Add a test for HashStable stability

Author: Zoxc

src/test/ui-fulldeps/hash-stable-is-unstable.rs

@@ -0,0 +1,15 @@
+// ignore-stage1
+
+extern crate rustc_data_structures;
+//~^ use of unstable library feature 'rustc_private'
+extern crate rustc;
+//~^ use of unstable library feature 'rustc_private'
+extern crate rustc_macros;
+//~^ use of unstable library feature 'rustc_private'
+
+use rustc_macros::HashStable;
+//~^ use of unstable library feature 'rustc_private'
+
+#[derive(HashStable)]
+//~^ use of unstable library feature 'rustc_private'
+struct Test;

src/test/ui-fulldeps/hash-stable-is-unstable.stderr

@@ -0,0 +1,48 @@
+error[E0601]: `main` function not found in crate `hash_stable_is_unstable`
+   |
+   = note: consider adding a `main` function to `$DIR/hash-stable-is-unstable.rs`
+
+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? (see issue #27812)
+  --> $DIR/hash-stable-is-unstable.rs:3:1
+   |
+LL | extern crate rustc_data_structures;
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = help: add #![feature(rustc_private)] to the crate attributes to enable
+
+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? (see issue #27812)
+  --> $DIR/hash-stable-is-unstable.rs:5:1
+   |
+LL | extern crate rustc;
+   | ^^^^^^^^^^^^^^^^^^^
+   |
+   = help: add #![feature(rustc_private)] to the crate attributes to enable
+
+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? (see issue #27812)
+  --> $DIR/hash-stable-is-unstable.rs:7:1
+   |
+LL | extern crate rustc_macros;
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = help: add #![feature(rustc_private)] to the crate attributes to enable
+
+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? (see issue #27812)
+  --> $DIR/hash-stable-is-unstable.rs:10:5
+   |
+LL | use rustc_macros::HashStable;
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = help: add #![feature(rustc_private)] to the crate attributes to enable
+
+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? (see issue #27812)
+  --> $DIR/hash-stable-is-unstable.rs:13:10
+   |
+LL | #[derive(HashStable)]
+   |          ^^^^^^^^^^
+   |
+   = help: add #![feature(rustc_private)] to the crate attributes to enable
+
+error: aborting due to 6 previous errors
+
+Some errors occurred: E0601, E0658.
+For more information about an error, try `rustc --explain E0601`.