x/tools/gopls: downrank slice as second "append()" param

[muirdm]

type foo struct { i int }
var bar []foo
bar = append(bar, <>)

Currently in the above example gopls prefers "bar" at "<>", but it's rare that you append a slice to itself. I think we should adjust scoring so that the literal candidate "foo{}" is ranked above "bar".

We could add a special case to downrank the first append param as the second append param, but I'm not sure the best way to do it. If we just check types.Object equality, we will incorrectly dowrank in the following case:

type foo struct { i []int }
var a, b foo
a.i = append(a.i, <>)

At "<>" we would downrank "b.i" in addition to "a.i". To avoid this we would need to use objectpath or AST/text equality check.

[gopherbot]

Change https://golang.org/cl/247519 mentions this issue: internal/lsp/source: improve completion in append()