fix(unexported-return): stabilize results when a directory mixes package foo and foo_test

[far4599]

Closes #1711

Motivation

The unexported-return rule produces non-deterministic and incorrect results when a directory contains both package foo source/internal-test files and package foo_test external-test files:

1. Wrong package qualifier — the returned type is reported as *foo_test.T instead of *foo.T.
2. Flaky detection — in ~15% of runs the issue is silently dropped.

Root cause: dots.ResolvePackages collapses GoFiles + TestGoFiles + XTestGoFiles into a single []string per directory. lint.Linter.lintPackage then bundles all of them into one lint.Package whose TypeCheck picks anyFile by iterating a Go map (non-deterministic order) and passes that file's package name to go/types.Config.Check. When anyFile is an XTestGoFile the whole package is type-checked under the foo_test name; additionally, mixing files from two different Go packages in a single types.Config.Check call causes some types to fail to resolve, so TypeOf returns nil and the issue is dropped.

Downstream impact: this race surfaces in golangci-lint as nolintlint flagging //nolint:revive directives as "unused" on the runs where revive drops the issue, producing intermittent CI failures.

Change

lint.Linter.lintPackage now groups files by their parsed package name before creating lint.Package instances. Each Go package (e.g. foo and foo_test) gets its own lint.Package and is type-checked independently. Files belonging to different Go packages no longer share a types.Package.

Tests

Added test/unexported_return_mixed_pkg_test.go — a RED/GREEN regression test that exercises the previously flaky path 50 times with a minimal reproduction (one generic constructor + internal test + external test).

• Without the fix the test fails on the first iteration (either wrong package qualifier or zero issues).
• With the fix all 50 iterations pass.
• All existing tests continue to pass: go test -race ./... -count=1 — OK.
• revive --config revive.toml ./... — 0 issues.
• golangci-lint run — 0 issues.

Style

Follows the existing coding style of the repository — errgroup.Group for concurrent package linting mirrors the pattern already used in Package.lint.

[Copilot]

Pull request overview

Fixes nondeterministic and incorrect unexported-return results when a single directory contains both package foo files and package foo_test external test files by ensuring type-checking is performed per distinct Go package.

Changes:

• Update lint.Linter.lintPackage to group files by parsed package name and lint/type-check each group independently.
• Add a regression test that repeatedly exercises the mixed-package directory scenario to verify stable output.
• Add minimal testdata fixtures representing pub, pub internal test, and pub_test external test files.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
lint/linter.go	Groups directory files by Go package name and lints each package separately to avoid mixed-package type-checking nondeterminism.
test/unexported_return_mixed_pkg_test.go	Regression test running the rule multiple times to ensure deterministic, correct failure text when mixed packages are present.
testdata/unexported_return_mixed_pkg/pub.go	Repro fixture: exported generic function returning an unexported generic type (expected to trigger unexported-return).
testdata/unexported_return_mixed_pkg/pub_internal_test.go	Repro fixture: internal test file in package pub to mirror real-world directories.
testdata/unexported_return_mixed_pkg/pub_external_test.go	Repro fixture: external test file in package pub_test to trigger the mixed-package scenario.

[alexandear]

I'm still working on this. Currently, I'm writing a benchmark suite for Revive to be sure this change won't affect anything.