[451891358]

A -Fd value with a trailing separator makes cl.exe write a shared
vc<NNN>.pdb inside that directory, but sccache recorded the artifact as
<dir>.pdb, a file cl.exe never creates. Writing the cache entry then failed
to open it, and that failure is fatal:

$ sccache cl /c /Zi /Fdpdb\ a.cpp
sccache: encountered fatal error
sccache: error: failed to zip up compiler outputs
sccache: caused by: failed to open file `...\pdb.pdb`
# exit code -2, although cl.exe succeeded and wrote a.obj and pdb\vc140.pdb

Because CacheWrite::from_objects treats a missing artifact with
optional: false as an error, and that error propagates out of
get_cached_or_compile, any build that passes a -Fd directory (for example
cl /c /Zi /Fd"$(IntDir)\") could not complete at all.

Report such compilations as not cacheable instead, with the shared pdb
reason already used when -Zi/-ZI is given without -Fd: the PDB cl.exe
writes into that directory is shared by every translation unit compiled into
it, so it cannot be tracked as a per-compilation artifact. The object file is
still produced, the build succeeds, and --show-stats names the reason.

Note that this also stops the previous silent Cacheable::No path for
directory names that happen to contain a dot (CMake's
CMakeFiles/<target>.dir/), which used to be classified as "the PDB already
exists, so it might be shared". The existing heuristic is unchanged for real
-Fd <file>.pdb paths.

Testing

New unit test test_parse_arguments_pdb_directory covers:

• -Fdpdb\ and -Fdpdb/ with -Zi → CannotCache("shared pdb")
• -Fdpdb\ without debug information → cacheable, no pdb artifact
  (cl.exe ignores -Fd in that case)
• -Fdpdb\foo.pdb → cacheable, pdb artifact is pdb\foo.pdb

End to end on Windows with MSVC 19.51:

command	before	after
sccache cl /c /Zi /Fd"pdb\" /Foa.obj a.cpp	exit -2, cache error	exit 0, a.obj written, Non-cacheable reasons: shared pdb 1
sccache cl /c /Zi /Fd"pdb2\a.pdb" /Foa2.obj a.cpp (twice)	—	miss → hit, both a2.obj and pdb2\a.pdb restored

cargo test --lib: 583 passed, 0 failed (default features; 530 with
--no-default-features). cargo fmt --check is clean.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 90.00000% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 78.21%. Comparing base (2eeea91) to head (ed71929).

Files with missing lines	Patch %	Lines
src/compiler/msvc.rs	90.00%	2 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2933   +/-   ##
=======================================
  Coverage   78.20%   78.21%           
=======================================
  Files          74       74           
  Lines       43708    43728   +20     
=======================================
+ Hits        34184    34202   +18     
- Misses       9524     9526    +2     

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[sylvestre]

Thanks for your PR