internal/lsp/source: fix literal completions in variadic args

We now properly offer literal completions when completing the variadic
parameter. For example:

    func foo(...[]int) {}
    foo(<>) // now offers "[]int{}"

Updates golang/go#37656.

Change-Id: I91c47d455ae3f8051078c82a308f2b5ad4b3d4cd
Reviewed-on: https://go-review.googlesource.com/c/tools/+/222199
Run-TryBot: Muir Manders <muir@mnd.rs>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Rebecca Stambler <rstambler@golang.org>

internal/lsp/source/completion.go

@@ -1052,12 +1052,18 @@ func (c *completer) lexical() error {
 		}
 	}
 
-	if c.inference.objType != nil {
+	if t := c.inference.objType; t != nil {
+		// Use variadic element type if we are completing variadic position.
+		if c.inference.variadicType != nil {
+			t = c.inference.variadicType
+		}
+
+		t = deref(t)
+
 		// If we have an expected type and it is _not_ a named type, see
 		// if an object literal makes a good candidate. For example, if
 		// our expected type is "[]int", this will add a candidate of
 		// "[]int{}".
-		t := deref(c.inference.objType)
 		if _, named := t.(*types.Named); !named {
 			c.literal(t, nil)
 		}

internal/lsp/testdata/lsp/primarymod/snippets/literal_snippets.go.in

@@ -192,3 +192,8 @@ func _() {
 		p: &ptrSt, //@rank(",", litPtrStruct)
 	}
 }
+
+func _() {
+	f := func(...[]int) {}
+	f() //@snippet(")", litIntSlice, "[]int{$0\\}", "[]int{$0\\}")
+}

internal/lsp/testdata/lsp/summary.txt.golden

@@ -1,7 +1,7 @@
 -- summary --
 CodeLensCount = 2
 CompletionsCount = 237
-CompletionSnippetCount = 74
+CompletionSnippetCount = 75
 UnimportedCompletionsCount = 11
 DeepCompletionsCount = 5
 FuzzyCompletionsCount = 8