Merge pull request #266971 from amjoseph-nixpkgs/pr/rustc/no-more-occult-nonsense

rustc: allow building/git-bisecting without tarballs

3 files changed, 85 insertions, 4 deletions

diff --git a/doc/languages-frameworks/rust.section.md b/doc/languages-frameworks/rust.section.md
index 3bd8e1c7651..d18b048b911 100644
--- a/doc/languages-frameworks/rust.section.md
+++ b/doc/languages-frameworks/rust.section.md
@@ -939,3 +939,68 @@ 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.
+
diff --git a/pkgs/development/compilers/rust/default.nix b/pkgs/development/compilers/rust/default.nix
index 0a0af783236..efd7042c230 100644
--- a/pkgs/development/compilers/rust/default.nix
+++ b/pkgs/development/compilers/rust/default.nix
@@ -73,7 +73,7 @@ in
         patches = rustcPatches;
 
         # Use boot package set to break cycle
-        inherit (bootstrapRustPackages) cargo rustc;
+        inherit (bootstrapRustPackages) cargo rustc rustfmt;
       });
       rustfmt = self.callPackage ./rustfmt.nix {
         inherit Security;
diff --git a/pkgs/development/compilers/rust/rustc.nix b/pkgs/development/compilers/rust/rustc.nix
index 3a649bde95d..1550887c356 100644
--- a/pkgs/development/compilers/rust/rustc.nix
+++ b/pkgs/development/compilers/rust/rustc.nix
@@ -1,7 +1,7 @@
 { lib, stdenv, removeReferencesTo, pkgsBuildBuild, pkgsBuildHost, pkgsBuildTarget, targetPackages
 , llvmShared, llvmSharedForBuild, llvmSharedForHost, llvmSharedForTarget, llvmPackages
 , fetchurl, file, python3
-, darwin, cargo, cmake, rust, rustc
+, darwin, cargo, cmake, rust, rustc, rustfmt
 , pkg-config, openssl, xz
 , libiconv
 , which, libffi
@@ -24,13 +24,15 @@
 let
   inherit (lib) optionals optional optionalString concatStringsSep;
   inherit (darwin.apple_sdk.frameworks) Security;
-in stdenv.mkDerivation rec {
+in stdenv.mkDerivation (finalAttrs: {
   pname = "${targetPackages.stdenv.cc.targetPrefix}rustc";
   inherit version;
 
   src = fetchurl {
     url = "https://static.rust-lang.org/dist/rustc-${version}-src.tar.gz";
     inherit sha256;
+    # See https://nixos.org/manual/nixpkgs/stable/#using-git-bisect-on-the-rust-compiler
+    passthru.isReleaseTarball = true;
   };
 
   __darwinAllowLocalNetworking = true;
@@ -79,6 +81,12 @@ in stdenv.mkDerivation rec {
     "--release-channel=stable"
     "--set=build.rustc=${rustc}/bin/rustc"
     "--set=build.cargo=${cargo}/bin/cargo"
+  ] ++ lib.optionals (!(finalAttrs.src.passthru.isReleaseTarball or false)) [
+    # release tarballs vendor the rustfmt source; when
+    # git-bisect'ing from upstream's git repo we must prevent
+    # attempts to download the missing source tarball
+    "--set=build.rustfmt=${rustfmt}/bin/rustfmt"
+  ] ++ [
     "--tools=rustc,rust-analyzer-proc-macro-srv"
     "--enable-rpath"
     "--enable-vendor"
@@ -203,6 +211,14 @@ in stdenv.mkDerivation rec {
     # See https://github.com/jemalloc/jemalloc/issues/1997
     # Using a value of 48 should work on both emulated and native x86_64-darwin.
     export JEMALLOC_SYS_WITH_LG_VADDR=48
+  '' + lib.optionalString (!(finalAttrs.src.passthru.isReleaseTarball or false)) ''
+    mkdir .cargo
+    cat > .cargo/config <<\EOF
+    [source.crates-io]
+    replace-with = "vendored-sources"
+    [source.vendored-sources]
+    directory = "vendor"
+    EOF
   '';
 
   # rustc unfortunately needs cmake to compile llvm-rt but doesn't
@@ -281,4 +297,4 @@ in stdenv.mkDerivation rec {
       "i686-windows" "x86_64-windows"
     ];
   };
-}
+})