Add strings.remove{pre,suf}fix methods (google#395)

* Add strings.remove{pre,suf}fix methods

* Avoid constant 6

doc/spec.md

@@ -175,6 +175,8 @@ reproducibility is paramount, such as build tools.
     * [string·lower](#string·lower)
     * [string·lstrip](#string·lstrip)
     * [string·partition](#string·partition)
+    * [string·removeprefix](#string·removeprefix)
+    * [string·removesuffix](#string·removesuffix)
     * [string·replace](#string·replace)
     * [string·rfind](#string·rfind)
     * [string·rindex](#string·rindex)
@@ -680,6 +682,8 @@ Strings have several built-in methods:
 * [`lstrip`](#string·lstrip)
 * [`partition`](#string·partition)
 * [`replace`](#string·replace)
+* [`removeprefix`](#string·removeprefix)
+* [`removesuffix`](#string·removesuffix)
 * [`rfind`](#string·rfind)
 * [`rindex`](#string·rindex)
 * [`rpartition`](#string·rpartition)
@@ -4026,6 +4030,30 @@ If S does not contain `x`, `partition` returns `(S, "", "")`.
 "one/two/three".partition("/")		# ("one", "/", "two/three")
 ```
 
+<a id='string·removeprefix'></a>
+### string·removeprefix
+
+`S.removeprefix(prefix)` returns a copy of string S with the prefix `prefix`
+removed if S starts with `prefix`, otherwise it returns S.
+
+```python
+"banana".removeprefix("ban")          # "ana"
+"banana".removeprefix("foo")          # "banana"
+"foofoobar".removeprefix("foo")       # "foobar"
+```
+
+<a id='string·removesuffix'></a>
+### string·removesuffix
+
+`S.removesuffix(suffix)` returns a copy of string S with the suffix `suffix`
+removed if S ends with `suffix`, otherwise it returns S.
+
+```python
+"banana".removesuffix("nana")         # "ba"
+"banana".removesuffix("foo")          # "banana"
+"banana".removesuffix("na")           # "bana"
+```
+
 <a id='string·replace'></a>
 ### string·replace
 

starlark/library.go

@@ -123,6 +123,8 @@ var (
 		"lower":          NewBuiltin("lower", string_lower),
 		"lstrip":         NewBuiltin("lstrip", string_strip), // sic
 		"partition":      NewBuiltin("partition", string_partition),
+		"removeprefix":   NewBuiltin("removeprefix", string_removefix),
+		"removesuffix":   NewBuiltin("removesuffix", string_removefix),
 		"replace":        NewBuiltin("replace", string_replace),
 		"rfind":          NewBuiltin("rfind", string_rfind),
 		"rindex":         NewBuiltin("rindex", string_rindex),
@@ -1889,6 +1891,22 @@ func string_partition(_ *Thread, b *Builtin, args Tuple, kwargs []Tuple) (Value,
 	return tuple, nil
 }
 
+// https://github.com/google/starlark-go/blob/master/doc/spec.md#string·removeprefix
+// https://github.com/google/starlark-go/blob/master/doc/spec.md#string·removesuffix
+func string_removefix(_ *Thread, b *Builtin, args Tuple, kwargs []Tuple) (Value, error) {
+	recv := string(b.Receiver().(String))
+	var fix string
+	if err := UnpackPositionalArgs(b.Name(), args, kwargs, 1, &fix); err != nil {
+		return nil, err
+	}
+	if b.name[len("remove")] == 'p' {
+		recv = strings.TrimPrefix(recv, fix)
+	} else {
+		recv = strings.TrimSuffix(recv, fix)
+	}
+	return String(recv), nil
+}
+
 // https://github.com/google/starlark-go/blob/master/doc/spec.md#string·replace
 func string_replace(_ *Thread, b *Builtin, args Tuple, kwargs []Tuple) (Value, error) {
 	recv := string(b.Receiver().(String))

starlark/testdata/string.star

@@ -470,3 +470,24 @@ assert.true(not "DŽenan LJubović".istitle())
 assert.fails(lambda: "".starts_with, "no .starts_with field.*did you mean .startswith")
 assert.fails(lambda: "".StartsWith, "no .StartsWith field.*did you mean .startswith")
 assert.fails(lambda: "".fin, "no .fin field.*.did you mean .find")
+
+
+# removesuffix
+assert.eq("Apricot".removesuffix("cot"), "Apri")
+assert.eq("Apricot".removesuffix("Cot"), "Apricot")
+assert.eq("Apricot".removesuffix("t"), "Aprico")
+assert.eq("a".removesuffix(""), "a")
+assert.eq("".removesuffix(""), "")
+assert.eq("".removesuffix("a"), "")
+assert.eq("Apricot".removesuffix("co"), "Apricot")
+assert.eq("Apricotcot".removesuffix("cot"), "Apricot")
+
+# removeprefix
+assert.eq("Apricot".removeprefix("Apr"), "icot")
+assert.eq("Apricot".removeprefix("apr"), "Apricot")
+assert.eq("Apricot".removeprefix("A"), "pricot")
+assert.eq("a".removeprefix(""), "a")
+assert.eq("".removeprefix(""), "")
+assert.eq("".removeprefix("a"), "")
+assert.eq("Apricot".removeprefix("pr"), "Apricot")
+assert.eq("AprApricot".removeprefix("Apr"), "Apricot")