const_eval: implies_by in rustc_const_unstable

Extend support for `implies_by` (from `#[stable]` and `#[unstable]`)
to `#[rustc_const_stable]` and `#[rustc_const_unstable]`.

Signed-off-by: David Wood <david.wood@huawei.com>

tests/ui/stability-attribute/auxiliary/const-stability-attribute-implies.rs

@@ -0,0 +1,12 @@
+#![crate_type = "lib"]
+#![feature(staged_api)]
+#![stable(feature = "stability_attribute_implies", since = "1.0.0")]
+#![rustc_const_stable(feature = "stability_attribute_implies", since = "1.0.0")]
+
+#[stable(feature = "stability_attribute_implies", since = "1.0.0")]
+#[rustc_const_stable(feature = "const_foo", since = "1.62.0")]
+pub const fn foo() {}
+
+#[stable(feature = "stability_attribute_implies", since = "1.0.0")]
+#[rustc_const_unstable(feature = "const_foobar", issue = "1", implied_by = "const_foo")]
+pub const fn foobar() {}

tests/ui/stability-attribute/const-stability-attribute-implies-missing.rs

@@ -0,0 +1,16 @@
+#![crate_type = "lib"]
+#![feature(staged_api)]
+#![stable(feature = "stability_attribute_implies", since = "1.0.0")]
+#![rustc_const_stable(feature = "stability_attribute_implies", since = "1.0.0")]
+
+// Tests that `implied_by = "const_bar"` results in an error being emitted if `const_bar` does not
+// exist.
+
+#[stable(feature = "stability_attribute_implies", since = "1.0.0")]
+#[rustc_const_unstable(feature = "const_foobar", issue = "1", implied_by = "const_bar")]
+//~^ ERROR feature `const_bar` implying `const_foobar` does not exist
+pub const fn foobar() -> u32 {
+    0
+}
+
+const VAR: u32 = foobar();

tests/ui/stability-attribute/const-stability-attribute-implies-missing.stderr

@@ -0,0 +1,8 @@
+error: feature `const_bar` implying `const_foobar` does not exist
+  --> $DIR/const-stability-attribute-implies-missing.rs:10:1
+   |
+LL | #[rustc_const_unstable(feature = "const_foobar", issue = "1", implied_by = "const_bar")]
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: aborting due to previous error
+

tests/ui/stability-attribute/const-stability-attribute-implies-no-feature.rs

@@ -0,0 +1,16 @@
+// aux-build:const-stability-attribute-implies.rs
+#![crate_type = "lib"]
+
+// Tests that despite the `const_foobar` feature being implied by now-stable feature `const_foo`,
+// if `const_foobar` isn't allowed in this crate then an error will be emitted.
+
+extern crate const_stability_attribute_implies;
+use const_stability_attribute_implies::{foo, foobar};
+
+pub const fn bar() -> u32 {
+    foo(); // no error - stable
+    foobar(); //~ ERROR `foobar` is not yet stable as a const fn
+    0
+}
+
+pub const VAR: u32 = bar();

tests/ui/stability-attribute/const-stability-attribute-implies-no-feature.stderr

@@ -0,0 +1,10 @@
+error: `foobar` is not yet stable as a const fn
+  --> $DIR/const-stability-attribute-implies-no-feature.rs:12:5
+   |
+LL |     foobar();
+   |     ^^^^^^^^
+   |
+   = help: add `#![feature(const_foobar)]` to the crate attributes to enable
+
+error: aborting due to previous error
+

tests/ui/stability-attribute/const-stability-attribute-implies-using-stable.rs

@@ -0,0 +1,19 @@
+// aux-build:const-stability-attribute-implies.rs
+#![crate_type = "lib"]
+#![deny(stable_features)]
+#![feature(const_foo)]
+//~^ ERROR the feature `const_foo` has been partially stabilized since 1.62.0 and is succeeded by the feature `const_foobar`
+
+// Tests that the use of `implied_by` in the `#[rustc_const_unstable]` attribute results in a
+// diagnostic mentioning partial stabilization, and that given the implied unstable feature is
+// unused (there is no `foobar` call), that the compiler suggests removing the flag.
+
+extern crate const_stability_attribute_implies;
+use const_stability_attribute_implies::foo;
+
+pub const fn bar() -> u32 {
+    foo();
+    0
+}
+
+pub const VAR: u32 = bar();

tests/ui/stability-attribute/const-stability-attribute-implies-using-stable.stderr

@@ -0,0 +1,22 @@
+error: the feature `const_foo` has been partially stabilized since 1.62.0 and is succeeded by the feature `const_foobar`
+  --> $DIR/const-stability-attribute-implies-using-stable.rs:4:12
+   |
+LL | #![feature(const_foo)]
+   |            ^^^^^^^^^
+   |
+note: the lint level is defined here
+  --> $DIR/const-stability-attribute-implies-using-stable.rs:3:9
+   |
+LL | #![deny(stable_features)]
+   |         ^^^^^^^^^^^^^^^
+help: if you are using features which are still unstable, change to using `const_foobar`
+   |
+LL | #![feature(const_foobar)]
+   |            ~~~~~~~~~~~~
+help: if you are using features which are now stable, remove this line
+   |
+LL - #![feature(const_foo)]
+   |
+
+error: aborting due to previous error
+

tests/ui/stability-attribute/const-stability-attribute-implies-using-unstable.rs

@@ -0,0 +1,21 @@
+// aux-build:const-stability-attribute-implies.rs
+#![crate_type = "lib"]
+#![deny(stable_features)]
+#![feature(const_foo)]
+//~^ ERROR the feature `const_foo` has been partially stabilized since 1.62.0 and is succeeded by the feature `const_foobar`
+
+// Tests that the use of `implied_by` in the `#[rustc_const_unstable]` attribute results in a
+// diagnostic mentioning partial stabilization and that given the implied unstable feature is
+// used (there is a `const_foobar` call), that the compiler suggests changing to that feature and
+// doesn't error about its use.
+
+extern crate const_stability_attribute_implies;
+use const_stability_attribute_implies::{foo, foobar};
+
+pub const fn bar() -> u32 {
+    foo();
+    foobar(); // no error!
+    0
+}
+
+pub const VAR: u32 = bar();

tests/ui/stability-attribute/const-stability-attribute-implies-using-unstable.stderr

@@ -0,0 +1,22 @@
+error: the feature `const_foo` has been partially stabilized since 1.62.0 and is succeeded by the feature `const_foobar`
+  --> $DIR/const-stability-attribute-implies-using-unstable.rs:4:12
+   |
+LL | #![feature(const_foo)]
+   |            ^^^^^^^^^
+   |
+note: the lint level is defined here
+  --> $DIR/const-stability-attribute-implies-using-unstable.rs:3:9
+   |
+LL | #![deny(stable_features)]
+   |         ^^^^^^^^^^^^^^^
+help: if you are using features which are still unstable, change to using `const_foobar`
+   |
+LL | #![feature(const_foobar)]
+   |            ~~~~~~~~~~~~
+help: if you are using features which are now stable, remove this line
+   |
+LL - #![feature(const_foo)]
+   |
+
+error: aborting due to previous error
+