Add Default implementation for &CStr, CString, Path

[tbu-]

No description provided.

[rust-highfive]

r? @aturon

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[tbu-]

Split off from #32785. Some motivation: #32785 (comment).

[alexcrichton]

CString seems fine to me personally as CString::new is the "clear and obvious constructor", the others seem conventionally fine but not perhaps well motivated.

[tbu-]

"" is the default value for &str, I'd argue that the equivalent value should be the default value for &CStr.

[GuillaumeGomez]

We just hit 1.8.0 release, so it becomes since = 1.10.0 (if you could confirm @alexcrichton?).

[nagisa]

I’m not sure about "" as the default path. You can’t even do anything sensible with it (at least on linux), other than join to other paths.

[tbu-]

Dunno, maybe base.join(config_option.unwrap_or_default())

[nagisa]

@tbu- for that purpose an equivalent of UNIX’s . has the same behaviour.

EDIT: in other words, I think this is too much a bikesheddable part to be included without an RFC.

[tbu-]

@nagisa Not for verbatim paths on Windows.

[aturon]

I'm fine with the CString-related defaults, but agree that the one for Path is probably more problematic than it's worth.

[alexcrichton]

The libs team discussed this PR during triage today and the conclusion was that the Default implementations for CStr and CString seem fine, but as mentioned previously we'd prefer that Path were left out for now.

Could you make those changes @tbu- and I'll r+? Thanks again for the PR!

[tbu-]

@alexcrichton Changed.

[alexcrichton]

@bors: r+ 5efe083

[bors]

⌛ Testing commit 5efe083 with merge 77987ba...

[bors]

☀️ Test successful - auto-linux-32-nopt-t, auto-linux-32-opt, auto-linux-32cross-opt, auto-linux-64-cargotest, auto-linux-64-cross-armhf, auto-linux-64-cross-armsf, auto-linux-64-cross-freebsd, auto-linux-64-cross-netbsd, auto-linux-64-debug-opt, auto-linux-64-nopt-t, auto-linux-64-opt, auto-linux-64-opt-mir, auto-linux-64-opt-rustbuild, auto-linux-64-x-android-t, auto-linux-cross-opt, auto-linux-musl-64-opt, auto-mac-32-opt, auto-mac-64-nopt-t, auto-mac-64-opt, auto-mac-64-opt-rustbuild, auto-mac-cross-ios-opt, auto-win-gnu-32-nopt-t, auto-win-gnu-32-opt, auto-win-gnu-32-opt-rustbuild, auto-win-gnu-64-nopt-t, auto-win-gnu-64-opt, auto-win-msvc-32-cross-opt, auto-win-msvc-32-opt, auto-win-msvc-64-cargotest, auto-win-msvc-64-opt, auto-win-msvc-64-opt-mir, auto-win-msvc-64-opt-rustbuild