x/tools/stringer: Generate platform specific code

[holyhope]

stringer version

v0.1.11

What did you do?

1. I define a enumeration in a platform specific go file: https://go.dev/play/p/1cKkWu9ngsD

   //go:build darwin && !linux && !freebsd && !netbsd && !openbsd && !windows && !js
   // +build darwin,!linux,!freebsd,!netbsd,!openbsd,!windows,!js
   
   package mypackage
   
   //go:generate stringer -type=MyEnum
   type MyEnum uint32
   
   const (
   	DarwinValue1Enum MyEnum = iota
   	DarwinValue2Enum
   )

2. I run stringer -type=MyEnum to generate a new file myenum_string.go: https://go.dev/play/p/eR6E7A9A7aU

What did you expect to see?

I expect to get a valid code for all platform. Which here means the MyEnum.String() should only exists on darwin platform.

What did you see instead?

The new generated file is only valid when compiling for darwin, and not for other GOOS:
GOOS=linux go vet ./...

./myenum_string.go:11:8: undefined: DarwinValue1Enum
./myenum_string.go:12:8: undefined: DarwinValue2Enum
./myenum_string.go:19:9: undefined: MyEnum
./myenum_string.go:20:10: undefined: MyEnum

Suggestions

I suggest to either:

1. stringer copies the header from the current file so it inject automatically the right //go:build comments.
2. Add --platform flag as a list of string so stringer automatically adds the header.
3. Add --template flag as a path to a template, so stringer use it to generate go files (and I can add my own //go:build comment).

[holyhope]

Note: I looked for this kind of issue, but I may missed it. If so, I am sorry for the duplication;;;

[seankhliao]

Duplicate of #47179