Rollup merge of rust-lang#37190 - QuietMisdreavus:rustdoc-where-newline, r=GuillaumeGomez

rustdoc: add line breaks to where clauses a la rustfmt

Much like my last PR for rustdoc (rust-lang#36679), this adds line breaks to certain statements based on their line length. Here the focus was on where clauses.

Some examples:
- [Where clause in a trait function](https://shiva.icesoldier.me/custom-std/std/iter/trait.Iterator.html?search=#method.unzip) (also in the trait header block at the top of the page)
- [Where clause on a bare function](https://shiva.icesoldier.me/doc-custom2/petgraph/visit/fn.depth_first_search.html)
- [Where clauses in trait impls on a struct](https://shiva.icesoldier.me/custom-std/std/collections/struct.HashMap.html) (scroll to the bottom) These are regularly not on their own line, but will be given their own line now if their "prefix text" doesn't give them enough room to sensibly print their constraints. HashMap's trait impls provide some examples of both behaviors.

The libstd links above are the whole docs rendered with this, and the "bare function" link above is in another set that pulls some notable crates together. `petgraph` was the one that brought this request up, and that collection also includes [itertools](https://shiva.icesoldier.me/doc-custom2/itertools/trait.Itertools.html) which provided an easy sample to test with.

r? @GuillaumeGomez

Author: eddyb

src/librustdoc/html/format.rs

@@ -42,15 +42,15 @@ pub struct UnsafetySpace(pub hir::Unsafety);
 #[derive(Copy, Clone)]
 pub struct ConstnessSpace(pub hir::Constness);
 /// Wrapper struct for properly emitting a method declaration.
-pub struct Method<'a>(pub &'a clean::FnDecl, pub &'a str);
+pub struct Method<'a>(pub &'a clean::FnDecl, pub usize);
 /// Similar to VisSpace, but used for mutability
 #[derive(Copy, Clone)]
 pub struct MutableSpace(pub clean::Mutability);
 /// Similar to VisSpace, but used for mutability
 #[derive(Copy, Clone)]
 pub struct RawMutableSpace(pub clean::Mutability);
 /// Wrapper struct for emitting a where clause from Generics.
-pub struct WhereClause<'a>(pub &'a clean::Generics);
+pub struct WhereClause<'a>(pub &'a clean::Generics, pub usize);
 /// Wrapper struct for emitting type parameter bounds.
 pub struct TyParamBounds<'a>(pub &'a [clean::TyParamBound]);
 /// Wrapper struct for emitting a comma-separated list of items
@@ -157,52 +157,71 @@ impl fmt::Display for clean::Generics {
 
 impl<'a> fmt::Display for WhereClause<'a> {
     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
-        let &WhereClause(gens) = self;
+        let &WhereClause(gens, pad) = self;
         if gens.where_predicates.is_empty() {
             return Ok(());
         }
+        let mut clause = String::new();
         if f.alternate() {
-            f.write_str(" ")?;
+            clause.push_str(" where ");
         } else {
-            f.write_str(" <span class='where'>where ")?;
+            clause.push_str(" <span class='where'>where ");
         }
         for (i, pred) in gens.where_predicates.iter().enumerate() {
             if i > 0 {
-                f.write_str(", ")?;
+                if f.alternate() {
+                    clause.push_str(", ");
+                } else {
+                    clause.push_str(",<br>");
+                }
             }
             match pred {
                 &clean::WherePredicate::BoundPredicate { ref ty, ref bounds } => {
                     let bounds = bounds;
                     if f.alternate() {
-                        write!(f, "{:#}: {:#}", ty, TyParamBounds(bounds))?;
+                        clause.push_str(&format!("{:#}: {:#}", ty, TyParamBounds(bounds)));
                     } else {
-                        write!(f, "{}: {}", ty, TyParamBounds(bounds))?;
+                        clause.push_str(&format!("{}: {}", ty, TyParamBounds(bounds)));
                     }
                 }
                 &clean::WherePredicate::RegionPredicate { ref lifetime,
                                                           ref bounds } => {
-                    write!(f, "{}: ", lifetime)?;
+                    clause.push_str(&format!("{}: ", lifetime));
                     for (i, lifetime) in bounds.iter().enumerate() {
                         if i > 0 {
-                            f.write_str(" + ")?;
+                            clause.push_str(" + ");
                         }
 
