Feature gate extern prelude additions from extern crate items

Fix rustdoc and fulldeps tests

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

@@ -0,0 +1,39 @@
+// edition:2018
+
+#![feature(alloc)]
+
+extern crate alloc;
+
+mod in_scope {
+    fn check() {
+        let v = alloc::vec![0];
+        //~^ ERROR use of extern prelude names introduced with `extern crate` items is unstable
+        type A = alloc::boxed::Box<u8>;
+        //~^ ERROR use of extern prelude names introduced with `extern crate` items is unstable
+    }
+}
+
+mod absolute {
+    fn check() {
+        let v = ::alloc::vec![0];
+        //~^ ERROR use of extern prelude names introduced with `extern crate` items is unstable
+        type A = ::alloc::boxed::Box<u8>;
+        //~^ ERROR use of extern prelude names introduced with `extern crate` items is unstable
+    }
+}
+
+mod import_in_scope {
+    use alloc;
+    //~^ ERROR use of extern prelude names introduced with `extern crate` items is unstable
+    use alloc::boxed;
+    //~^ ERROR use of extern prelude names introduced with `extern crate` items is unstable
+}
+
+mod import_absolute {
+    use ::alloc;
+    //~^ ERROR use of extern prelude names introduced with `extern crate` items is unstable
+    use ::alloc::boxed;
+    //~^ ERROR use of extern prelude names introduced with `extern crate` items is unstable
+}
+
+fn main() {}

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

@@ -0,0 +1,67 @@
+error[E0658]: use of extern prelude names introduced with `extern crate` items is unstable (see issue #54658)
+  --> $DIR/feature-gate-extern_crate_item_prelude.rs:26:9
+   |
+LL |     use alloc;
+   |         ^^^^^
+   |
+   = help: add #![feature(extern_crate_item_prelude)] to the crate attributes to enable
+
+error[E0658]: use of extern prelude names introduced with `extern crate` items is unstable (see issue #54658)
+  --> $DIR/feature-gate-extern_crate_item_prelude.rs:28:9
+   |
+LL |     use alloc::boxed;
+   |         ^^^^^
+   |
+   = help: add #![feature(extern_crate_item_prelude)] to the crate attributes to enable
+
+error[E0658]: use of extern prelude names introduced with `extern crate` items is unstable (see issue #54658)
+  --> $DIR/feature-gate-extern_crate_item_prelude.rs:33:11
+   |
+LL |     use ::alloc;
+   |           ^^^^^
+   |
+   = help: add #![feature(extern_crate_item_prelude)] to the crate attributes to enable
+
+error[E0658]: use of extern prelude names introduced with `extern crate` items is unstable (see issue #54658)
+  --> $DIR/feature-gate-extern_crate_item_prelude.rs:35:11
+   |
+LL |     use ::alloc::boxed;
+   |           ^^^^^
+   |
+   = help: add #![feature(extern_crate_item_prelude)] to the crate attributes to enable
+
+error[E0658]: use of extern prelude names introduced with `extern crate` items is unstable (see issue #54658)
+  --> $DIR/feature-gate-extern_crate_item_prelude.rs:9:17
+   |
+LL |         let v = alloc::vec![0];
+   |                 ^^^^^
+   |
+   = help: add #![feature(extern_crate_item_prelude)] to the crate attributes to enable
+
+error[E0658]: use of extern prelude names introduced with `extern crate` items is unstable (see issue #54658)
+  --> $DIR/feature-gate-extern_crate_item_prelude.rs:11:18
+   |
+LL |         type A = alloc::boxed::Box<u8>;
+   |                  ^^^^^
+   |
+   = help: add #![feature(extern_crate_item_prelude)] to the crate attributes to enable
+
+error[E0658]: use of extern prelude names introduced with `extern crate` items is unstable (see issue #54658)
+  --> $DIR/feature-gate-extern_crate_item_prelude.rs:18:19
+   |
+LL |         let v = ::alloc::vec![0];
+   |                   ^^^^^
+   |
+   = help: add #![feature(extern_crate_item_prelude)] to the crate attributes to enable
+
+error[E0658]: use of extern prelude names introduced with `extern crate` items is unstable (see issue #54658)
+  --> $DIR/feature-gate-extern_crate_item_prelude.rs:20:20
+   |
+LL |         type A = ::alloc::boxed::Box<u8>;
+   |                    ^^^^^
+   |
+   = help: add #![feature(extern_crate_item_prelude)] to the crate attributes to enable
+
+error: aborting due to 8 previous errors
+
+For more information about this error, try `rustc --explain E0658`.

src/test/ui/imports/extern-prelude-extern-crate-cfg.rs

@@ -1,6 +1,7 @@
 // compile-pass
 // compile-flags:--cfg my_feature
 
+#![feature(extern_crate_item_prelude)]
 #![no_std]
 
 #[cfg(my_feature)]

src/test/ui/imports/extern-prelude-extern-crate-pass.rs

@@ -1,6 +1,8 @@
 // compile-pass
 // aux-build:two_macros.rs
 
+#![feature(extern_crate_item_prelude)]
+
 extern crate two_macros;
 
 mod m {

src/test/ui/imports/extern-prelude-extern-crate-restricted-shadowing.rs

@@ -1,5 +1,7 @@
 // aux-build:two_macros.rs
 
+#![feature(extern_crate_item_prelude)]
+
 macro_rules! define_vec {
     () => {
         extern crate std as Vec;

src/test/ui/imports/extern-prelude-extern-crate-restricted-shadowing.stderr

@@ -1,11 +1,11 @@
 error[E0659]: `Vec` is ambiguous
-  --> $DIR/extern-prelude-extern-crate-restricted-shadowing.rs:13:9
+  --> $DIR/extern-prelude-extern-crate-restricted-shadowing.rs:15:9
    |
 LL |         Vec::panic!(); //~ ERROR `Vec` is ambiguous
    |         ^^^ ambiguous name
    |
 note: `Vec` could refer to the name defined here
-  --> $DIR/extern-prelude-extern-crate-restricted-shadowing.rs:5:9
+  --> $DIR/extern-prelude-extern-crate-restricted-shadowing.rs:7:9
    |
 LL |         extern crate std as Vec;
    |         ^^^^^^^^^^^^^^^^^^^^^^^^