std: add fmt option to escape non-printable characters

lib/std/fmt.zig

@@ -66,6 +66,7 @@ fn peekIsAlign(comptime fmt: []const u8) bool {
 ///   - output numeric value in hexadecimal notation
 /// - `s`: print a pointer-to-many as a c-string, use zero-termination
 /// - `B` and `Bi`: output a memory size in either metric (1000) or power-of-two (1024) based notation. works for both float and integer values.
+/// - `e` and `E`: if printing a string, escape non-printable characters
 /// - `e`: output floating point value in scientific notation
 /// - `d`: output numeric value in decimal notation
 /// - `b`: output integer value in binary notation
@@ -599,6 +600,16 @@ pub fn formatText(
             try formatInt(c, 16, fmt[0] == 'X', FormatOptions{ .width = 2, .fill = '0' }, writer);
         }
         return;
+    } else if (comptime (std.mem.eql(u8, fmt, "e") or std.mem.eql(u8, fmt, "E"))) {
+        for (bytes) |c| {
+            if (std.ascii.isPrint(c)) {
+                try writer.writeByte(c);
+            } else {
+                try writer.writeAll("\\x");
+                try formatInt(c, 16, fmt[0] == 'E', FormatOptions{ .width = 2, .fill = '0' }, writer);
+            }
+        }
+        return;
     } else {
         @compileError("Unknown format string: '" ++ fmt ++ "'");
     }
@@ -1319,6 +1330,12 @@ test "slice" {
     try testFmt("buf: Test\n Other text", "buf: {s}\n Other text", .{"Test"});
 }
 
+test "escape non-printable" {
+    try testFmt("abc", "{e}", .{"abc"});
+    try testFmt("ab\\xffc", "{e}", .{"ab\xffc"});
+    try testFmt("ab\\xFFc", "{E}", .{"ab\xffc"});
+}
+
 test "pointer" {
     {
         const value = @intToPtr(*align(1) i32, 0xdeadbeef);