rustdoc: use remap-path-prefix

[nmattia]

The rust compiler has an option --remap-path-prefix for remapping path prefixes. It's useful to avoid runtime messages that are independent of where the source code was located during the build.

As far as I can tell that doesn't apply to rustdoc's <meta> info "Source to the Rust file ...", which (to me) is unexpected.

Meta

$ rustc --version --verbose
rustc 1.40.0
binary: rustc
commit-hash: unknown
commit-date: unknown
host: x86_64-apple-darwin
release: 1.40.0
LLVM version: 8.0

[jyn514]

This should be fixed since https://github.com/rust-lang/rust/pull/83813/files#diff-04002bbcf90fc0a3356db4344dee8f1e03d868fa6c0f666713affa7fa2679b6f, I don't think it's tested through.

[weihanglo]

To add one more item to the list, --remap-path-prefix on rustc also applies to diagnostic messages. However, it is not the case on rustdoc. See the zulip topic.

This is found when trying to resolve issues in #111540. I expect rustdoc to match the behavior of rustc.

[weihanglo]

From Zulip t-rustdoc:

If rustc supports it (and IIUC it seems like it does), then I think we should match rustc in that.
Rustdoc diagnostics should normally behave the same as rustc.

So we kinda get a green light for working on this.

[GuillaumeGomez]

Taking a look.