[Issue 24015] New: C#-style indented delimited strings

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=24015

          Issue ID: 24015
           Summary: C#-style indented delimited strings
           Product: D
           Version: D2
          Hardware: All
                OS: All
            Status: NEW
          Severity: enhancement
          Priority: P1
         Component: dmd
          Assignee: nobody at puremagic.com
          Reporter: qs.il.paperinik at gmail.com

https://dlang.org/spec/lex.html#delimited_strings allows identifier-delimited
strings like
```
q"EOS
This
is a multi-line
heredoc string
EOS"
```
However, that “The closing identifier must be placed on its own line at the
leftmost column” is annoying and ugly in code w.r.t. indentation. For example:
```d
void f()
{
    string str = q"EOS
This
is a multi-line
heredoc string
EOS";
}
```

C# has triple-quote strings that are similar to D’s identifier-delimited
strings as if `""` were the identifier, however, C# allows the string to be in
one line and if it is not, the whitespace between the last newline and the
closing """ is not considered part of the string – and, of course, must be
present at the beginning of every line of the string.

For the sake of readability, D should have something similar: If the sequence
`q"` *end-of-string-identifier* *newline* is followed by whitespace (a sequence
of whitespace characters), the whitespace is repeated on every line up until a
line consists of the whitespace and the *end-of-string-identifier* `"` string
terminator, that is considered a string literal. The line-initial whitespace is
not part of the literal.

E.g.:
```d
void f()
{
    string str = q"EOS
        This
        is a multi-line
        heredoc string
        EOS";
    assert(str == "This\nis a multi-line\nheredoc string\n");
}
```

Theoretically, this is a breaking change, but it will be very unlikely that the
string termination sequence will be anywhere in the string, let alone after
whitespace that is repeated on every line before.

Alternatively, D can go “full C#” and just implement `"""` delimited strings
with the exact same semantics as C#:
https://learn.microsoft.com/en-us/dotnet/csharp/language-reference/tokens/raw-string

--