This change wouldn't be limited to gopls, all go tooling which don't operate directly on .go files (like some linters do) would need to be changed.

The suggestion of a folder like "_folder" (starts with an underscore) not being considered by the the go tool by default is already implemented in Go. See the last paragraph of https://pkg.go.dev/cmd/go#hdr-Package_lists_and_patterns:

Directory and file names that begin with "." or "_" are ignored by the go tool, as are directories named "testdata".

An example of that is in the x/website repository: it has a "_content" directory for website content rather than Go packages.

@dmitshur it does not behave as they suggest:

a> tree
.
|-- _folder
|   `-- b.go
|-- a.go
`-- go.mod

// a.go
package main

func main() {
	a()
}

// b.go
package main

import "fmt"

func a() {
	fmt.Println("Hello world")
}

a> go run .
# aaa
./a.go:4:2: undefined: a

https://pkg.go.dev/cmd/go#hdr-Package_lists_and_patterns is thus more breaking changes.

@Jorropo I misunderstood the original proposal, sorry. Indeed, the behavior of having a prefix in front of a folder name makes the go tool ignore its contents, not act as if its contents are in the parent directory as your described.

Just name your files aaa-foo.go aaa-bar.go bbb-foo.go bbb-bar.go instead of aaa/foo.go aaa/bar.go bbb/foo.go bbb/bar.go. Adding a sub-folder makes everything more complicated for no net benefit. Put the files where they are supposed to be. We've seen the alternative in other languages, and it is bad.

This proposal has been declined as infeasible.
— rsc for the proposal review group