[Issue 22776] New: string literal printing fails on non-ASCII/non-printable chars

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=22776

          Issue ID: 22776
           Summary: string literal printing fails on
                    non-ASCII/non-printable chars
           Product: D
           Version: D2
          Hardware: All
                OS: All
            Status: NEW
          Severity: normal
          Priority: P1
         Component: dmd
          Assignee: nobody at puremagic.com
          Reporter: dkorpel at live.nl

When dmd prints a string literal in an error message it doesn't handle
non-trivial cases properly:
```
"before \0 after";
"\n";
```

Error: declaration expected, not `"before "`
Error: declaration expected, not `"\x0a"`

It truncates at zero bytes and uses \x00 codes for everything non-printable,
even when there are more readable escape sequences available (most commonly \n
for \x0A).

non-ASCII code units in wstrings and dstrings also get messed up in semantic
errors:

```
pragma(msg, "\u1234"w.stringof); // "\x34\x12"w
pragma(msg, "\u1234"d.stringof); // "\x34\x12"d
```

The lexer handles these correctly by emitting \u sequences instead of \x
sequences.

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