Rustdoc lints on duplicate, unused, and stable features

Author: jyn514

src/librustdoc/core.rs

@@ -232,6 +232,9 @@ pub(crate) fn create_config(
         rustc_lint::builtin::RENAMED_AND_REMOVED_LINTS.name.to_string(),
         rustc_lint::builtin::UNKNOWN_LINTS.name.to_string(),
         rustc_lint::builtin::UNEXPECTED_CFGS.name.to_string(),
+        rustc_lint::builtin::DUPLICATE_FEATURES.name.to_string(),
+        rustc_lint::builtin::UNUSED_FEATURES.name.to_string(),
+        rustc_lint::builtin::STABLE_FEATURES.name.to_string(),
         // this lint is needed to support `#[expect]` attributes
         rustc_lint::builtin::UNFULFILLED_LINT_EXPECTATIONS.name.to_string(),
     ];

tests/rustdoc-ui/issues/issue-79494.rs

@@ -1,6 +1,4 @@
 //@ only-64bit
 
-#![feature(const_transmute)]
-
 pub const ZST: &[u8] = unsafe { std::mem::transmute(1usize) };
 //~^ ERROR transmuting from 8-byte type to 16-byte type

tests/rustdoc-ui/issues/issue-79494.stderr

@@ -1,5 +1,5 @@
 error[E0080]: transmuting from 8-byte type to 16-byte type: `usize` -> `&[u8]`
-  --> $DIR/issue-79494.rs:5:33
+  --> $DIR/issue-79494.rs:3:33
    |
 LL | pub const ZST: &[u8] = unsafe { std::mem::transmute(1usize) };
    |                                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^ evaluation of `ZST` failed here

tests/rustdoc-ui/rustc-check-passes.rs

@@ -1,4 +1,6 @@
+//@ check-pass
+
 #![feature(rustdoc_internals)]
-#![feature(rustdoc_internals)] //~ ERROR
+#![feature(rustdoc_internals)] //~ WARN duplicate
 
 pub fn foo() {}

tests/rustdoc-ui/rustc-check-passes.stderr

@@ -1,9 +1,10 @@
-error[E0636]: the feature `rustdoc_internals` has already been enabled
-  --> $DIR/rustc-check-passes.rs:2:12
+warning: the feature `rustdoc_internals` has already been enabled
+  --> $DIR/rustc-check-passes.rs:4:12
    |
 LL | #![feature(rustdoc_internals)]
    |            ^^^^^^^^^^^^^^^^^
+   |
+   = note: `#[warn(duplicate_features)]` on by default
 
-error: aborting due to 1 previous error
+warning: 1 warning emitted
 
-For more information about this error, try `rustc --explain E0636`.