workspace inheritiance slowing down cargo startup times

[Muscraft]

Problem

@ehuss noticed that workspace inheritance caused a significant increase in startup times when using workspace inheritance.

When running the same test (as described here) I got

Benchmark 1: cd rust; cargo +nightly metadata
  Time (mean ± σ):     156.7 ms ±   2.3 ms    [User: 110.0 ms, System: 35.3 ms]
  Range (min … max):   152.5 ms … 161.0 ms    18 runs
 
Benchmark 2: cd rust2; cargo +nightly metadata
  Time (mean ± σ):     199.9 ms ±   1.2 ms    [User: 150.0 ms, System: 38.8 ms]
  Range (min … max):   196.8 ms … 202.5 ms    14 runs
 
Summary
  'cd rust; cargo +nightly metadata' ran
    1.28 ± 0.02 times faster than 'cd rust2; cargo +nightly metadata'

This is a large increase in time and should be addressed before stabilization

Steps

Original steps

@ehuss's testing notes

Modified steps using +nightly

1. Extract this file in benches/workspaces
   rust2.zip

   This is just a quick hack that adds inheritance to a bunch of packages, but I didn't really put much thought into setting it up.

2. Extract rust.tgz in that directory, too.

3. Prime the target directory with a cache of rustc info. In rust and rust2, run: cargo +nightly c -p linkchecker. Otherwise it would be measuring rustc overhead.

4. Test out some command that spends most of its time loading the workspace. There aren't very many good candidates, but cargo metadata or cargo tree can work. I use hyperfine for quick measurements. For example:

hyperfine "cd rust; cargo +nightly metadata" "cd rust2; cargo +nightly metadata"

Possible Solution(s)

One possible solution would be to record paths of workspace roots that can be checked before walking the filesystem to look for a workspace root. The biggest problem with this is where to record everything. Config is one place and there is some precedence for a cache being put in there.

Notes

No response

Version

cargo 1.63.0-nightly (4d92f07f3 2022-06-09)
release: 1.63.0-nightly
commit-hash: 4d92f07f34ba7fb7d7f207564942508f46c225d3
commit-date: 2022-06-09
host: aarch64-apple-darwin
libgit2: 1.4.2 (sys:0.14.2 vendored)
libcurl: 7.79.1 (sys:0.4.55+curl-7.83.1 system ssl:(SecureTransport) LibreSSL/3.3.6)
os: Mac OS 12.4.0 [64-bit]