Negative LitFloat seems to parse incorrectly in procedural macro

[sagacity]

I have come across a weird issue in my procedural macro:

I parse a float, like so:

let float = syn::Lit::Float(syn::LitFloat::new("-10.0", Span::call_site()));

I would expect float to be: Float(LitFloat { token: - 10.0 }). However, it is Float(LitFloat { token: - 10 }) instead. This causes the code generated by my macro to miscompile, since -10 is not interpreted as a float.

Weirdly, this only happens when the parsing is done inside a procedural macro. If I do the parsing in a regular binary it works fine. I have no idea what is different about executing in a procedural macro that could cause this, and it also makes it quite hard to debug.

I have a reproduction case here. Run it with cargo run --example example and it will panic with the parsed float showing as: Float(LitFloat { token: - 10 }). I would have expected Float(LitFloat { token: - 10.0 }) instead.

I've tried this both on Rust 1.55.0 and the currently nightly, on MacOS.

[dtolnay]

I believe this is a bug in the compiler's proc_macro implementation, which is why you only saw it affecting procedural macros and not regular binaries. I've put up a fix in rust-lang/rust#90297.

[sagacity]

Great, thanks so much!

[dtolnay]

The compiler fix is still needed but I implemented a workaround in #1087 + #1088 as long as the compiler being used is 1.56+.

  = help: message: float is parsed as: Float(LitFloat { token: -10.0 })