Adds range subscripting for strings.

test/string/subscript_not_num.wren

@@ -1,2 +1,2 @@
 var a = "123"
-a["2"] // expect runtime error: Subscript must be a number.
+a["2"] // expect runtime error: Subscript must be a number or a range.

test/string/subscript_range.wren

@@ -0,0 +1,34 @@
+var string = "abcde"
+IO.print(string[0..0]) // expect: a
+IO.print(string[1...1] == "") // expect: true
+IO.print(string[1..2]) // expect: bc
+IO.print(string[1...2]) // expect: b
+IO.print(string[2..4]) // expect: cde
+IO.print(string[2...5]) // expect: cde
+
+// A backwards range reverses.
+IO.print(string[3..1]) // expect: dcb
+IO.print(string[3...1]) // expect: dc
+IO.print(string[3...3] == "") // expect: true
+
+// Negative ranges index from the end.
+IO.print(string[-5..-2]) // expect: abcd
+IO.print(string[-5...-2]) // expect: abc
+IO.print(string[-3..-5]) // expect: cba
+IO.print(string[-3...-6]) // expect: cba
+
+// Half-negative ranges are treated like the negative value is fixed before
+// walking the range.
+IO.print(string[-5..3]) // expect: abcd
+IO.print(string[-3...5]) // expect: cde
+IO.print(string[-2..1]) // expect: dcb
+IO.print(string[-2...0]) // expect: dcb
+
+IO.print(string[1..-2]) // expect: bcd
+IO.print(string[2...-1]) // expect: cd
+IO.print(string[4..-5]) // expect: edcba
+IO.print(string[3...-6]) // expect: dcba
+
+// An empty range at zero is allowed on an empty string.
+IO.print(""[0...0] == "") // expect: true
+IO.print(""[0..-1] == "") // expect: true

test/string/subscript_range_from_not_int.wren

@@ -0,0 +1,2 @@
+var a = "string"
+a[1.5..2] // expect runtime error: Range start must be an integer.

test/string/subscript_range_from_too_large.wren

@@ -0,0 +1,2 @@
+var a = "123"
+a[3..2] // expect runtime error: Range start out of bounds.

test/string/subscript_range_from_too_small.wren

@@ -0,0 +1,2 @@
+var a = "123"
+a[-4..2] // expect runtime error: Range start out of bounds.

test/string/subscript_range_to_exclusive_too_large.wren

@@ -0,0 +1,2 @@
+var a = "123"
+a[1...4] // expect runtime error: Range end out of bounds.

test/string/subscript_range_to_exclusive_too_small.wren

@@ -0,0 +1,2 @@
+var a = "123"
+a[0...-5] // expect runtime error: Range end out of bounds.

test/string/subscript_range_to_not_int.wren

@@ -0,0 +1,2 @@
+var a = "string"
+a[1..2.5] // expect runtime error: Range end must be an integer.

test/string/subscript_range_to_too_large.wren

@@ -0,0 +1,2 @@
+var a = "123"
+a[1..3] // expect runtime error: Range end out of bounds.

test/string/subscript_range_to_too_small.wren

@@ -0,0 +1,2 @@
+var a = "123"
+a[0..-4] // expect runtime error: Range end out of bounds.