rust: remap debuginfo

[dan.mcgregor_usask.ca]

Summary

Prior to this update, when the rust compiler generates warnings
or errors that refer to source code for the standard library, its
messaging use the build time path, rather than the installed path.

It's more useful for most users of rust to not have to
mentally rename files to find the files in question.

[dan.mcgregor_usask.ca]

A bit more explanation, this is the old output:

note: tuple variant defined here
   --> /wrkdirs/usr/ports/lang/rust/work/rustc-1.77.0-src/library/core/src/result.rs:506:5
help: you can convert an `i64` to a `u64` and panic if the converted value doesn't fit

And the new output for the same error:

note: tuple variant defined here
   --> /usr/local/lib/rustlib/src/rust/library/core/src/result.rs:506:5
    |
506 |     Ok(#[stable(feature = "rust1", since = "1.0.0")] T),
    |     ^^
help: you can convert an `i64` to a `u64` and panic if the converted value doesn't fit

Notice that the new error message even includes context from the library source file.

[dutra]

Very useful!!