Rollup merge of rust-lang#48481 - Manishearth:dyn-paren, r=petrochenkov

Allow parentheses in `dyn (Trait)`

r? @eddyb @nikomatsakis

src/libsyntax/parse/parser.rs

@@ -405,11 +405,14 @@ impl TokenType {
     }
 }
 
-// Returns true if `IDENT t` can start a type - `IDENT::a::b`, `IDENT<u8, u8>`,
-// `IDENT<<u8 as Trait>::AssocTy>`, `IDENT(u8, u8) -> u8`.
-fn can_continue_type_after_ident(t: &token::Token) -> bool {
+/// Returns true if `IDENT t` can start a type - `IDENT::a::b`, `IDENT<u8, u8>`,
+/// `IDENT<<u8 as Trait>::AssocTy>`.
+///
+/// Types can also be of the form `IDENT(u8, u8) -> u8`, however this assumes
+/// that IDENT is not the ident of a fn trait
+fn can_continue_type_after_non_fn_ident(t: &token::Token) -> bool {
     t == &token::ModSep || t == &token::Lt ||
-    t == &token::BinOp(token::Shl) || t == &token::OpenDelim(token::Paren)
+    t == &token::BinOp(token::Shl)
 }
 
 /// Information about the path to a module.
@@ -1619,7 +1622,8 @@ impl<'a> Parser<'a> {
             impl_dyn_multi = bounds.len() > 1 || self.prev_token_kind == PrevTokenKind::Plus;
             TyKind::ImplTrait(bounds)
         } else if self.check_keyword(keywords::Dyn) &&
-                  self.look_ahead(1, |t| t.can_begin_bound() && !can_continue_type_after_ident(t)) {
+                  self.look_ahead(1, |t| t.can_begin_bound() &&
+                                         !can_continue_type_after_non_fn_ident(t)) {
             self.bump(); // `dyn`
             // Always parse bounds greedily for better error recovery.
             let bounds = self.parse_ty_param_bounds()?;

src/test/compile-fail/dyn-trait-compatibility.rs

@@ -20,10 +20,5 @@ type A3 = dyn<<dyn as dyn>::dyn>;
 //~^ ERROR cannot find type `dyn` in this scope
 //~| ERROR cannot find type `dyn` in this scope
 //~| ERROR Use of undeclared type or module `dyn`
-type A4 = dyn(dyn, dyn) -> dyn;
-//~^ ERROR cannot find type `dyn` in this scope
-//~| ERROR cannot find type `dyn` in this scope
-//~| ERROR cannot find type `dyn` in this scope
-//~| ERROR cannot find type `dyn` in this scope
 
 fn main() {}

src/test/run-pass/dyn-trait.rs

@@ -17,6 +17,8 @@ static BYTE: u8 = 33;
 fn main() {
     let x: &(dyn 'static + Display) = &BYTE;
     let y: Box<dyn Display + 'static> = Box::new(BYTE);
+    let _: &dyn (Display) = &BYTE;
+    let _: &dyn (::std::fmt::Display) = &BYTE;
     let xstr = format!("{}", x);
     let ystr = format!("{}", y);
     assert_eq!(xstr, "33");