[Issue 17985] Implement -stdin for rdmd

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=17985

--- Comment #5 from hsteoh at quickfur.ath.cx ---
One advantage `echo ... | rdmd -` has over `echo ... | dmd --run -` is that
rdmd inserts a bunch of standard imports into the code, so that you don't have
to keep typing `import std.stdio; ...` in every echo command.

So yes, this is still a valid ER.

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