std: Tweak String implementations

This commit performs a pass over the implementations of the new `String` trait
in the formatting module. Some implementations were removed as a conservative
move pending an upcoming convention about `String` implementations, and some
were added in order to retain consistency across the libraries. Specifically:

* All "smart pointers" implement `String` now, adding missing implementations
  for `Arc` and `Rc`.
* The `Vec<T>` and `[T]` types no longer implement `String`.
* The `*const T` and `*mut T` type no longer implement `String`.
* The `()` type no longer implements `String`.
* The `Path` type's `Show` implementation does not surround itself with `Path
  {}` (a minor tweak).

All implementations of `String` in this PR were also marked `#[stable]` to
indicate that the types will continue to implement the `String` trait regardless
of what it looks like.

src/librustdoc/html/format.rs

@@ -51,6 +51,8 @@ pub struct ConciseStability<'a>(pub &'a Option<clean::Stability>);
 pub struct WhereClause<'a>(pub &'a clean::Generics);
 /// 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
+pub struct CommaSep<'a, T: 'a>(pub &'a [T]);
 
 impl VisSpace {
     pub fn get(&self) -> Option<ast::Visibility> {
@@ -64,6 +66,16 @@ impl UnsafetySpace {
     }
 }
 
+impl<'a, T: fmt::String> fmt::String for CommaSep<'a, T> {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        for (i, item) in self.0.iter().enumerate() {
+            if i != 0 { try!(write!(f, ", ")); }
+            try!(write!(f, "{}", item));
+        }
+        Ok(())
+    }
+}
+
 //NOTE(stage0): remove impl after snapshot
 #[cfg(stage0)]
 impl<'a> fmt::Show for TyParamBounds<'a> {
@@ -530,7 +542,8 @@ impl fmt::String for clean::Type {
                        lifetimes = if decl.lifetimes.len() == 0 {
                            "".to_string()
                        } else {
-                           format!("for &lt;{:#}&gt;", decl.lifetimes)
+                           format!("for &lt;{}&gt;",
+                                   CommaSep(decl.lifetimes.as_slice()))
                        },
                        args = decl.decl.inputs,
                        arrow = decl.decl.output,
@@ -562,7 +575,8 @@ impl fmt::String for clean::Type {
                        lifetimes = if decl.lifetimes.len() == 0 {
                            "".to_string()
                        } else {
-                           format!("for &lt;{:#}&gt;", decl.lifetimes)
+                           format!("for &lt;{}&gt;",
+                                   CommaSep(decl.lifetimes.as_slice()))
                        },
                        args = decl.decl.inputs,
                        bounds = if decl.bounds.len() == 0 {
@@ -592,7 +606,8 @@ impl fmt::String for clean::Type {
                 primitive_link(f, clean::PrimitiveTuple,
                                match typs.as_slice() {
                                     [ref one] => format!("({},)", one),
-                                    many => format!("({:#})", many)
+                                    many => format!("({})",
+                                                    CommaSep(many.as_slice()))
                                }.as_slice())
             }
             clean::Vector(ref t) => {