use error_block_no_opening_brace more

src/librustc_parse/parser/stmt.rs

@@ -304,7 +304,11 @@ pub(super) fn parse_inner_attrs_and_block(
         maybe_whole!(self, NtBlock, |x| (Vec::new(), x));
 
         let lo = self.token.span;
-        self.expect(&token::OpenDelim(token::Brace))?;
+
+        if !self.eat(&token::OpenDelim(token::Brace)) {
+            return self.error_block_no_opening_brace();
+        }
+
         Ok((self.parse_inner_attributes()?, self.parse_block_tail(lo, BlockCheckMode::Default)?))
     }
 

src/test/ui/parser/block-no-opening-brace.rs

@@ -0,0 +1,30 @@
+// edition:2018
+
+#![feature(try_blocks)]
+
+fn main() {}
+
+fn f1() {
+   loop
+     let x = 0; //~ ERROR expected `{`, found keyword `let`
+   }
+
+fn f2() {
+   while true
+     let x = 0; //~ ERROR expected `{`, found keyword `let`
+   }
+
+fn f3() {
+   for x in 0..1
+     let x = 0; //~ ERROR expected `{`, found keyword `let`
+   }
+
+fn f4() {
+   try //~ ERROR expected expression, found reserved keyword `try`
+     let x = 0;
+   }
+
+fn f5() {
+   async //~ ERROR async closures are unstable
+     let x = 0; //~ ERROR expected one of `move`, `|`, or `||`, found keyword `let`
+   }

src/test/ui/parser/block-no-opening-brace.stderr

@@ -0,0 +1,53 @@
+error: expected `{`, found keyword `let`
+  --> $DIR/block-no-opening-brace.rs:9:6
+   |
+LL |      let x = 0;
+   |      ^^^-------
+   |      |
+   |      expected `{`
+   |      help: try placing this code inside a block: `{ let x = 0; }`
+
+error: expected `{`, found keyword `let`
+  --> $DIR/block-no-opening-brace.rs:14:6
+   |
+LL |      let x = 0;
+   |      ^^^-------
+   |      |
+   |      expected `{`
+   |      help: try placing this code inside a block: `{ let x = 0; }`
+
+error: expected `{`, found keyword `let`
+  --> $DIR/block-no-opening-brace.rs:19:6
+   |
+LL |      let x = 0;
+   |      ^^^-------
+   |      |
+   |      expected `{`
+   |      help: try placing this code inside a block: `{ let x = 0; }`
+
+error: expected expression, found reserved keyword `try`
+  --> $DIR/block-no-opening-brace.rs:23:4
+   |
+LL |    try
+   |    ^^^ expected expression
+
+error: expected one of `move`, `|`, or `||`, found keyword `let`
+  --> $DIR/block-no-opening-brace.rs:29:6
+   |
+LL |    async
+   |         - expected one of `move`, `|`, or `||`
+LL |      let x = 0;
+   |      ^^^ unexpected token
+
+error[E0658]: async closures are unstable
+  --> $DIR/block-no-opening-brace.rs:28:4
+   |
+LL |    async
+   |    ^^^^^
+   |
+   = note: see issue #62290 <https://github.com/rust-lang/rust/issues/62290> for more information
+   = help: add `#![feature(async_closure)]` to the crate attributes to enable
+
+error: aborting due to 6 previous errors
+
+For more information about this error, try `rustc --explain E0658`.