These examples should compile according to the rust reference:

fn compiles() -> i32 { ({2}) + ({2}) } fn fails() -> i32 { {2} + {2} } fn compiles2() -> i32 { 2 + {2} } fn fails2() -> i32 { {2} + 2 }

(Playground)

Errors:

 Compiling playground v0.0.1 (file:///playground) error: expected expression, found `+` --> src/lib.rs:6:9 | 6 | {2} + {2} | ^ expected expression error: expected expression, found `+` --> src/lib.rs:14:9 | 14 | {2} + 2 | ^ expected expression error: aborting due to 2 previous errors error: Could not compile `playground`. To learn more, run the command again with --verbose. 

Rust reference snippet (thanks @Moongoodboy-K for the help here):

Expression : BlockExpression | OperatorExpression | ..
OperatorExpression : ArithmeticOrLogicalExpression | ..
ArithmeticOrLogicalExpression : Expression + Expression | ..
BlockExpression : { InnerAttribute* Statement* Expression? }

cc: @Moongoodboy-K