failed to mmap rmeta metadata

[ehuss]

Since #89634, rustc has been spewing this warning:

WARN rustc_metadata::locator no metadata found: failed to mmap rmeta metadata: '/home/eric/Proj/rust/build/x86_64-unknown-linux-gnu/stage0-sysroot/lib/rustlib/x86_64-unknown-linux-gnu/lib/librustc_ast-83f5770a611ead91.rmeta'

This happens particularly when using -L to load a sysroot crate, and using --emit metadata. This can happen with x.py check (using latest master as stage0 rustc), though I imagine it can happen in other scenarios. x.py check uses cargo's --all-targets which ends up including tests in the checks.

Reproduction:

touch shared.rs
rustc shared.rs --crate-type=dylib
rustc shared.rs --test --emit=metadata
echo "extern crate shared;" > a.rs
rustc a.rs --crate-type=rlib -L .

The cause of this particular error is that memmap2 0.2.1 returns an error it attempts to mmap a file of 0 length (though IIRC, the behavior is platform-specific). .rmeta files produced for binaries (such as with --test) have 0 length (#67293).

Possible Solutions

• Update memmap2 to 0.5.0, which does not return an error for 0 sized files. However, the rest of the crate loading code would need to be updated, as it then generates an error since the file is missing the metadata header.
• Skip rmeta files that are empty in crate loading.
• Downgrade that warn! to info! or debug!

Meta

rustc --version --verbose:

rustc 1.57.0-nightly (41dfaaa3c 2021-10-10)
binary: rustc
commit-hash: 41dfaaa3c66759395835b3af59b22f4f22175dc8
commit-date: 2021-10-10
host: x86_64-unknown-linux-gnu
release: 1.57.0-nightly
LLVM version: 13.0.0

[petrochenkov]

cc #88368

This is not an erroneous situation, the warn! is "overreacting".
I'd personally remove this logging, but @bjorn3 asked to keep it in #88368 (comment), so demoting it to info!/debug! is also an alternative.

[bjorn3]

I think ignoring 0 sized rmeta files specifically is fine. I would prefer to keep it a warning for bigger files. In any case please don't make it a debug!(). That requires recompiling rustc to be able to enable it.

[gilescope]

get_metadata_section's error is a String at the moment. That would need to be changed to an enum so that we can match on the right sort of error. If we uncompress something and its 0 long that presumably wants to stay as a warning...