-                        write!(f, "{}", lifetime)?;
+                        clause.push_str(&format!("{}", lifetime));
                     }
                 }
                 &clean::WherePredicate::EqPredicate { ref lhs, ref rhs } => {
                     if f.alternate() {
-                        write!(f, "{:#} == {:#}", lhs, rhs)?;
+                        clause.push_str(&format!("{:#} == {:#}", lhs, rhs));
                     } else {
-                        write!(f, "{} == {}", lhs, rhs)?;
+                        clause.push_str(&format!("{} == {}", lhs, rhs));
                     }
                 }
             }
         }
         if !f.alternate() {
-            f.write_str("</span>")?;
+            clause.push_str("</span>");
+            let plain = format!("{:#}", self);
+            if plain.len() > 80 {
+                //break it onto its own line regardless, but make sure method impls and trait
+                //blocks keep their fixed padding (2 and 9, respectively)
+                let padding = if pad > 10 {
+                    clause = clause.replace("class='where'", "class='where fmt-newline'");
+                    repeat("&nbsp;").take(8).collect::<String>()
+                } else {
+                    repeat("&nbsp;").take(pad + 6).collect::<String>()
+                };
+                clause = clause.replace("<br>", &format!("<br>{}", padding));
+            } else {
+                clause = clause.replace("<br>", " ");
+            }
         }
-        Ok(())
+        write!(f, "{}", clause)
     }
 }
 
@@ -718,30 +737,43 @@ impl fmt::Display for clean::Type {
 }
 
 fn fmt_impl(i: &clean::Impl, f: &mut fmt::Formatter, link_trait: bool) -> fmt::Result {
+    let mut plain = String::new();
+
     if f.alternate() {
         write!(f, "impl{:#} ", i.generics)?;
     } else {
         write!(f, "impl{} ", i.generics)?;
     }
+    plain.push_str(&format!("impl{:#} ", i.generics));
+
     if let Some(ref ty) = i.trait_ {
-        write!(f, "{}",
-               if i.polarity == Some(clean::ImplPolarity::Negative) { "!" } else { "" })?;
+        if i.polarity == Some(clean::ImplPolarity::Negative) {
+            write!(f, "!")?;
+            plain.push_str("!");
+        }
+
         if link_trait {
             fmt::Display::fmt(ty, f)?;
+            plain.push_str(&format!("{:#}", ty));
         } else {
             match *ty {
                 clean::ResolvedPath{ typarams: None, ref path, is_generic: false, .. } => {
                     let last = path.segments.last().unwrap();
                     fmt::Display::fmt(&last.name, f)?;
                     fmt::Display::fmt(&last.params, f)?;
+                    plain.push_str(&format!("{:#}{:#}", last.name, last.params));
                 }
                 _ => unreachable!(),
             }
         }
         write!(f, " for ")?;
+        plain.push_str(" for ");
     }
+
     fmt::Display::fmt(&i.for_, f)?;
-    fmt::Display::fmt(&WhereClause(&i.generics), f)?;
+    plain.push_str(&format!("{:#}", i.for_));
+
+    fmt::Display::fmt(&WhereClause(&i.generics, plain.len() + 1), f)?;
     Ok(())
 }
 
@@ -870,24 +902,30 @@ impl<'a> fmt::Display for Method<'a> {
 
         let mut output: String;
         let plain: String;
+        let pad = repeat(" ").take(indent).collect::<String>();
         if arrow.is_empty() {
             output = format!("({})", args);
-            plain = format!("{}({})", indent.replace("&nbsp;", " "), args_plain);
+            plain = format!("{}({})", pad, args_plain);
         } else {
             output = format!("({args})<br>{arrow}", args = args, arrow = arrow);
-            plain = format!("{indent}({args}){arrow}",
-                            indent = indent.replace("&nbsp;", " "),
+            plain = format!("{pad}({args}){arrow}",
+                            pad = pad,
                             args = args_plain,
                             arrow = arrow_plain);
         }
 
         if plain.len() > 80 {
-            let pad = format!("<br>{}", indent);
+            let pad = repeat("&nbsp;").take(indent).collect::<String>();
+            let pad = format!("<br>{}", pad);
             output = output.replace("<br>", &pad);
         } else {
             output = output.replace("<br>", "");
         }
-        write!(f, "{}", output)
+        if f.alternate() {
+            write!(f, "{}", output.replace("<br>", "\n"))
+        } else {
+            write!(f, "{}", output)
+        }
     }
 }