Auto merge of #57428 - alexreg:associated_type_bounds, r=nikomatsakis,Centril

Implementation of RFC 2289 (associated_type_bounds)

This PR implements the [`asociated_type_bounds` feature](https://github.com/rust-lang/rfcs/blob/master/text/2289-associated-type-bounds.md).

Associated type bounds are implemented in:
   - function/method arguments and return types
   - structs, enums, unions
   - associated items in traits
   - type aliases
   - type parameter defaults
   - trait objects
   - let bindings

CC @nikomatsakis @centril

1 files changed, 15 insertions, 8 deletions

diff --git a/src/libsyntax/print/pprust.rs b/src/libsyntax/print/pprust.rs
index b81dc53ef68..57c01e9e3ef 100644
--- a/src/libsyntax/print/pprust.rs
+++ b/src/libsyntax/print/pprust.rs
@@ -1715,7 +1715,7 @@ impl<'a> State<'a> {
         match els {
             Some(_else) => {
                 match _else.node {
-                    // "another else-if"
+                    // Another `else if` block.
                     ast::ExprKind::If(ref i, ref then, ref e) => {
                         self.cbox(INDENT_UNIT - 1)?;
                         self.ibox(0)?;
@@ -1725,7 +1725,7 @@ impl<'a> State<'a> {
                         self.print_block(then)?;
                         self.print_else(e.as_ref().map(|e| &**e))
                     }
-                    // "another else-if-let"
+                    // Another `else if let` block.
                     ast::ExprKind::IfLet(ref pats, ref expr, ref then, ref e) => {
                         self.cbox(INDENT_UNIT - 1)?;
                         self.ibox(0)?;
@@ -1738,14 +1738,14 @@ impl<'a> State<'a> {
                         self.print_block(then)?;
                         self.print_else(e.as_ref().map(|e| &**e))
                     }
-                    // "final else"
+                    // Final `else` block.
                     ast::ExprKind::Block(ref b, _) => {
                         self.cbox(INDENT_UNIT - 1)?;
                         self.ibox(0)?;
                         self.s.word(" else ")?;
                         self.print_block(b)
                     }
-                    // BLEAH, constraints would be great here
+                    // Constraints would be great here!
                     _ => {
                         panic!("print_if saw if with weird alternative");
                     }
@@ -2450,14 +2450,21 @@ impl<'a> State<'a> {
 
                 let mut comma = data.args.len() != 0;
 
-                for binding in data.bindings.iter() {
+                for constraint in data.constraints.iter() {
                     if comma {
                         self.word_space(",")?
                     }
-                    self.print_ident(binding.ident)?;
+                    self.print_ident(constraint.ident)?;
                     self.s.space()?;
-                    self.word_space("=")?;
-                    self.print_type(&binding.ty)?;
+                    match constraint.kind {
+                        ast::AssocTyConstraintKind::Equality { ref ty } => {
+                            self.word_space("=")?;
+                            self.print_type(ty)?;
+                        }
+                        ast::AssocTyConstraintKind::Bound { ref bounds } => {
+                            self.print_type_bounds(":", &*bounds)?;
+                        }
+                    }
                     comma = true;
                 }