Musl cross compiling example can't build cross-platform

[UebelAndre]

While working on #2841 I noticed that I was unable to produce a crate_universe output that worked on both Linux and MacOS for the musl-cross_compiling example. On MacOS, I generated the following diff to the lock file.

diff --git a/examples/musl_cross_compiling/Cargo.Bazel.lock.json b/examples/musl_cross_compiling/Cargo.Bazel.lock.json
index 5c3474a8..e9227b46 100644
--- a/examples/musl_cross_compiling/Cargo.Bazel.lock.json
+++ b/examples/musl_cross_compiling/Cargo.Bazel.lock.json
@@ -1,5 +1,5 @@
 {
-  "checksum": "fc8d0cce009897974859e6a3690ba7dade4f6ad225670f87eafb72a9c14632b2",
+  "checksum": "012c077f40c6def2a7ba6a1f1a8235c5b9adb0beaee7b2486697fec8e2c58f2e",
   "crates": {
     "aes 0.8.4": {
       "name": "aes",
@@ -4625,7 +4625,7 @@
         {
           "Library": {
             "crate_name": "musl_cross_compiling",
-            "crate_root": "../../../../../../dev/null",
+            "crate_root": "../../../../../../../../dev/null",
             "srcs": {
               "allow_empty": true,
               "include": [

However, this fails CI with

ERROR: An error occurred during the fetch of repository 'cu':
   Traceback (most recent call last):
	File "/var/lib/buildkite-agent/.cache/bazel/_bazel_buildkite-agent/4521dda2175db4332f201db7aa8123f5/external/rules_rust/crate_universe/private/crates_repository.bzl", line 45, column 28, in _crates_repository_impl
		repin = determine_repin(
	File "/var/lib/buildkite-agent/.cache/bazel/_bazel_buildkite-agent/4521dda2175db4332f201db7aa8123f5/external/rules_rust/crate_universe/private/generate_utils.bzl", line 422, column 13, in determine_repin
		fail(msg)
Error in fail: Error: Digests do not match: Current Digest("012c077f40c6def2a7ba6a1f1a8235c5b9adb0beaee7b2486697fec8e2c58f2e") != Expected Digest("e5e2509cc5dbd6ae948c2c9260f6cc04d60cbcc9b1ace79e80c841983de3556a")
The current `lockfile` is out of date for 'cu'. Please re-run bazel using `CARGO_BAZEL_REPIN=true` if this is expected and the lockfile should be updated.
ERROR: no such package '@@cu//': Error: Digests do not match: Current Digest("012c077f40c6def2a7ba6a1f1a8235c5b9adb0beaee7b2486697fec8e2c58f2e") != Expected Digest("e5e2509cc5dbd6ae948c2c9260f6cc04d60cbcc9b1ace79e80c841983de3556a")
The current `lockfile` is out of date for 'cu'. Please re-run bazel using `CARGO_BAZEL_REPIN=true` if this is expected and the lockfile should be updated.

When enabling CARGO_BAZEL_DEBUG I can see a difference between the outputs on MacOS vs Linux

--- macos.log	2024-09-06 11:49:20
+++ linux.log	2024-09-06 11:49:09
@@ -1,7 +1,7 @@
 Query DEBUG cargo_bazel::lockfile: cargo-bazel version hash: ef009193262ad048e8890de3f6c4c66f8f26584396e9b6f544894b337ab951ce
-Query DEBUG cargo_bazel::lockfile: lockfile context hash: b1f6708c723a384ff6c57202fd845f7155bac978a83216999c465c346d0b9010
+Query DEBUG cargo_bazel::lockfile: lockfile context hash: de05fe88c0a9d32ae90936e16ecb1f23ee22b0b0d0aaa06cc4d045c72a5b2570
 Query DEBUG cargo_bazel::lockfile: workspace config hash: cb23d5c520f46a52d4eafdbb886b34c5b49dbe161d69fd7274c40d96677e4968
 Query DEBUG cargo_bazel::lockfile: splicing manifest hash: 70a890c01bcd729ab8a9b456562da0c45d83ac6c4064366a563c14793fe301c6
-Query DEBUG cargo_bazel::lockfile: Cargo version hash: 8520bbc057b3e7174845066f361879e5e75abb81f9e41dcb63d6be39dad482b5
-Query DEBUG cargo_bazel::lockfile: Rustc version hash: 4a88d82bb95e896d4afa722e04ca05dabee5ec69f52722b038e8d8880ef2462c
-Query DEBUG cargo_bazel::lockfile: Digest hash: 012c077f40c6def2a7ba6a1f1a8235c5b9adb0beaee7b2486697fec8e2c58f2e
+Query DEBUG cargo_bazel::lockfile: Cargo version hash: 56a097b157ad136a3e188cdf46a07188e79b59ad7c279058015c00d6c2d95100
+Query DEBUG cargo_bazel::lockfile: Rustc version hash: a80571782099c29396b91f5eee1a2f6733618054b0a07ad7c066c7801400df38
+Query DEBUG cargo_bazel::lockfile: Digest hash: 43c81042a860df9293da9de1b4f3eae93ff5367ca1b0eef59ade1cdd65aedbf2

We should understand this issue.

[UebelAndre]

I wonder if this is related to #2775

cc @illicitonion

[marvin-hansen]

I ran full tilt into this issue while porting the MUSL example from WORKSPACE to Bazelmod.
Same error related to lockfile.

Repro:
https://github.com/marvin-hansen/rules_rust_fork/tree/bzlmod-musl/examples/bzlmod/musl_hello_world

Furthermore, another example I made today that is a bit more complex than hello world also fails to cross compile.

Repro:
https://github.com/marvin-hansen/rules_rust_fork/tree/bzlmod-musl/examples/bzlmod/musl_scratch

[illicitonion]

It looks like there are two differences:

1. We're currently using /dev/null as a placeholder fake source file in a few Cargo.toml files. We shouldn't. We should use a package-relative path like fake.rs as we do elsewhere. Otherwise the lockfile is sensitive to how deep in your filesystem your git clone is. Use fake.rs not /dev/null as fake source file #2851 fixes this.
2. It looks from your log like cargo and rustc are giving different --version outputs, which reminds me of

   rules_rust/crate_universe/src/lockfile.rs

   Lines 180 to 196 in ab3a688

   	// TODO: There is a bug in the linux binary for Cargo 1.60.0 where
   	// the commit hash reported by the version is shorter than what's
   	// reported on other platforms. This conditional here is a hack to
   	// correct for this difference and ensure lockfile hashes can be
   	// computed consistently. If a new binary is released then this
   	// condition should be removed
   	// https://github.com/rust-lang/cargo/issues/10547
   	let corrections = BTreeMap::from([
   	(
   	"cargo 1.60.0 (d1fd9fe 2022-03-01)",
   	"cargo 1.60.0 (d1fd9fe2c 2022-03-01)",
   	),
   	(
   	"cargo 1.61.0 (a028ae4 2022-04-29)",
   	"cargo 1.61.0 (a028ae42f 2022-04-29)",
   	),
   	]);

   - I manually ran cargo --version and rustc --version on macOS and Linux for 1.81.0 and they were identical... Can you try debugging that on your end?

[illicitonion]

Fixed in #2841 :)