Auto merge of #40043 - petrochenkov:objpars, r=nikomatsakis

Refactor parsing of trait object types

Bugs are fixed and code is cleaned up.

User visible changes:
- `ty` matcher in macros accepts trait object types like `Write + Send` (#39080)
- Buggy priority of `+` in trait object types starting with `for` is fixed (#39317). `&for<'a> Trait<'a> + Send` is now parsed as `(&for<'a> Trait<'a>) + Send` and requires parens `&(for<'a> Trait<'a> + Send)`. For comparison, `&Send + for<'a> Trait<'a>` was parsed like this since [Nov 27, 2014](#19298).
- Trailing `+`s are supported in trait objects, like in other bounds.
- Better error reporting for trait objects starting with `?Sized`.

Fixes #39080
Fixes #39317 [breaking-change]
Closes #39298
cc #39085 (fixed, then reverted #40043 (comment))
cc #39318 (fixed, then reverted #40043 (comment))

r? @nikomatsakis

src/libsyntax/ast.rs

@@ -17,7 +17,7 @@ pub use self::PathParameters::*;
 pub use symbol::Symbol as Name;
 pub use util::ThinVec;
 
-use syntax_pos::{mk_sp, Span, DUMMY_SP, ExpnId};
+use syntax_pos::{mk_sp, BytePos, Span, DUMMY_SP, ExpnId};
 use codemap::{respan, Spanned};
 use abi::Abi;
 use ext::hygiene::SyntaxContext;
@@ -1716,6 +1716,16 @@ pub struct PolyTraitRef {
     pub span: Span,
 }
 
+impl PolyTraitRef {
+    pub fn new(lifetimes: Vec<LifetimeDef>, path: Path, lo: BytePos, hi: BytePos) -> Self {
+        PolyTraitRef {
+            bound_lifetimes: lifetimes,
+            trait_ref: TraitRef { path: path, ref_id: DUMMY_NODE_ID },
+            span: mk_sp(lo, hi),
+        }
+    }
+}
+
 #[derive(Clone, PartialEq, Eq, RustcEncodable, RustcDecodable, Hash, Debug)]
 pub enum Visibility {
     Public,

src/libsyntax/ext/expand.rs

@@ -657,7 +657,7 @@ impl<'a> Parser<'a> {
             }
             ExpansionKind::Expr => Expansion::Expr(self.parse_expr()?),
             ExpansionKind::OptExpr => Expansion::OptExpr(Some(self.parse_expr()?)),
-            ExpansionKind::Ty => Expansion::Ty(self.parse_ty_no_plus()?),
+            ExpansionKind::Ty => Expansion::Ty(self.parse_ty()?),
             ExpansionKind::Pat => Expansion::Pat(self.parse_pat()?),
         })
     }

src/libsyntax/ext/quote.rs

@@ -414,7 +414,7 @@ pub fn parse_arm_panic(parser: &mut Parser) -> Arm {
 }
 
 pub fn parse_ty_panic(parser: &mut Parser) -> P<Ty> {
-    panictry!(parser.parse_ty_no_plus())
+    panictry!(parser.parse_ty())
 }
 
 pub fn parse_stmt_panic(parser: &mut Parser) -> Option<Stmt> {

src/libsyntax/ext/tt/macro_parser.rs

@@ -512,7 +512,7 @@ fn parse_nt<'a>(p: &mut Parser<'a>, sp: Span, name: &str) -> Nonterminal {
         },
         "pat" => token::NtPat(panictry!(p.parse_pat())),
         "expr" => token::NtExpr(panictry!(p.parse_expr())),
-        "ty" => token::NtTy(panictry!(p.parse_ty_no_plus())),
+        "ty" => token::NtTy(panictry!(p.parse_ty())),
         // this could be handled like a token, since it is one
         "ident" => match p.token {
             token::Ident(sn) => {