std.path: Fix tilde expansion for HOME=/

[kinke]

That's the case for CirrusCI/FreeBSD - the user is root, the USER env var unset and HOME set to /, and the tilde expansion for ~ in that case led to an empty string and according failures in

phobos/std/path.d

Lines 4158 to 4163 in 95e4fb4

	immutable tildeUser = "~" ~ environment.get("USER");
	immutable path = executeShell("echo " ~ tildeUser).output.strip();
	immutable expTildeUser = expandTilde(tildeUser);
	assert(expTildeUser == path, expTildeUser);
	immutable expTildeUserSlash = expandTilde(tildeUser ~ "/");
	assert(expTildeUserSlash == path ~ "/", expTildeUserSlash);

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[Geod24]

Looks like a no-brainer. Don't you want it in stable ? Also I'm not sure copying bash's behavior for ~/ is the most sensible, as it will be the only place where // is produced, will it not ?

[kinke]

stable would be somewhat easier for me/LDC, but as that bug has probably been there for ages, I thought it'd be safer to target master. No strong opinion though.

As to 'copying bash's behavior for ~/' - I took care of that first, but then realized that the failing test would then fail again, because it unconditionally expects an added slash compared to ~ alone. While the test can of course be adapted for this (or that whole test only exected if USER is set - although it revealed this bug), I've only later seen that bash doesn't try to get rid of duplicate dir separators at all, unlike Phobos here, so HOME=/ is no special case for bash. Open for suggestions wrt. how to proceed.

[kinke]

D:\a\1\dmd\test>"D:\a\1\dmd\dmd2\windows\bin\dmd.exe" -m32mscoff -i run.d || exit /B 6
D:\a\1\dmd\test>run.exe all || exit /B 6
core.exception.AssertError@D:\a\1\dmd\dmd2\windows\bin....\src\phobos\std\algorithm\mutation.d(2790): Swap: lhs internal pointer.

This also happens consistently when running the run testdriver for LDC's AArch64 CI. Definitely needs some investigation.