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rust: add tutorial on git-bisecting rustc

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This commits adds to the manual a tutorial showing how to
troubleshoot breakage caused by a rustc upgrade in *downstream*
packages.
This commit is contained in:
Adam Joseph 2023-11-11 22:12:01 -08:00
parent b6a3fabcc1
commit ae7b2678f3
1 changed files with 64 additions and 0 deletions
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64
doc/languages-frameworks/rust.section.md
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@ -939,3 +939,67 @@ Fenix also has examples with `buildRustPackage`,
[crane](https://github.com/ipetkov/crane),
[naersk](https://github.com/nix-community/naersk),
and cross compilation in its [Examples](https://github.com/nix-community/fenix#examples) section.
## Using `git bisect` on the Rust compiler {#using-git-bisect-on-the-rust-compiler}
Sometimes an upgrade of the Rust compiler (`rustc`) will break a
downstream package. In these situations, being able to `git bisect`
the `rustc` version history to find the offending commit is quite
useful. Nixpkgs makes it easy to do this.
First, roll back your nixpkgs to a commit in which its `rustc` used
*the most recent one which doesn't have the problem.* You'll need
to do this because of `rustc`'s extremely aggressive
version-pinning.
Next, add the following overlay, updating the Rust version to the
one in your rolled-back nixpkgs, and replacing `/git/scratch/rust`
with the path into which you have `git clone`d the `rustc` git
repository:
```nix
(final: prev: /*lib.optionalAttrs prev.stdenv.targetPlatform.isAarch64*/ {
rust_1_72 =
lib.updateManyAttrsByPath [{
path = [ "packages" "stable" ];
update = old: old.overrideScope(final: prev: {
rustc = prev.rustc.overrideAttrs (_: {
src = lib.cleanSource /git/scratch/rust;
# do *not* put passthru.isReleaseTarball=true here
});
});
}]
prev.rust_1_72;
})
```
If the problem you're troubleshooting only manifests when
cross-compiling you can uncomment the `lib.optionalAttrs` in the
example above, and replace `isAarch64` with the target that is
having problems. This will speed up your bisect quite a bit, since
the host compiler won't need to be rebuilt.
Now, you can start a `git bisect` in the directory where you checked
out the `rustc` source code. It is recommended to select the
endpoint commits by searching backwards from `origin/master` for the
*commits which added the release notes for the versions in
question.* If you set the endpoints to commits on the release
branches (i.e. the release tags), git-bisect will often get confused
by the complex merge-commit structures it will need to traverse.
The command loop you'll want to use for bisecting looks like this:
```bash
git bisect {good,bad} # depending on result of last build
git submodule update --init
CARGO_NET_OFFLINE=false cargo vendor \
--sync ./src/tools/cargo/Cargo.toml \
--sync ./src/tools/rust-analyzer/Cargo.toml \
--sync ./compiler/rustc_codegen_cranelift/Cargo.toml \
--sync ./src/bootstrap/Cargo.toml
nix-build $NIXPKGS -A package-broken-by-rust-changes
```
The `git submodule update --init` and `cargo vendor` commands above
require network access, so they can't be performed from within the
`rustc` derivation, unfortunately.