Some cleanup to maybe_parse_struct_expr

src/libsyntax/parse/parser.rs

@@ -2910,27 +2910,22 @@ impl<'a> Parser<'a> {
         path: &ast::Path,
         attrs: &ThinVec<Attribute>,
     ) -> Option<PResult<'a, P<Expr>>> {
-        // We don't want to assume it's a struct when encountering `{ <ident>: <ident> }` because
-        // it could be type ascription, like in `{ ident: u32 }`.
-        let isnt_ascription = self.look_ahead(1, |t| t.is_ident()) &&
-            self.look_ahead(2, |t| *t == token::Colon) && (
-                (self.look_ahead(3, |t| t.is_ident()) &&
-                 self.look_ahead(4, |t| *t == token::Comma)) ||
-                self.look_ahead(3, |t| t.is_lit()) ||
-                self.look_ahead(3, |t| *t == token::BinOp(token::Minus)) &&
-                self.look_ahead(4, |t| t.is_lit())
-            );
-        let could_be_struct = self.look_ahead(1, |t| t.is_ident()) && (
-            self.look_ahead(2, |t| *t == token::Colon) && isnt_ascription
-            || self.look_ahead(2, |t| *t == token::Comma)
-            // We could also check for `token::CloseDelim(token::Brace)`, but that would
-            // have false positives in the case of `if x == y { z } { a }`.
+        let struct_allowed = !self.restrictions.contains(Restrictions::NO_STRUCT_LITERAL);
+        let certainly_not_a_block = || self.look_ahead(1, |t| t.is_ident()) && (
+            // `{ ident, ` cannot start a block
+            self.look_ahead(2, |t| t == &token::Comma) ||
+            self.look_ahead(2, |t| t == &token::Colon) && (
+                // `{ ident: token, ` cannot start a block
+                self.look_ahead(4, |t| t == &token::Comma) ||
+                // `{ ident: ` cannot start a block unless it's a type ascription `ident: Type`
+                self.look_ahead(3, |t| !t.can_begin_type())
+            )
         );
-        let bad_struct = self.restrictions.contains(Restrictions::NO_STRUCT_LITERAL);
-        if !bad_struct || could_be_struct {
+
+        if struct_allowed || certainly_not_a_block() {
             // This is a struct literal, but we don't can't accept them here
             let expr = self.parse_struct_expr(lo, path.clone(), attrs.clone());
-            if let (Ok(expr), true) = (&expr, bad_struct) {
+            if let (Ok(expr), false) = (&expr, struct_allowed) {
                 let mut err = self.diagnostic().struct_span_err(
                     expr.span,
                     "struct literals are not allowed here",

src/test/ui/parser/struct-literal-in-match-discriminant.stderr

@@ -14,3 +14,4 @@ LL |     }) {
    |
 
 error: aborting due to previous error
+

src/test/ui/struct-literal-variant-in-if.rs

@@ -13,9 +13,7 @@ fn test_E(x: E) {
     if x == E::I { field1: true, field2: 42 } {}
     //~^ ERROR struct literals are not allowed here
     if x == E::V { field: false } {}
-    //~^ ERROR expected identifier, found keyword `false`
-    //~| ERROR expected type, found keyword `false`
-    //~| ERROR expected value, found struct variant `E::V`
+    //~^ ERROR struct literals are not allowed here
     if x == E::J { field: -42 } {}
     //~^ ERROR struct literals are not allowed here
     if x == E::K { field: "" } {}

src/test/ui/struct-literal-variant-in-if.stderr

@@ -8,32 +8,18 @@ help: surround the struct literal with parenthesis
 LL |     if x == (E::I { field1: true, field2: 42 }) {}
    |             ^                                 ^
 
-error: expected identifier, found keyword `false`
-  --> $DIR/struct-literal-variant-in-if.rs:15:27
-   |
-LL |     if x == E::V { field: false } {}
-   |                           ^^^^^ expected identifier, found keyword
-help: you can escape reserved keywords to use them as identifiers
-   |
-LL |     if x == E::V { field: r#false } {}
-   |                           ^^^^^^^
-
-error: expected type, found keyword `false`
-  --> $DIR/struct-literal-variant-in-if.rs:15:27
+error: struct literals are not allowed here
+  --> $DIR/struct-literal-variant-in-if.rs:15:13
    |
 LL |     if x == E::V { field: false } {}
-   |                           ^^^^^ expecting a type here because of type ascription
-   |
-   = note: type ascription is a nightly-only feature that lets you annotate an expression with a type: `<expr>: <type>`
-note: this expression expects an ascribed type after the colon
-  --> $DIR/struct-literal-variant-in-if.rs:15:20
+   |             ^^^^^^^^^^^^^^^^^^^^^
+help: surround the struct literal with parenthesis
    |
-LL |     if x == E::V { field: false } {}
-   |                    ^^^^^
-   = help: this might be indicative of a syntax error elsewhere
+LL |     if x == (E::V { field: false }) {}
+   |             ^                     ^
 
 error: struct literals are not allowed here
-  --> $DIR/struct-literal-variant-in-if.rs:19:13
+  --> $DIR/struct-literal-variant-in-if.rs:17:13
    |
 LL |     if x == E::J { field: -42 } {}
    |             ^^^^^^^^^^^^^^^^^^^
@@ -43,7 +29,7 @@ LL |     if x == (E::J { field: -42 }) {}
    |             ^                   ^
 
 error: struct literals are not allowed here
-  --> $DIR/struct-literal-variant-in-if.rs:21:13
+  --> $DIR/struct-literal-variant-in-if.rs:19:13
    |
 LL |     if x == E::K { field: "" } {}
    |             ^^^^^^^^^^^^^^^^^^
@@ -60,14 +46,6 @@ LL |     if x == E::V { field } {}
    |             |
    |             help: surround the struct literal with parenthesis: `(E::V { field })`
 
-error[E0423]: expected value, found struct variant `E::V`
-  --> $DIR/struct-literal-variant-in-if.rs:15:13
-   |
-LL |     if x == E::V { field: false } {}
-   |             ^^^^-----------------
-   |             |
-   |             help: surround the struct literal with parenthesis: `(E::V { field: false })`
-
 error[E0308]: mismatched types
   --> $DIR/struct-literal-variant-in-if.rs:10:20
    |
@@ -81,15 +59,15 @@ LL |     if x == E::V { field } {}
               found type `bool`
 
 error[E0308]: mismatched types
-  --> $DIR/struct-literal-variant-in-if.rs:23:20
+  --> $DIR/struct-literal-variant-in-if.rs:21:20
    |
 LL |     let y: usize = ();
    |                    ^^ expected usize, found ()
    |
    = note: expected type `usize`
               found type `()`
 
-error: aborting due to 9 previous errors
+error: aborting due to 7 previous errors
 
-Some errors occurred: E0308, E0423.
+Some errors have detailed explanations: E0308, E0423.
 For more information about an error, try `rustc --explain E0308`.