cmd/go: always pass -lang to cmd/compile

[zigo101]

Go version

go version go1.22.0 linux/amd64

Output of go env in your module/workspace:

.

What did you do?

main.go:

//go:build go1.21

package main

func main() {
	for i, p := 0, (*int)(nil); p == nil; println(p == &i) {
		p = &i
	}
}

go run main.go

What did you see happen?

true

What did you expect to see?

false

Note that when the -gcflags=-lang=go1.22 compiler flag is specified, then the "//go:build go1.xy" comment directive is respected (go run . with a go.mod file also respects it).

$ gotv 1.22. run main.go 
[Run]: $HOME/.cache/gotv/tag_go1.22.0/bin/go run main.go
false
$ gotv 1.22. run -gcflags=-lang=go1.22 main.go 
[Run]: $HOME/.cache/gotv/tag_go1.22.0/bin/go run -gcflags=-lang=go1.22 main.go
true
$ gotv 1.22. run .
[Run]: $HOME/.cache/gotv/tag_go1.22.0/bin/go run .
true
$ gotv 1.22. run -gcflags=-lang=go1.22 .
[Run]: $HOME/.cache/gotv/tag_go1.22.0/bin/go run -gcflags=-lang=go1.22 .
true

[bcmills]

This is closely related to

• cmd/go: always pass a GoVersion to cmd/vet #65612
  and https://go.dev/cl/567435 (CC @samthanawalla @matloob), in that cmd/go sometimes doesn't pass a language version for a main.go file when it could.

However, in the absence of any -lang flag, I expect cmd/compile to still apply whatever version information it has inferred from the //go:build directives, and it appears not to be doing so. This looks like a compiler bug.

[mknyszek]

CC @golang/compiler

[zigo101]

@bcmills
Just a btw question: it looks the -gcflags=-lang=go1.xy compiler flag is only applied to the seed files being passed to the compiler. Other involved source files in building will not be affected by the flag. Is this the intended design?

[mknyszek]

In runtime/compiler triage, we think this is a Go command issue? Is that right? Thanks.

[bcmills]

I think it is a composite of a cmd/go bug (not passing the -lang flag) and a cmd/compile bug (not using //go:build version hints when -lang is not explicitly set).

The cmd/go bug should be fixed in #65612, so I suggest we focus the investigation in this issue on the cmd/compile side.