rustdoc: Hide -> () in cross crate inlined Fn* bounds

src/librustdoc/clean/simplify.rs

@@ -106,7 +106,9 @@ pub fn where_clauses(cx: &DocContext, clauses: Vec<WP>) -> Vec<WP> {
                 }
                 PP::Parenthesized { ref mut output, .. } => {
                     assert!(output.is_none());
-                    *output = Some(rhs.clone());
+                    if *rhs != clean::Type::Tuple(Vec::new()) {
+                        *output = Some(rhs.clone());
+                    }
                 }
             };
             true

src/test/rustdoc/auxiliary/unit-return.rs

@@ -0,0 +1,13 @@
+// Copyright 2018 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+pub fn f2<F: FnMut(u32) + Clone>(f: F) {}
+
+pub fn f3<F: FnMut(u64) -> () + Clone>(f: F) {}

src/test/rustdoc/unit-return.rs

@@ -0,0 +1,27 @@
+// Copyright 2018 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+// aux-build:unit-return.rs
+
+#![crate_name = "foo"]
+
+extern crate unit_return;
+
+// @has 'foo/fn.f0.html' '//*[@class="rust fn"]' 'F: FnMut(u8) + Clone'
+pub fn f0<F: FnMut(u8) + Clone>(f: F) {}
+
+// @has 'foo/fn.f1.html' '//*[@class="rust fn"]' 'F: FnMut(u16) + Clone'
+pub fn f1<F: FnMut(u16) -> () + Clone>(f: F) {}
+
+// @has 'foo/fn.f2.html' '//*[@class="rust fn"]' 'F: FnMut(u32) + Clone'
+pub use unit_return::f2;
+
+// @has 'foo/fn.f3.html' '//*[@class="rust fn"]' 'F: FnMut(u64) + Clone'
+pub use unit_return::f3;