macros: Remove 'r#' prefix from raw identifiers in field names

[dtolnay]

Motivation

Currently a call like info!(..., r#type = "...") ends up emitting a field named "r#type", which is never desirable. Previous work in #2925 made the desired behavior expressible as info!(..., type = "..."), and even before that PR one could write it as info!(..., "type" = "...").

However, when dealing with macro-generated use of tracing, we do still sometimes end up with raw identifiers. For example, if user-provided identifiers are used for both declaring (or instantiating, or accessing) a struct, and also for tracing. Something like this:

example!(name, r#type);

// expands to:

...
info!(..., name = GLOBAL.name, r#type = GLOBAL.r#type);
...

If we ask the user of example! to pass type (unraw), there is no way for the macro to access GLOBAL.r#type from that. But if we ask the user of example! to pass r#type (raw), there is no straightforward way for the macro to trace the field name as "type". The best option would have been something like:

info!(
    ...
    $(
        {
            (const {
                if let Some((b"r#", unraw)) = stringify!($field).as_bytes().split_first_chunk() {
                    unsafe { core::str::from_utf8_unchecked(unraw) }
                } else {
                    stringify!($field)
                }
            })
        } = GLOBAL.$field,
    )*
);

but this gets even more ridiculous when dealing with field names that are not just a single identifier. (prefix.r#keyword.suffix).

Solution

r#keyword = $value is made equivalent to keyword = $value. The field will be emitted with the name "keyword", not "r#keyword" as before.