debugging - How do I see the expanded macro code that's causing my compile error?

Question

I have a compile error involving a macro:

<mdo macros>:6:19: 6:50 error: cannot move out of captured outer variable in an `FnMut` closure
<mdo macros>:6 bind ( $ e , move | $ p | mdo ! { $ ( $ t ) * } ) ) ; (
                                 ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
<mdo macros>:1:1: 14:36 note: in expansion of mdo!
<mdo macros>:6:27: 6:50 note: expansion site
<mdo macros>:1:1: 14:36 note: in expansion of mdo!
<mdo macros>:6:27: 6:50 note: expansion site
<mdo macros>:1:1: 14:36 note: in expansion of mdo!
src/parser.rs:30:42: 37:11 note: expansion site
error: aborting due to previous error

Unfortunately, the macro is recursive so it's hard to figure out what the compiler is complaining about, plus it seems like the line numbers are for the expanded macro rather than my code.

How can I see the expanded macro? Is there a flag I can pass to rustc (or even better, cargo) to dump this out?

(This macro is from rust-mdo, though I don't think it matters.)

Answer 1

cargo rustc -- -Zunstable-options --pretty=expanded, but a more concise alternative is the cargo-expand crate. It provides a Cargo subcommand cargo expand which prints the result of macro expansion. It also passes the expanded code through rustfmt which generally results in much more readable code than the default output from rustc.

Install by running cargo install cargo-expand.