rustc: Ignore fs::canonicalize errors in metadata

[alexcrichton]

This commit updates the metadata location logic to ignore errors when
calling fs::canonicalize. Canonicalization was added historically so
multiple -L paths to the same directory don't print errors about
multiple candidates (since rustc can deduplicate same-named paths), but
canonicalization doesn't work on all filesystems. Cargo, for example,
always uses this sort of fallback where it will opportunitistically try
to canonicalize but fall back to using the input path if it otherwise
doesn't work.

If rustc is run on a filesystem that doesn't support canonicalization
then the effect of this change will be that -L paths which logically
point to the same directory will cause errors, but that's a rare enough
occurrence it shouldn't cause much issue in practice. Otherwise rustc
doesn't work at all today on those sorts of filesystem where
canonicalization isn't supported!

[rust-highfive]

r? @estebank

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[Mark-Simulacrum]

It looks like

rust/src/librustc_metadata/creader.rs

Line 239 in 584e83d

let l = fs::canonicalize(l).ok();

and

rust/src/librustc_session/filesearch.rs

Line 127 in 584e83d

Err(e) => panic!("failed to get realpath: {}", e),

might also need to be fixed for rustc itself to work well on canonicalize-unsupporting systems.

I didn't look at tools or bootstrap but those probably have more cases like this as well.

r=me if you don't want to fix those in this PR though!

[alexcrichton]

Ah good catch! I've gone ahead and fixed those too.

@bors: r=Mark-Simulacrum

[bors]

📌 Commit cd8bdb5 has been approved by Mark-Simulacrum

[estebank]

r? @Mark-Simulacrum