Repository: rust
Issue: `rustc --version` is slow even without the rustup wrapper
Labels: C-enhancement,T-compiler

## Problem Description

Running `rustc --version` without the rustup wrapper takes 11ms on my linux machine (See https://github.com/rust-lang/rustup/issues/2626 for the rustup side of this).

This is an issue for [uv](https://github.com/astral-sh/uv), as we've [been asked](https://github.com/astral-sh/uv/issues/1958) to include the output of `rustc --version` in our user agent when making requests to the python package index so the python ecosystem gets usage stats. A minimal resolution with a network request (revalidation request) takes ~100ms on machine, so 20ms extra before the first network request is noticeable. I'd also be happy to read the default rustc version from another location, given that this works with alternative ways of installation.

## Benchmarks

The benchmark runs from my user home on ubuntu, and i've include rustc with and without rustup, python without shim and node with volta shim and without for comparison. Tested with rustc 1.76.0 (07dca489a 2024-02-04).

```
$ hyperfine --warmup 10 --shell=none "rustc --version" ".rustup/toolchains/nightly-x86_64-unknown-linux-gnu/bin/rustc --version" "python --version" "node --version" ".volta/tools/image/node/18.18.2/bin/node --version"
Benchmark 1: rustc --version
  Time (mean ± σ):      19.9 ms ±   1.5 ms    [User: 14.8 ms, System: 5.0 ms]
  Range (min … max):    17.5 ms …  26.3 ms    157 runs
 
Benchmark 2: .rustup/toolchains/nightly-x86_64-unknown-linux-gnu/bin/rustc --version
  Time (mean ± σ):      10.6 ms ±   3.0 ms    [User: 4.8 ms, System: 5.6 ms]
  Range (min … max):     4.4 ms …  17.6 ms    240 runs
 
Benchmark 3: python --version
  Time (mean ± σ):       1.4 ms ±   0.5 ms    [User: 0.9 ms, System: 0.4 ms]
  Range (min … max):     0.3 ms …   2.3 ms    1635 runs
 
Benchmark 4: node --version
  Time (mean ± σ):       9.7 ms ±   3.1 ms    [User: 3.9 ms, System: 5.8 ms]
  Range (min … max):     2.8 ms …  14.3 ms    229 runs
 
Benchmark 5: .volta/tools/image/node/18.18.2/bin/node --version
  Time (mean ± σ):       7.2 ms ±   2.2 ms    [User: 2.4 ms, System: 4.6 ms]
  Range (min … max):     1.7 ms …  12.0 ms    796 runs
 
Summary
  python --version ran
    5.28 ± 2.54 times faster than .volta/tools/image/node/18.18.2/bin/node --version
    7.14 ± 3.50 times faster than node --version
    7.77 ± 3.64 times faster than .rustup/toolchains/nightly-x86_64-unknown-linux-gnu/bin/rustc --version
   14.62 ± 5.57 times faster than rustc --version
```

On a low-end server and a shared server the contrast to python becomes even more stark:

```
$ hyperfine --warmup 10 --shell=none ".rustup/toolchains/nightly-x86_64-unknown-linux-gnu/bin/rustc --version" "python3.11 --version"
Benchmark 1: .rustup/toolchains/nightly-x86_64-unknown-linux-gnu/bin/rustc --version
  Time (mean ± σ):      20.8 ms ±   2.3 ms    [User: 7.8 ms, System: 12.7 ms]
  Range (min … max):    18.3 ms …  35.9 ms    136 runs
 
  Warning: Statistical outliers were detected. Consider re-running this benchmark on a quiet system without any interferences from other programs. It might help to use the '--warmup' or '--prepare' options.
 
Benchmark 2: python3.11 --version
  Time (mean ± σ):       1.8 ms ±   0.3 ms    [User: 1.1 ms, System: 0.6 ms]
  Range (min … max):     1.3 ms …   5.9 ms    1882 runs
 
  Warning: Statistical outliers were detected. Consider re-running this benchmark on a quiet system without any interferences from other programs. It might help to use the '--warmup' or '--prepare' options.
 
Summary
  python3.11 --version ran
   11.36 ± 2.12 times faster than .rustup/toolchains/nightly-x86_64-unknown-linux-gnu/bin/rustc --version
```

```
$ hyperfine --warmup 10 --shell=none ".rustup/toolchains/nightly-x86_64-unknown-linux-gnu/bin/rustc --version" "/usr/bin/python3.11 --version"
Benchmark 1: .rustup/toolchains/nightly-x86_64-unknown-linux-gnu/bin/rustc --version
  Time (mean ± σ):      34.3 ms ±   7.1 ms    [User: 12.1 ms, System: 21.2 ms]
  Range (min … max):    26.2 ms …  64.8 ms    80 runs
 
Benchmark 2: /usr/bin/python3.11 --version
  Time (mean ± σ):       6.6 ms ±   3.1 ms    [User: 1.5 ms, System: 4.7 ms]
  Range (min … max):     4.1 ms …  64.7 ms    476 runs
 
  Warning: Statistical outliers were detected. Consider re-running this benchmark on a quiet system without any interferences from other programs. It might help to use the '--warmup' or '--prepare' options.
 
Summary
  /usr/bin/python3.11 --version ran
    5.20 ± 2.65 times faster than .rustup/toolchains/nightly-x86_64-unknown-linux-gnu/bin/rustc --version
```