Add support to use rustc --print=cfg as fallback to resolve target information by mhatzl · Pull Request #3331 · nextest-rs/nextest

mhatzl · 2026-05-08T15:57:02Z

Uses rustc --print=cfg --target=<given target> in nextest internally to resolve the platform in case target is not built-in or custom JSON target.
If this fails, heuristic check is done as before.

bumps target-spec to v3.6.0 to get support for rustc --print=cfg.
(Cargo.lock was already at v3.6.0, but Cargo.toml wasn't)

To test this feature with de upstream rustc, I created a shell script and set it for RUSTC only for this one test case.
The shell script forwards all rustc calls except when --print=cfg is the first argument.
In this case it returns a fixed cfg that is then compared in the test case.

Wouldn't know how else to test this, because rustc --print=cfg requires to pass the target as well, but upstream rustc only knows targets that would get resolved by the built-in case, so would never reach the --print=cfg fallback.

codecov · 2026-05-08T16:09:41Z

Codecov Report

❌ Patch coverage is 76.47059% with 8 lines in your changes missing coverage. Please review.
✅ Project coverage is 85.58%. Comparing base (d2bdbe1) to head (0a3e63e).

Files with missing lines	Patch %	Lines
nextest-runner/src/cargo_config/target_triple.rs	75.00%	6 Missing ⚠️
integration-tests/test-helpers/rustc-shim.rs	80.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3331      +/-   ##
==========================================
- Coverage   85.58%   85.58%   -0.01%     
==========================================
  Files         160      160              
  Lines       47893    47927      +34     
==========================================
+ Hits        40990    41019      +29     
- Misses       6903     6908       +5

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sunshowers · 2026-05-10T00:58:06Z

Thanks for doing this! Have a look at the rustc-shim binary, you should be able to fake a rustc invocation using that: https://github.com/nextest-rs/nextest/blob/a2c4c13e16946c0c5ad06a9fedeabf10f83e4fe3/integration-tests/test-helpers/rustc-shim.rs

mhatzl · 2026-05-11T17:41:01Z

thanks @sunshowers
will try to use rustc-shim

Needed, because `to_string` is used in `aquire_graph_data` in the call to cargo metadata. This requires to pass the target triple.

mhatzl · 2026-05-13T17:26:40Z

switched to rustc-shim and removed the script.
CI is now failing, because I expect NEXTEST_BIN_EXE_rustc_shim to be set and pointing to the rustc-shim binary.

I don't see how this is set for the integration-test, because inside set_env_vars_for_test, the env is also only checked but not set.
Since this is the only place where I could find a mention of this env, I guess it is set in GitHub environments?

Should I hardcode the path using NEXTEST_WORKSPACE_ROOT and then target/release/rustc-shim?

sunshowers · 2026-05-13T17:28:14Z

Hmm, nextest is responsible for setting it. I'll have a look.

mhatzl · 2026-05-13T17:36:21Z

Hmm, nextest is responsible for setting it. I'll have a look.

my added test is still under nextest-runner/tests/integration/target_triple.rs.
didn't want to move it to integration-tests/tests, because the test is about the target triple. Maybe NEXTEST_BIN_EXE_rustc_shim is only available for integration-tests/tests?

sunshowers · 2026-05-13T17:40:50Z

Ah yeah it has to live under integration-tests.

I would have a look at cargo nextest debug build-platforms -- you may need to add a mode which just prints out all the platform settings. The debug commands aren't stable so you can make whatever changes you need to them.

Adding a new debug command to check target arg output

mhatzl · 2026-05-13T18:38:29Z

added cargo debug print-cargo-target-arg to compare the CargoTargetArg output of a set target and that seems to work quite good now.

not sure about the MSRV fails, because edition=2024 was already set.
the macos-latest fails are also strange

Prerequisite for #3331.

sunshowers · 2026-05-13T18:59:40Z

The nextest-metadata MSRV thing will be addressed by #3342.

This generally looks great, thanks! I'll take care of the rest (want to make a few tweaks to the output format before landing).

mhatzl · 2026-05-13T19:08:45Z

perfect. thank you.

and thanks for the pointers

Prerequisite for #3331.

…ty} (#3343) Going to use this in #3331.

sunshowers · 2026-05-13T23:00:17Z

+            triple_str,
+            "--output-format",
+            "json-pretty",
+        ])


I added JSON output to debug build-platforms in #3343 and used it here.

sunshowers · 2026-05-13T23:00:59Z

+debug_assertions
+panic="abort"
+target_abi="eabihf"
+target_arch="arm"
+target_endian="little"
+target_env=""
+target_has_atomic="16"
+target_has_atomic="32"
+target_has_atomic="8"
+target_has_atomic="ptr"
+target_os="none"
+target_pointer_width="32"
+target_vendor="unknown"


Moved this to a file to avoid duplicating it across the shim and the test.

sunshowers · 2026-05-13T23:01:34Z

+source: integration-tests/tests/integration/main.rs
+expression: "&stdout"
+---
+{


This is the new snapshot.

sunshowers · 2026-05-13T23:02:33Z

The macOS failures are definitely unrelated to the PR and I think are an infrastructure issue with GitHub.

mhatzl added 4 commits May 13, 2026 17:53

Use rustc --print=cfg to resolve non-builtin targets

6d28820

Add test for custom rustc cfg target

cefd9c1

Use target triple for display of custom rustc cfg target

18bbd64

Needed, because `to_string` is used in `aquire_graph_data` in the call to cargo metadata. This requires to pass the target triple.

Use rustc-shim for rustc-cfg target testing

8a4bb8d

mhatzl force-pushed the mh/rustc-cfg-target-support branch from 55c920b to 8a4bb8d Compare May 13, 2026 16:32

Move print-cfg test to integration-tests

e82399c

Adding a new debug command to check target arg output

sunshowers mentioned this pull request May 13, 2026

[nextest-metadata] bump target-spec to 3.6.0, MSRV to 1.86 #3342

Merged

sunshowers added a commit that referenced this pull request May 13, 2026

[nextest-metadata] bump target-spec to 3.6.0, MSRV to 1.86

35d903b

Prerequisite for #3331.

sunshowers added a commit that referenced this pull request May 13, 2026

[nextest-metadata] bump target-spec to 3.6.0, MSRV to 1.86 (#3342)

09012fb

Prerequisite for #3331.

sunshowers mentioned this pull request May 13, 2026

[cargo-nextest] add debug build-platforms --output-format json{,-pretty} #3343

Merged

sunshowers added a commit that referenced this pull request May 13, 2026

[cargo-nextest] add debug build-platforms --output-format json{,-pret…

d2bdbe1

…ty} (#3343) Going to use this in #3331.

merge main into branch, switch over to build-platforms

0a3e63e

sunshowers reviewed May 13, 2026

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sunshowers merged commit e760edc into nextest-rs:main May 13, 2026
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