rustdoc: Simplify predicates with paren notation

This change is aimed at improving cross-crate (inlined) notation of generic
closures. The change modifies `simplify::where_predicates` to handle
parenthesized notation as well as starting to handle supertrait bounds as well.
This was necessary because all output constraints of closures are bound to
`FnOnce` but most trait bounds are that of `FnMut`.

Close #21801

src/librustdoc/clean/mod.rs

@@ -958,7 +958,7 @@ impl<'a, 'tcx> Clean<Generics> for (&'a ty::Generics<'tcx>,
         Generics {
             type_params: simplify::ty_params(stripped_typarams),
             lifetimes: stripped_lifetimes,
-            where_predicates: simplify::where_clauses(where_predicates),
+            where_predicates: simplify::where_clauses(cx, where_predicates),
         }
     }
 }

src/librustdoc/clean/simplify.rs

@@ -29,11 +29,16 @@
 use std::mem;
 use std::collections::HashMap;
 
-use clean;
-use clean::WherePredicate as WP;
-use clean::PathParameters as PP;
+use rustc::middle::subst;
+use rustc::middle::ty;
+use syntax::ast;
 
-pub fn where_clauses(clauses: Vec<WP>) -> Vec<WP> {
+use clean::PathParameters as PP;
+use clean::WherePredicate as WP;
+use clean::{self, Clean};
+use core::DocContext;
+
+pub fn where_clauses(cx: &DocContext, clauses: Vec<WP>) -> Vec<WP> {
     // First, partition the where clause into its separate components
     let mut params = HashMap::new();
     let mut lifetimes = Vec::new();
@@ -90,16 +95,25 @@ pub fn where_clauses(clauses: Vec<WP>) -> Vec<WP> {
                 clean::ResolvedPath { did, ref mut path, ..} => (did, path),
                 _ => return false,
             };
-            if did != trait_did { return false }
+            // If this QPath's trait `trait_did` is the same as, or a supertrait
+            // of, the bound's trait `did` then we can keep going, otherwise
+            // this is just a plain old equality bound.
+            if !trait_is_same_or_supertrait(cx, did, trait_did) {
+                return false
+            }
             let last = path.segments.last_mut().unwrap();
-            let bindings = match last.params {
-                PP::AngleBracketed { ref mut bindings, .. } => bindings,
-                PP::Parenthesized { .. } => return false,
+            match last.params {
+                PP::AngleBracketed { ref mut bindings, .. } => {
+                    bindings.push(clean::TypeBinding {
+                        name: name.clone(),
+                        ty: rhs.clone(),
+                    });
+                }
+                PP::Parenthesized { ref mut output, .. } => {
+                    assert!(output.is_none());
+                    *output = Some(rhs.clone());
+                }
             };
-            bindings.push(clean::TypeBinding {
-                name: name.clone(),
-                ty: rhs.clone(),
-            });
             true
         })
     });
@@ -134,3 +148,33 @@ pub fn ty_params(mut params: Vec<clean::TyParam>) -> Vec<clean::TyParam> {
 fn ty_bounds(bounds: Vec<clean::TyParamBound>) -> Vec<clean::TyParamBound> {
     bounds
 }
+
+fn trait_is_same_or_supertrait(cx: &DocContext, child: ast::DefId,
+                               trait_: ast::DefId) -> bool {
+    if child == trait_ {
+        return true
+    }
+    let def = ty::lookup_trait_def(cx.tcx(), child);
+    let predicates = ty::lookup_predicates(cx.tcx(), child);
+    let generics = (&def.generics, &predicates, subst::TypeSpace).clean(cx);
+    generics.where_predicates.iter().filter_map(|pred| {
+        match *pred {
+            clean::WherePredicate::BoundPredicate {
+                ty: clean::Generic(ref s),
+                ref bounds
+            } if *s == "Self" => Some(bounds),
+            _ => None,
+        }
+    }).flat_map(|bounds| bounds.iter()).any(|bound| {
+        let poly_trait = match *bound {
+            clean::TraitBound(ref t, _) => t,
+            _ => return false,
+        };
+        match poly_trait.trait_ {
+            clean::ResolvedPath { did, .. } => {
+                trait_is_same_or_supertrait(cx, did, trait_)
+            }
+            _ => false,
+        }
+    })
+}

src/test/auxiliary/issue-21801.rs

@@ -0,0 +1,17 @@
+// Copyright 2015 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 struct Foo;
+
+impl Foo {
+    pub fn new<F>(f: F) -> Foo where F: FnMut() -> i32 {
+        loop {}
+    }
+}

src/test/rustdoc/issue-21801.rs

@@ -0,0 +1,18 @@
+// Copyright 2015 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:issue-21801.rs
+
+extern crate issue_21801;
+
+// @has issue_21801/struct.Foo.html
+// @has - '//*[@id="method.new"]' \
+//        'fn new<F>(f: F) -> Foo where F: FnMut() -> i32'
+pub use issue_21801::Foo;