feat(mockgen): add project generate mode

[YuheiTakagawa]

Summary

This adds a project-level generate mode to mockgen:

mockgen -generate ./...

The new mode discovers existing //go:generate directives that invoke mockgen across the requested package patterns and runs the matching mockgen invocations in-process. This keeps the existing go:generate directives as the source of truth while avoiding the cost of spawning one mockgen process per directive.

The implementation currently supports:

• direct mockgen ... directives
• go run ... mockgen ... directives, including common go run flags before the mockgen package
• file-local //go:generate -command ... aliases that resolve to mockgen
• generate build-tag files via go list -tags=generate
• internal and external test files, including GOPACKAGE expansion for package foo_test
• source, package, archive, and model gob modes through the existing mockgen paths

It also shares package name resolution across all discovered targets and uses go/format plus deterministic import grouping to avoid per-target goimports environment setup while preserving existing generated output.

Motivation

Large repositories can have hundreds or thousands of //go:generate mockgen ... directives. Running them through:

go generate --run=mockgen ./...

spawns one mockgen process per directive. Each process repeats package/import resolution and related go command work.

This mode provides a faster path for repositories that already maintain mockgen directives in source files, without introducing a separate config file or requiring users to manually maintain lists of source files.

Performance data

On a large repository with 1133 existing source-mode mockgen directives, I measured the following:

Existing workflow

go generate --run=mockgen ./...

• mockgen processes: 1133
• generated destinations: 1133
• missing destinations: 0
• changed files vs existing generated mocks: 0
• fresh copy elapsed: 518.162s
• hot rerun elapsed: 406.284s
• nested go commands from a previous instrumented run:
  • 3482 total
  • 2349 go list
  • 1133 go env

New workflow

mockgen -generate ./...

• mockgen processes: 1
• generated destinations: 1133
• missing destinations: 0
• changed files vs existing generated mocks: 0
• fresh copy elapsed: 182.753s
• hot rerun elapsed: 48.153s
• nested go commands:
  • 55 total
  • 55 go list
  • 0 go env

This is approximately:

• 2.8x faster on a fresh copied repository
• 8.4x faster on a hot rerun

Compatibility

I also verified the existing go generate --run=mockgen ./... workflow on the same large repository after these changes:

• all 1133 destinations were generated
• no outputs were missing
• changed files vs existing generated mocks: 0

Implementation notes

• -generate is a new top-level mode and is mutually exclusive with the existing single-target modes.
• Discovery uses go list -e -tags=generate -json=Dir,Name,GoFiles,TestGoFiles,XTestGoFiles.
• Directives are parsed and expanded with the go generate variables used by this mode: GOFILE, GOLINE, GOPACKAGE, and DOLLAR. Other variables are resolved from the process environment.
• Matching mockgen directives are converted into the same option paths used by the existing CLI.
• Package name resolution is primed in batches to reduce repeated go list calls and avoid command-line length issues.
• Existing output files are read before writing; if generated output is byte-for-byte identical, the file is left untouched.

Test plan

go test ./mockgen -run 'TestParseGenerateDirectiveRecognizesGoRunWithFlags|TestParseGenerateDirectiveRecognizesCommandAlias|TestDiscoverGenerateTargetsIncludesGenerateTagFiles|TestDiscoverGenerateTargetsIncludesExternalTestFiles' -count=1

go test ./mockgen ./mockgen/model

go test ./...

Additional manual validation:

• Ran mockgen -generate ./... on a large repository with 1133 existing mockgen directives.
• Verified all 1133 expected destinations exist.
• Verified there were no missing outputs.
• Verified generated files were byte-for-byte identical to existing generated mocks.
• Verified nested go command count was reduced to 55 total / 55 go list / 0 go env.

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[YuheiTakagawa]

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