Add new tests.

src/test/ui/feature-gates/feature-gate-rustc_private-libc.rs

@@ -0,0 +1,4 @@
+fn main() {
+    extern crate libc; //~ ERROR use of unstable
+    use libc::*; //~ ERROR unresolved import
+}

src/test/ui/feature-gates/feature-gate-rustc_private-libc.stderr

@@ -0,0 +1,18 @@
+error[E0432]: unresolved import `libc`
+  --> $DIR/feature-gate-rustc_private-libc.rs:3:9
+   |
+LL |     use libc::*;
+   |         ^^^^ maybe a missing `extern crate libc;`?
+
+error[E0658]: use of unstable library feature 'rustc_private': crate "libc" is being loaded from the sysroot, an unstable location; did you mean to load this crate from crates.io via `Cargo.toml` instead? (see issue #27812)
+  --> $DIR/feature-gate-rustc_private-libc.rs:2:5
+   |
+LL |     extern crate libc;
+   |     ^^^^^^^^^^^^^^^^^^
+   |
+   = help: add #![feature(rustc_private)] to the crate attributes to enable
+
+error: aborting due to 2 previous errors
+
+Some errors occurred: E0432, E0658.
+For more information about an error, try `rustc --explain E0432`.

src/test/ui/feature-gates/feature-gate-test.rs

@@ -0,0 +1,4 @@
+fn main() {
+    extern crate test; //~ ERROR use of unstable
+    use test::*; //~ ERROR unresolved import
+}

src/test/ui/feature-gates/feature-gate-test.stderr

@@ -0,0 +1,18 @@
+error[E0432]: unresolved import `test`
+  --> $DIR/feature-gate-test.rs:3:9
+   |
+LL |     use test::*;
+   |         ^^^^ maybe a missing `extern crate test;`?
+
+error[E0658]: use of unstable library feature 'test' (see issue #27812)
+  --> $DIR/feature-gate-test.rs:2:5
+   |
+LL |     extern crate test;
+   |     ^^^^^^^^^^^^^^^^^^
+   |
+   = help: add #![feature(test)] to the crate attributes to enable
+
+error: aborting due to 2 previous errors
+
+Some errors occurred: E0432, E0658.
+For more information about an error, try `rustc --explain E0432`.

src/test/ui/issues/issue-37887.stderr

@@ -4,7 +4,7 @@ error[E0432]: unresolved import `libc`
 LL |     use libc::*;
    |         ^^^^ maybe a missing `extern crate libc;`?
 
-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?
+error[E0658]: use of unstable library feature 'rustc_private': crate "libc" is being loaded from the sysroot, an unstable location; did you mean to load this crate from crates.io via `Cargo.toml` instead? (see issue #27812)
   --> $DIR/issue-37887.rs:2:5
    |
 LL |     extern crate libc;