--timings interacts poorly with cargo clean

[g2p]

Problem

The stabilization PR (#10245) for --timings / -Ztimings moved html reports under target/cargo-timings, which is wiped on cargo clean.

Getting reports for a clean build generally requires cargo clean, but this choice of directory makes it hard to keep reports over time, which one would need to measure progress.

Steps

Attempting to track build performance over time

1. cargo clean; cargo build --timings
2. Change something, try to impact build performance
3. cargo clean; cargo build --timings
4. Previous reports were wiped and can't be compared

Possible Solution(s)

Consider:
Using a directory outside target. reports/cargo-timings/ would work.

Making cargo clean delete only known subsets of the target directory.

A new flag, cargo clean --debug, similar to cargo clean --release, might provide a way to benchmark clean debug builds with less risk (as long as the flags are used every time).

Version

cargo +nightly version
cargo 1.61.0-nightly (ea2a21c 2022-02-15)

[ehuss]

A new flag, cargo clean --debug, similar to cargo clean --release, might provide a way to benchmark clean debug builds with less risk (as long as the flags are used every time).

cargo clean --profile=dev will only delete the dev profile directory.