Fix #6279

If the replacement rune was multi-byte, `os.replace_path_separators` would silently fail to replace anything.

Now it properly handles non-ASCII separator. Additionally added a fast path for when all runes in the input path as well as the replacement separator are simple ASCII.

Test added.

core/os/path.odin

@@ -29,22 +29,39 @@ with the `new_sep` rune.
 *Allocates Using Provided Allocator*
 */
 replace_path_separators :: proc(path: string, new_sep: rune, allocator: runtime.Allocator) -> (new_path: string, err: Error) {
-	buf := make([]u8, len(path), allocator) or_return
+	length       := len(path)
+	rep_b, rep_w := utf8.encode_rune(new_sep)
+
+	byte_oriented := rep_w == 1
 
-	i: int
 	for r in path {
-		replacement := r
 		if r == '/' || r == '\\' {
-			replacement = new_sep
+			length += rep_w - 1
 		}
+		if r > rune(0x7f) {
+			byte_oriented = false
+		}
+	}
 
-		if replacement <= rune(0x7F) {
-			buf[i] = u8(replacement)
-			i += 1
-		} else {
-			b, w := utf8.encode_rune(r)
-			copy(buf[i:], b[:w])
-			i += w
+	buf := make([]u8, length, allocator) or_return
+
+	if byte_oriented {
+		// Neither replacement rune or any other rune in the path takes up more than 1 byte
+		str := transmute([]byte)path
+		#no_bounds_check for b, i in str {
+			buf[i] = u8(new_sep) if b == '/' || b == '\\' else b
+		}
+	} else {
+		i: int
+		for r in path {
+			if r == '/' || r == '\\' {
+				copy(buf[i:], rep_b[:rep_w])
+				i += rep_w
+			} else {
+				r_b, r_w := utf8.encode_rune(r)
+				copy(buf[i:], r_b[:r_w])
+				i += r_w
+			}
 		}
 	}
 	return string(buf), nil

tests/core/os/path.odin

@@ -99,6 +99,17 @@ test_clean_path :: proc(t: ^testing.T) {
 	}
 }
 
+@(test)
+replace_path_separators :: proc(t: ^testing.T) {
+	P :: "W:/Odin/core/os/path.odin"
+
+	p1, _ := os.replace_path_separators(P, '😀', context.temp_allocator)
+	testing.expect_value(t, p1, "W:😀Odin😀core😀os😀path.odin")
+
+	p2, _ := os.replace_path_separators(P, '|', context.temp_allocator)
+	testing.expect_value(t, p2, "W:|Odin|core|os|path.odin")
+}
+
 @(test)
 test_is_absolute_path :: proc(t: ^testing.T) {
 	when ODIN_OS == .Windows {