cmd/go: provide an option to print toolchain switch

[hyangah]

Sometimes we want to trace how and where go picks up the target toolchain.
(is it from module cache? or is it from one of my PATH that may have a symlink?)
Either a debug environment variable or a flag (-x or -v) to enable tool switch activity logging can be helpful.

[heschi]

cc @bcmills @matloob

[bcmills]

I think the existing -x flag is the right fit — it should emit a comment when we switch toolchains.

[raghvenders]

@bcmills , @hyangah - I would like to work on this feature change.

Currently I did a small local setup on this go.mod file.

Which are those go commands we can show the information about switch?

module change-tc

go 1.21.1

toolchain go1.21.4

by local go env modifies as below

set GOROOT=C:\Users\R\go\pkg\mod\golang.org\toolchain@v0.0.1-go1.21.4.windows-amd64
set GOSUMDB=sum.golang.org
set GOTMPDIR=
set GOTOOLCHAIN=auto
set GOTOOLDIR=C:\Users\R\go\pkg\mod\golang.org\toolchain@v0.0.1-go1.21.4.windows-amd64\pkg\tool\windows_amd64

Note : Current relevant go [...] -x shows the updated toolchain location.

[raghvenders]

Are we refering situations mentioned - https://go.dev/doc/toolchain - Go toolchain switches, especially after it has been switched , but not showing the message during subsequent calls ?

[raghvenders]

Can we fix this issue - cmd/go: go env GOMOD tries to download newer toolchain #61455 along with this. It has to make an appropriate switch here?

[raghvenders]

@hyangah - I did try to address for "toolchain not available" issue and switching to go version executing this command or local available go in PATH.
Please let me know if I can raise a CL for this.

C:\Users\R\golang\analyse-bugs>cat go.mod
module example.com/analyze-bugs

go 1.23

C:\Users\R\golang\analyse-bugs>"C:\Program Files\Go\bin\go" run .
go: downloading go1.23 (windows/amd64)
go: download go1.23 for windows/amd64: toolchain not available

C:\Users\R\golang\analyse-bugs>go run .
go: downloading go1.23 (windows/amd64)
go: download go1.23 for windows/amd64: toolchain not available
Switching to your local go version executing this command: devel go1.22-5f08b44799 Mon Nov 13 14:11:13 2023 +0000 windows/amd64
Swithing Go line in go.mod file to : go 1.22 ------ go.mod ------
module example.com/analyze-bugs

go 1.22
====================
Go.mod file is switched to go 1.22 . Kindly re-execute the relevant commands
C:\Users\R\golang\analyse-bugs>go run .
Hello World