starlark: treat {+1} in format string as key not number (google#115)

Fixes google#114

starlark/library.go

@@ -1724,7 +1724,7 @@ func string_format(fnname string, recv_ Value, args Tuple, kwargs []Tuple) (Valu
 			}
 			arg = args[index]
 			index++
-		} else if num, err := strconv.Atoi(name); err == nil && !strings.HasPrefix(name, "-") {
+		} else if num, ok := decimal(name); ok {
 			// positional argument
 			if auto {
 				return nil, fmt.Errorf("cannot switch from automatic field numbering to manual field specification")
@@ -1780,6 +1780,22 @@ func string_format(fnname string, recv_ Value, args Tuple, kwargs []Tuple) (Valu
 	return String(buf.String()), nil
 }
 
+// decimal interprets s as a sequence of decimal digits.
+func decimal(s string) (x int, ok bool) {
+	n := len(s)
+	for i := 0; i < n; i++ {
+		digit := s[i] - '0'
+		if digit > 9 {
+			return 0, false
+		}
+		x = x*10 + int(digit)
+		if x < 0 {
+			return 0, false // underflow
+		}
+	}
+	return x, true
+}
+
 // https://github.com/google/starlark-go/blob/master/doc/spec.md#string·index
 func string_index(fnname string, recv Value, args Tuple, kwargs []Tuple) (Value, error) {
 	return string_find_impl(fnname, string(recv.(String)), args, kwargs, false, false)

starlark/testdata/string.star

@@ -174,6 +174,10 @@ assert.eq("a{x}b{y}c{}".format(1, x=2, y=3), "a2b3c1")
 assert.fails(lambda: "a{z}b".format(x=1), "keyword z not found")
 assert.fails(lambda: "{-1}".format(1), "keyword -1 not found")
 assert.fails(lambda: "{-0}".format(1), "keyword -0 not found")
+assert.fails(lambda: "{+0}".format(1), "keyword \\+0 not found")
+assert.fails(lambda: "{+1}".format(1), "keyword \\+1 not found") # starlark-go/issues/114
+assert.eq("{0000000000001}".format(0, 1), "1")
+assert.eq("{012}".format(*range(100)), "12") # decimal, despite leading zeros
 assert.fails(lambda: '{0,1} and {1}'.format(1, 2), "keyword 0,1 not found")
 assert.fails(lambda: "a{123}b".format(), "tuple index out of range")
 assert.fails(lambda: "a{}b{}c".format(1), "tuple index out of range")