add rustc_private as a proper language feature gate

[matklad]

At the moment, rustc_private as a (library) feature exists by
accident: char::is_xid_start, char::is_xid_continue methods in
libcore define it.

cc https://rust-lang.zulipchat.com/#narrow/stream/131828-t-compiler/topic/How.20to.20declare.20new.20langauge.20feature.3F

I don't know if this is at all reasonable, but at least tests seem to pass locally. That probably means that we can remove/rename to something more resonable the feature in libcore in the next release?

[Mark-Simulacrum]

Did you check if cfg_attr(bootstrap, unstable(...)) would work instead of the current unstable attributes on the xid functions? I'd prefer to merge with that rather than without, or at least understand why it doesn't work to remove them in stage1+.

[matklad]

@Mark-Simulacrum sorry, I am afraid I don't fully understand what you are suggesting. Do you mean applying this patch?

diff --git a/src/libcore/char/methods.rs b/src/libcore/char/methods.rs
index e843303380..6709b9c75b 100644
--- a/src/libcore/char/methods.rs
+++ b/src/libcore/char/methods.rs
@@ -553,9 +553,10 @@ impl char {
     /// 'XID_Start' is a Unicode Derived Property specified in
     /// [UAX #31](http://unicode.org/reports/tr31/#NFKC_Modifications),
     /// mostly similar to `ID_Start` but modified for closure under `NFKx`.
-    #[unstable(feature = "rustc_private",
-               reason = "mainly needed for compiler internals",
-               issue = "27812")]
+    #[cfg_attr(bootstrap,
+               unstable(feature = "rustc_private",
+                        reason = "mainly needed for compiler internals",
+                        issue = "27812"))]
     #[inline]
     pub fn is_xid_start(self) -> bool {
         derived_property::XID_Start(self)
@@ -567,9 +568,10 @@ impl char {
     /// 'XID_Continue' is a Unicode Derived Property specified in
     /// [UAX #31](http://unicode.org/reports/tr31/#NFKC_Modifications),
     /// mostly similar to 'ID_Continue' but modified for closure under NFKx.
-    #[unstable(feature = "rustc_private",
-               reason = "mainly needed for compiler internals",
-               issue = "27812")]
+    #[cfg_attr(bootstrap,
+               unstable(feature = "rustc_private",
+                        reason = "mainly needed for compiler internals",
+                        issue = "27812"))]
     #[inline]
     pub fn is_xid_continue(self) -> bool {
         derived_property::XID_Continue(self)

I am testing this right now, but I am not sure how this'd work: if not bootstrapping, this will make these methods stable? Should I add an cfg_attr(not(bootstrap)) which uses a different feature name (unicode_internals probably)?

[Mark-Simulacrum]

Oh, right, I sort of forgot that we do in fact want that feature gate (i.e., this is not a stabilization). Yeah, let's do not(bootstrap) with unicode_internals or some similar name.

[matklad]

@Mark-Simulacrum should be good now I think

[Mark-Simulacrum]

@bors r+

[bors]

📌 Commit 27b703d has been approved by Mark-Simulacrum

[matklad]

bors r=Mark-Simulacrum

[matklad]

@bors r=Mark-Simulacrum

[bors]

📌 Commit 7e612c1 has been approved by Mark-Simulacrum