The trace_macros! stringifier isn't inserting parens when required: due to the use of 'expr' the macro is treating 3 + 7 as a single node X, and then the multip

The trace_macros! stringifier isn't inserting parens when required: due to the use of 'expr' the macro is treating 3 + 7 as a single node X, and then the multiplication with 4 is, in the AST, X * 4 (i.e. correct). The thing that is being printed there is the recursive rpn invocation in the @op branch, and specifically the '$a $op $b', which is a "disconnected" sequence of AST items/token trees. Once it's forced to be parsed as an expression (that is, when matched against the recursive '$... : expr' rules), the printer handles it correctly. For instance, the last 'to' line of the output in the example you quote, or, if that example is changed to 'rpn!(2 3 7 + 4 * 5)', the trace_macros output includes: ... = note: to `rpn ! ( [ 5 , (3 + 7) * 4 , 2 ] )` = note: expanding `rpn! { [ 5 , (3 + 7) * 4 , 2 ] }` ...