VSCode - rust-analyzer - annoying re-builds due to RUSTC_BOOTSTRAP changed

editors and IDEs
1

TL/TR
what can I do to avoid the re-builds to happen and instead have consistent, reusable build results for these: cargo test, CodeLens, TestExplorer?

Backgound
(Note: this is a more specific spin of from Why are builds of my crate's test not consistent (cargo build vs cargo test vs VSCode-Test-Explorer))
Using: rust-analyzer extension for VSCode
Name: rust-analyzer
Id: rust-lang.rust-analyzer
Description: Rust language support for Visual Studio Code
Version: 0.3.2870
Publisher: The Rust Programming Language
VS Marketplace Link: https://marketplace.visualstudio.com/items?itemName=rust-lang.rust-analyzer

Reprocue:

  1. cargo new ra_rebuild_test
  2. cargo add proc-macros2
  3. use rust-analyzer.testExplorer": true (e.g. in ./vscode/settings.json)
  4. add any test into main.rs, e.g.
#[cfg(test)]
mod tests {
    #[test]
    fn it_works() {
        assert_eq!(2 + 2, 4);
    }
}
  1. Use VSCode's TestExplorer to run it_works
  2. cargo test --no-run --verbose

Expected: no rebuild necessary.
Observe: it says this and rebuilds:

 Dirty proc-macro2 v1.0.106: the env variable RUSTC_BOOTSTRAP changed
   Compiling proc-macro2 v1.0.106
[...]

This is no big deal for a simple project, however gets ugly when the project grows larger, as described in Why are builds of my crate's test not consistent (cargo build vs cargo test vs VSCode-Test-Explorer)).

Note: I don't observe this, when useing VSCode's CodeLense feature to run the individual test (which I suppose is also a feature brought in by rust-analyzer extension into VSCode)

Questions

  1. why is it necessary for rust-analyzer to make use of RUSTC_BOOTSTRAP in TestExplorer, but - as it seems - not in the CodeLens feature?
  2. why is this environment variable relevant for rebuilding proc-macro2 (and as it seems not for some other dependencies)?
  3. what can I do to avoid the re-builds to happen and instead have consistent, reusable build results for these: cargo test, CodeLens, TestExplorer?
2

I reported that issue years ago to proc-macro2. It still might be worth to add a comment there that this is now triggering rebuilds due to rust-analyzer + also report it to rust-analyzer as well.

3

The test explorer needs to parse the output of cargo test, so it asks for JSON output, which is an unstable feature. The code lens does not parse test output.