ast: use string builder for StringInterLiteral.str() (#10207)

vlib/v/ast/str.v

@@ -362,26 +362,26 @@ pub fn (x Expr) str() string {
 			return '__offsetof($x.struct_type, $x.field)'
 		}
 		StringInterLiteral {
-			mut res := []string{}
-			res << "'"
+			mut res := strings.new_builder(50)
+			res.write_string("'")
 			for i, val in x.vals {
-				res << val
+				res.write_string(val)
 				if i >= x.exprs.len {
 					break
 				}
-				res << '$'
+				res.write_string('$')
 				fspec_str, needs_braces := x.get_fspec_braces(i)
 				if needs_braces {
-					res << '{'
-					res << x.exprs[i].str()
-					res << fspec_str
-					res << '}'
+					res.write_string('{')
+					res.write_string(x.exprs[i].str())
+					res.write_string(fspec_str)
+					res.write_string('}')
 				} else {
-					res << x.exprs[i].str()
+					res.write_string(x.exprs[i].str())
 				}
 			}
-			res << "'"
-			return res.join('')
+			res.write_string("'")
+			return res.str()
 		}
 		StringLiteral {
 			return "'$x.val'"

vlib/v/fmt/fmt.v

@@ -2389,7 +2389,6 @@ pub fn (mut f Fmt) string_literal(node ast.StringLiteral) {
 }
 
 pub fn (mut f Fmt) string_inter_literal(node ast.StringInterLiteral) {
-	// TODO: this code is very similar to ast.Expr.str()
 	mut quote := "'"
 	for val in node.vals {
 		if val.contains('\\"') {
@@ -2407,6 +2406,9 @@ pub fn (mut f Fmt) string_inter_literal(node ast.StringInterLiteral) {
 			quote = '"'
 		}
 	}
+	// TODO: this code is very similar to ast.Expr.str()
+	// serkonda7: it can not fully be replaced tho as ´f.expr()´ and `ast.Expr.str()`
+	//	work too different for the various exprs that are interpolated
 	f.write(quote)
 	for i, val in node.vals {
 		f.write(val)