lsp/completion: offer candidates converting arrays to slices

For example:

    var b [4]byte
    var s []byte = <>

At <> we now prefer "b" and insert as "b[:]".

Fixes golang/go#40277.

Change-Id: I5fe9d153813dac7218edf31c7c33610130eef9bc
Reviewed-on: https://go-review.googlesource.com/c/tools/+/311169
Run-TryBot: Muir Manders <muir@mnd.rs>
Run-TryBot: Rebecca Stambler <rstambler@golang.org>
gopls-CI: kokoro <noreply+kokoro@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Rebecca Stambler <rstambler@golang.org>
Trust: Rebecca Stambler <rstambler@golang.org>
Trust: Robert Findley <rfindley@google.com>

internal/lsp/source/completion/completion.go

@@ -386,6 +386,9 @@ type candidate struct {
 	// For example, dereference=2 turns "foo" into "**foo" when formatting.
 	dereference int
 
+	// takeSlice is true if obj is an array that should be converted to a slice.
+	takeSlice bool
+
 	// variadic is true if this candidate fills a variadic param and
 	// needs "..." appended.
 	variadic bool
@@ -2464,6 +2467,13 @@ func (c *candidate) anyCandType(f func(t types.Type, addressable bool) bool) boo
 		return true
 	}
 
+	if array, ok := objType.Underlying().(*types.Array); ok {
+		if f(types.NewSlice(array.Elem()), false) {
+			c.takeSlice = true
+			return true
+		}
+	}
+
 	return false
 }
 

internal/lsp/source/completion/format.go

@@ -150,6 +150,11 @@ func (c *completer) item(ctx context.Context, cand candidate) (CompletionItem, e
 		prefix = typeName + "(" + prefix
 		suffix = ")"
 	}
+
+	if cand.takeSlice {
+		suffix += "[:]"
+	}
+
 	// Add variadic "..." only if snippets if enabled or cand is not a function
 	if cand.variadic && (c.opts.snippets || !cand.expandFuncCall) {
 		suffix += "..."

internal/lsp/testdata/arraytype/array_type.go.in

@@ -41,3 +41,8 @@ func _() {
 	var ds [][]myInt
 	ds = [][]m //@complete(" //", atMyInt)
 }
+
+func _() {
+	var b [0]byte //@item(atByte, "b", "[0]byte", "var")
+	var _ []byte = b //@snippet(" //", atByte, "b[:]", "b[:]")
+}

internal/lsp/testdata/summary.txt.golden

@@ -2,7 +2,7 @@
 CallHierarchyCount = 2
 CodeLensCount = 5
 CompletionsCount = 262
-CompletionSnippetCount = 94
+CompletionSnippetCount = 95
 UnimportedCompletionsCount = 5
 DeepCompletionsCount = 5
 FuzzyCompletionsCount = 8