x/tools/cmd/deadcode: add new command

[adonovan]

The golang.org/x/tools/internal/cmd/deadcode command reports dead code, that is, functions not reachable by a sequence of calls from main, even through reflection, based on an RTA-based call graph. We have found it useful for identifying orphaned bits of code in x/tools and we imagine others might find it useful too. We propose to publish it at cmd/deadcode in the same repo.

This is an example of the tool in action, showing functions in x/tools not reachable from gopls' main or any of its tests, and suppressing functions in generated source files.

xtools$ deadcode -test ./gopls/...
golang.org/x/tools/gopls/internal/lsp
	openClientEditor

golang.org/x/tools/gopls/internal/lsp/cache
	(fileLoadScope).aScope
	(packageLoadScope).aScope
	(moduleLoadScope).aScope
	(viewLoadScope).aScope

golang.org/x/tools/gopls/internal/lsp/debug/log
	(Level).Log
	(Level).Logf

golang.org/x/tools/gopls/internal/lsp/filecache
	SetBudget

golang.org/x/tools/gopls/internal/lsp/regtest
	(State).outstandingWork

golang.org/x/tools/gopls/internal/lsp/source
	WidestPackageForFile

golang.org/x/tools/gopls/internal/lsp/template
	(*Parsed).WriteNode
	(wrNode).writeNode

xtools$ deadcode -test -json ./gopls/...
[
	{
		"Path": "golang.org/x/tools/gopls/internal/lsp",
		"Funcs": [
			{
				"Name": "golang.org/x/tools/gopls/internal/lsp.openClientEditor",
				"RelName": "openClientEditor",
				"Posn": "/Users/adonovan/w/xtools/gopls/internal/lsp/command.go:1178:6",
				"Generated": false
			}
		]
	},
...

[earthboundkid]

Cf @bep's https://github.com/bep/punused, which I have used to check for dead code before. That one can have false positive though.

[adonovan]

I'd particularly interested in feedback on how we could improve the user interface to make it clearer. For example, the first command I ran to produce the example above used just ./gopls, so it did not include tests from subpackages, causing it to report some functions that are only used from those tests. Perhaps better defaults would make such mistakes less likely.

Please try it out and let us know what you find.

[earthboundkid]

-generated=false is awkward. -exclude-generated would be less awkward.

[hdonnay]

One thing I noticed is that it needs some logic around Example* functions in tests.

[adonovan]

it needs some logic around Example* functions in tests.

Logic to do what? The help message says:

Bear in mind that an Example test
function without an "Output:" comment is merely documentation:
it is dead code, and does not contribute coverage.

Are you suggesting that it should treat non-executed Example functions as somehow reachable? I think its current behavior is more sensible. It's also unlikely that anyone would casually delete an Example based on the report produced by this tool.

[rsc]

Why does the output print package "path"? Why not just print path, or perhaps like the go command # path?
package "path" is weird because that's not the syntax of a package declaration.

[rsc]

This proposal has been added to the active column of the proposals project
and will now be reviewed at the weekly proposal review meetings.
— rsc for the proposal review group