheses Fn notation error

src/librustc_typeck/astconv.rs

@@ -1151,9 +1151,16 @@ impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
                             .as_ref()
                             .and_then(|args| args.args.get(0))
                             .and_then(|arg| match arg {
-                                hir::GenericArg::Type(ty) => {
-                                    sess.source_map().span_to_snippet(ty.span).ok()
+                                hir::GenericArg::Type(ty) => match ty.kind {
+                                    hir::TyKind::Tup(t) => t
+                                        .iter()
+                                        .map(|e| sess.source_map().span_to_snippet(e.span))
+                                        .collect::<Result<Vec<_>, _>>()
+                                        .map(|a| a.join(", ")),
+                                    _ => sess.source_map().span_to_snippet(ty.span),
                                 }
+                                .map(|s| format!("({})", s))
+                                .ok(),
                                 _ => None,
                             })
                             .unwrap_or_else(|| "()".to_string()),

src/test/ui/suggestions/fn-trait-notation.fixed

@@ -0,0 +1,19 @@
+// run-rustfix
+fn e0658<F, G, H>(f: F, g: G, h: H) -> i32
+where
+    F: Fn(i32) -> i32, //~ ERROR E0658
+    G: Fn(i32, i32) -> (i32, i32), //~ ERROR E0658
+    H: Fn(i32) -> i32, //~ ERROR E0658
+{
+    f(3);
+    g(3, 4);
+    h(3)
+}
+
+fn main() {
+    e0658(
+        |a| a,
+        |a, b| (b, a),
+        |a| a,
+    );
+}

src/test/ui/suggestions/fn-trait-notation.rs

@@ -0,0 +1,19 @@
+// run-rustfix
+fn e0658<F, G, H>(f: F, g: G, h: H) -> i32
+where
+    F: Fn<i32, Output = i32>, //~ ERROR E0658
+    G: Fn<(i32, i32, ), Output = (i32, i32)>, //~ ERROR E0658
+    H: Fn<(i32,), Output = i32>, //~ ERROR E0658
+{
+    f(3);
+    g(3, 4);
+    h(3)
+}
+
+fn main() {
+    e0658(
+        |a| a,
+        |a, b| (b, a),
+        |a| a,
+    );
+}

src/test/ui/suggestions/fn-trait-notation.stderr

@@ -0,0 +1,30 @@
+error[E0658]: the precise format of `Fn`-family traits' type parameters is subject to change
+  --> $DIR/fn-trait-notation.rs:4:8
+   |
+LL |     F: Fn<i32, Output = i32>,
+   |        ^^^^^^^^^^^^^^^^^^^^^ help: use parenthetical notation instead: `Fn(i32) -> i32`
+   |
+   = note: see issue #29625 <https://github.com/rust-lang/rust/issues/29625> for more information
+   = help: add `#![feature(unboxed_closures)]` to the crate attributes to enable
+
+error[E0658]: the precise format of `Fn`-family traits' type parameters is subject to change
+  --> $DIR/fn-trait-notation.rs:5:8
+   |
+LL |     G: Fn<(i32, i32, ), Output = (i32, i32)>,
+   |        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: use parenthetical notation instead: `Fn(i32, i32) -> (i32, i32)`
+   |
+   = note: see issue #29625 <https://github.com/rust-lang/rust/issues/29625> for more information
+   = help: add `#![feature(unboxed_closures)]` to the crate attributes to enable
+
+error[E0658]: the precise format of `Fn`-family traits' type parameters is subject to change
+  --> $DIR/fn-trait-notation.rs:6:8
+   |
+LL |     H: Fn<(i32,), Output = i32>,
+   |        ^^^^^^^^^^^^^^^^^^^^^^^^ help: use parenthetical notation instead: `Fn(i32) -> i32`
+   |
+   = note: see issue #29625 <https://github.com/rust-lang/rust/issues/29625> for more information
+   = help: add `#![feature(unboxed_closures)]` to the crate attributes to enable
+
+error: aborting due to 3 previous errors
+
+For more information about this error, try `rustc --explain E0658`.