x/tools/gopls: completion offers too many irrelevant candidates in struct literal

What version of Go are you using (go version)?

Does this issue reproduce with the latest release?

Yes

What operating system and processor architecture are you using (go env)?

What did you do?

Consider the following example:

package main

import (
	"fmt"

	"playground.com/p"
)

func main() {
	s := p.S{
		// attempt completion
	}
	fmt.Println(s)
}


-- go.mod --
module playground.com

-- p/p.go --
package p

type S struct {
	Name string
	Age  int
}

If completion is attempted at the position of the comment // attempt completion the following list of candidates is returned:

Name
Age
fmt
p
main()
string
append
bool
byte
cap
close
complex
complex128
complex64
copy
delete                                                                                                                                                                                         78% ☰   11/14 ㏑ :  7
error
false
float32
float64
imag
int
int16
int32
int64
int8
iota
len
make
new
nil
panic
print
println
real
recover
rune
true
uint
uint16
uint32
uint64
uint8
uintptr

In this case, because S is declared in another package, vet will enforce that use of the struct type in a composite literal must be keyed:

https://play.golang.org/p/-H5Fnm5c9zj

Hence I believe the only valid candidates are:

Name
Age

In any case, if S were declared in the same package, the list of candidates is not actually correct: it appears to be the valid key names plus all the predeclared identifiers, plus package-scope identifiers, regardless of whether they are applicable.

My proposal would be that regardless of whether S is declared in the current package or not, the list of candidates be limited to the valid key names. This feels like a more sensible default; far less noise in the majority of cases.

I realise this is subjective... so other thoughts welcomed!

---

cc @stamblerre @ianthehat