pantsbuild / pants / 28575780130

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Accept new `#cgo` directives introduced by go 1.24 (#23439)

(First of all thank you for the great build tool!)

Go 1.24 [introduced](https://go.dev/doc/go1.24#cgo) new `#cgo`
directives for performance improvements. Unfortunately the current pants
go plugin does not understand the directives and results in the
following error when building:

```
21:02:21.05 [ERROR] Failed to analyze github.com/pantsbuild/pants/testprojects/src/go/pants_test/cgo/directives for dependency inference:
invalid Go sources encountered
nocallback_noescape.go: cgo error: nocallback_noescape.go: invalid #cgo line: #cgo nocallback addNumbers
```

The error message was obtained by running the new test `./pants test
testprojects/src/go/pants_test/cgo/directives:` with the following diff
(to recognize code under `testprojects/src/go/`).

```diff
diff --git a/pants.toml b/pants.toml
index fde22eaf8e..f3e4f56313 100644
--- a/pants.toml
+++ b/pants.toml
@@ -72,7 +72,7 @@ pants_ignore.add = [

 build_ignore.add = [
   # Disable Go targets by default so Pants developers do not need Go installed.
-  "testprojects/src/go/**",
+  # "testprojects/src/go/**",
 ]

 unmatched_build_file_globs = "error"
```

This PR fixes the issue by changing `saveCgo` function in go package
analyzer. Specifically I updated `saveCgo` based on
https://github.com/golang/go/blob/go1.26.4/src/go/build/build.go#L1710-L1713
(i.e. catch up with upstream) so that `#cgo nocallback` directive is not
rejected.

I also added a simple cgo test case. Please let me know if you have a
better idea how to test this feature.

20 of 20 new or added lines in 1 file covered. (100.0%)

93774 of 101020 relevant lines covered (92.83%)

3.7 hits per line