[sylvestre]

rust: reuse dep-info and output lists across compiles of a crate

To compute the cache key of a crate, sccache runs rustc --emit=dep-info
to list its source files and env-deps, and rustc --print file-names to
list its outputs, even when the compile then hits the cache. The first one
parses and expands the whole crate, proc macros included, and was most of
the work of a Rust cache hit.

The server now remembers what both runs found, and reuses it for a later
compile with the same compiler, arguments, working directory, environment,
externs, static libraries and target spec, as long as every listed source
file still has the same digest and every env-dep the same value. The whole
environment is part of that key, except the jobserver variables, so that a
proc macro picking its inputs from an untracked variable can only cause a
miss. Entries live for the life of the server; nothing is written to the
cache.

All-hits debug build of sccache (332 Rust crates):
wall: 5.7 s -> 3.9 s
server-side CPU, including the rustc it runs: 26.3 s -> 4.1 s

[codecov-commenter]

Codecov Report

❌ Patch coverage is 98.11321% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 78.68%. Comparing base (4c9ad47) to head (4e4f932).

Files with missing lines	Patch %	Lines
src/compiler/rust.rs	98.11%	4 Missing ⚠️

Additional details and impacted files

@@                      Coverage Diff                      @@
##           preprocessor-entry-decode    #2942      +/-   ##
=============================================================
+ Coverage                      78.60%   78.68%   +0.07%     
=============================================================
  Files                             74       74              
  Lines                          44179    44364     +185     
=============================================================
+ Hits                           34728    34908     +180     
- Misses                          9451     9456       +5     

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