gopls/internal: check for variadic arguments from string to []byte/[]rune

gopls will correctly convert a string argument to a []byte/[]rune argument,
however it will not check if the the target is also variadic.

gopls/internal/golang/completion/completion.go

@@ -52,7 +52,6 @@ import (
 
 // A CompletionItem represents a possible completion suggested by the algorithm.
 type CompletionItem struct {
-
 	// Invariant: CompletionItem does not refer to syntax or types.
 
 	// Label is the primary text the user sees for this completion item.
@@ -2965,6 +2964,13 @@ func (ci *candidateInference) candTypeMatches(cand *candidate) bool {
 				// If candType doesn't otherwise match, consider if we can
 				// convert candType directly to expType.
 				if considerTypeConversion(candType, expType, cand.path) {
+					// special case: a string to a variadic []byte/[]rune
+					typ := deslice(expType)
+					isBytesOrRunes := typ != nil && (isBasicKind(typ, types.Byte) || isBasicKind(typ, types.Rune))
+					if isBasicType(candType, types.IsString) && isBytesOrRunes && expType == variadicType {
+						cand.mods = append(cand.mods, takeDotDotDot)
+					}
+
 					cand.convertTo = expType
 					// Give a major score penalty so we always prefer directly
 					// assignable candidates, all else equal.
@@ -3045,7 +3051,7 @@ func considerTypeConversion(from, to types.Type, path []types.Object) bool {
 
 	// Don't offer to convert ints to strings since that probably
 	// doesn't do what the user wants.
-	if isBasicKind(from, types.IsInteger) && isBasicKind(to, types.IsString) {
+	if isBasicType(from, types.IsInteger) && isBasicType(to, types.IsString) {
 		return false
 	}
 

gopls/internal/golang/completion/format.go

@@ -182,7 +182,6 @@ Suffixes:
 	// add the additional text edits needed.
 	if cand.imp != nil {
 		addlEdits, err := c.importEdits(cand.imp)
-
 		if err != nil {
 			return CompletionItem{}, err
 		}
@@ -214,7 +213,7 @@ Suffixes:
 		}
 
 		prefix = typeName + "(" + prefix
-		suffix = ")"
+		suffix = ")" + suffix
 	}
 
 	if prefix != "" {

gopls/internal/golang/completion/util.go

@@ -300,10 +300,15 @@ func formatZeroValue(T types.Type, qf types.Qualifier) string {
 	}
 }
 
-// isBasicKind returns whether t is a basic type of kind k.
-func isBasicKind(t types.Type, k types.BasicInfo) bool {
+// isBasicType returns whether t has property information i.
+func isBasicType(t types.Type, i types.BasicInfo) bool {
 	b, _ := t.Underlying().(*types.Basic)
-	return b != nil && b.Info()&k > 0
+	return b != nil && b.Info()&i > 0
+}
+
+func isBasicKind(t types.Type, k types.BasicKind) bool {
+	b, _ := t.Underlying().(*types.Basic)
+	return b != nil && b.Kind() == k
 }
 
 func (c *completer) editText(from, to token.Pos, newText string) ([]protocol.TextEdit, error) {