Rollup merge of #62869 - matklad:feature-gate, r=Mark-Simulacrum

add rustc_private as a proper language feature gate

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?

src/test/ui/feature-gate/rustc-private.rs

@@ -0,0 +1,5 @@
+// gate-test-rustc_private
+
+extern crate libc; //~ ERROR  use of unstable library feature 'rustc_private'
+
+fn main() {}

src/test/ui/feature-gate/rustc-private.stderr

@@ -0,0 +1,12 @@
+error[E0658]: use of unstable library feature 'rustc_private': this crate is being loaded from the sysroot, an unstable location; did you mean to load this crate from crates.io via `Cargo.toml` instead?
+  --> $DIR/rustc-private.rs:3:1
+   |
+LL | extern crate libc;
+   | ^^^^^^^^^^^^^^^^^^
+   |
+   = note: for more information, see https://github.com/rust-lang/rust/issues/27812
+   = help: add `#![feature(rustc_private)]` to the crate attributes to enable
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0658`.