cargo nightly build with RUSTFLAGS having --remap-path-prefix broke since last nightly on Windows

[pocesar]

Problem

When using remap-path-prefix before, I was able to set the paths to a common root like : "build/src/", now the generated .d files had wrong paths when trying to set the RUSTFLAGS with remap-path-prefix and cargo fails with:

   Compiling proc-macro2 v0.4.30
   Compiling unicode-xid v0.1.0
   Compiling serde v1.0.98
error: could not parse/generate dep info at: C:\ard\target\debug\deps\unicode_xid-d89e93a95ce7a9a5.d

Caused by:
  The system cannot find the path specified. (os error 3)
warning: build failed, waiting for other jobs to finish...
error: could not parse/generate dep info at: C:\ard\target\debug\build\serde-cbe4cec761dd61b5\build_script_build-cbe4cec761dd61b5.d

Caused by:
  The system cannot find the file specified. (os error 2)
warning: build failed, waiting for other jobs to finish...
error: could not parse/generate dep info at: C:\ard\target\debug\build\proc-macro2-c6a71a674b70c483\build_script_build-c6a71a674b70c483.d

Caused by:
  The system cannot find the file specified. (os error 2)
build failed, exit code 101 null

So at the moment, the builds are non reproducible (for me at least)

Steps

1. Setup a simple bin with any dependencies, put some code that uses it
2. set RUSTFLAGS=--remap-path-prefix=C:\ard\=\build\src\ on the console
3. try to cargo build

N.B: used to work fine before
PS: if you put "quotes" around the value as in "C:\ard\=\build\src\", the remap fails

Possible Solution(s)

It seems that some commits regarding the remap-path-prefix and dealing with Windows paths, might have broken it:

d99b7fe
1140c52
c8a9f88
65e3885
3595de3
4f6553a

Notes

Output of cargo version: cargo 1.38.0-nightly (26092da 2019-07-31)

Windows 10 x64
rustc 1.38.0-nightly (d3f8a0b5d 2019-08-04)

[ehuss]

Thanks for the report. I'm having a hard time reproducing the issue. Can you tell me which terminal you are using (cmd, powershell, mingw, etc.) and exactly which commands you ran? Also, does it happen with any project, or does it require certain dependencies?

[pocesar]

I'll try to make a minimal repro, meanwhile, if I set to --remap-path-prefix=C:\ard\=, it works as before (which has the same reproducible result but the paths are empty)

[alexcrichton]

@ehuss I think this may be caused by #7030 where we're expecting absolute paths coming out of the compiler but actually the dep-info files are likely lying since they're respecting remap-path-prefix which means they point to paths which don't exist...

cc @Mark-Simulacrum, you're likely interested in this as well

[Mark-Simulacrum]

That's quite likely, yes, I wasn't aware that dep-info files are remapped by --remap-path-prefix, in fact that feels a bit odd? Presumably independent of the mapped path in the binary and such you'd want Cargo to receive the real on-the-disk path...

[ehuss]

I see what's happening. It's due to this line I added to canonicalize the path. I'll post a PR to try to work around it.

As to whether or not the .d file should include remapped files or not is a rustc issue. I'll file an issue over there to track that.

[ehuss]

Posted #7219 with a fix.

Posted rust-lang/rust#63329 for the rustc side. Rebuild detection is broken in Cargo when using --remap-path-prefix, so if/when it is fixed, we should probably add a test for that.