Add `internal_features` lint

It lints against features that are inteded to be internal to the
compiler and standard library. Implements MCP #596.

We allow `internal_features` in the standard library and compiler as those
use many features and this _is_ the standard library from the "internal to the compiler and
standard library" after all.

Marking some features as internal wasn't exactly the most scientific approach, I just marked some
mostly obvious features. While there is a categorization in the macro,
it's not very well upheld (should probably be fixed in another PR).

We always pass `-Ainternal_features` in the testsuite
About 400 UI tests and several other tests use internal features.
Instead of throwing the attribute on each one, just always allow them.
There's nothing wrong with testing internal features^^

45 files changed, 169 insertions, 42 deletions

diff --git a/compiler/rustc_abi/src/lib.rs b/compiler/rustc_abi/src/lib.rs
index 1442747fe1e..12dd1542d79 100644
--- a/compiler/rustc_abi/src/lib.rs
+++ b/compiler/rustc_abi/src/lib.rs
@@ -1,4 +1,5 @@
 #![cfg_attr(feature = "nightly", feature(step_trait, rustc_attrs, min_specialization))]
+#![cfg_attr(all(not(bootstrap), feature = "nightly"), allow(internal_features))]
 
 use std::fmt;
 #[cfg(feature = "nightly")]
diff --git a/compiler/rustc_arena/src/lib.rs b/compiler/rustc_arena/src/lib.rs
index ba47ebd68cb..f4900ece1cc 100644
--- a/compiler/rustc_arena/src/lib.rs
+++ b/compiler/rustc_arena/src/lib.rs
@@ -23,6 +23,7 @@
 #![deny(unsafe_op_in_unsafe_fn)]
 #![deny(rustc::untranslatable_diagnostic)]
 #![deny(rustc::diagnostic_outside_of_impl)]
+#![cfg_attr(not(bootstrap), allow(internal_features))]
 #![allow(clippy::mut_from_ref)] // Arena allocators are one of the places where this pattern is fine.
 
 use smallvec::SmallVec;
diff --git a/compiler/rustc_borrowck/src/lib.rs b/compiler/rustc_borrowck/src/lib.rs
index 317af8ae1f6..efe525c224d 100644
--- a/compiler/rustc_borrowck/src/lib.rs
+++ b/compiler/rustc_borrowck/src/lib.rs
@@ -11,6 +11,7 @@
 #![feature(trusted_step)]
 #![feature(try_blocks)]
 #![recursion_limit = "256"]
+#![cfg_attr(not(bootstrap), allow(internal_features))]
 
 #[macro_use]
 extern crate rustc_middle;
diff --git a/compiler/rustc_data_structures/src/lib.rs b/compiler/rustc_data_structures/src/lib.rs
index 3deb9c5c2f5..33772089744 100644
--- a/compiler/rustc_data_structures/src/lib.rs
+++ b/compiler/rustc_data_structures/src/lib.rs
@@ -37,6 +37,7 @@
 #![allow(rustc::potential_query_instability)]
 #![deny(rustc::untranslatable_diagnostic)]
 #![deny(rustc::diagnostic_outside_of_impl)]
+#![cfg_attr(not(bootstrap), allow(internal_features))]
 #![deny(unsafe_op_in_unsafe_fn)]
 
 #[macro_use]
diff --git a/compiler/rustc_error_codes/src/error_codes/E0092.md b/compiler/rustc_error_codes/src/error_codes/E0092.md
index 5cbe2a188b0..84ec0656d1a 100644
--- a/compiler/rustc_error_codes/src/error_codes/E0092.md
+++ b/compiler/rustc_error_codes/src/error_codes/E0092.md
@@ -4,6 +4,7 @@ Erroneous code example:
 
 ```compile_fail,E0092
 #![feature(intrinsics)]
+#![allow(internal_features)]
 
 extern "rust-intrinsic" {
     fn atomic_foo(); // error: unrecognized atomic operation
@@ -17,6 +18,7 @@ functions are defined in `compiler/rustc_codegen_llvm/src/intrinsic.rs` and in
 
 ```
 #![feature(intrinsics)]
+#![allow(internal_features)]
 
 extern "rust-intrinsic" {
     fn atomic_fence_seqcst(); // ok!
diff --git a/compiler/rustc_error_codes/src/error_codes/E0093.md b/compiler/rustc_error_codes/src/error_codes/E0093.md
index b1683cf4fc4..2bda4d74f72 100644
--- a/compiler/rustc_error_codes/src/error_codes/E0093.md
+++ b/compiler/rustc_error_codes/src/error_codes/E0093.md
@@ -4,6 +4,7 @@ Erroneous code example:
 
 ```compile_fail,E0093
 #![feature(intrinsics)]
+#![allow(internal_features)]
 
 extern "rust-intrinsic" {
     fn foo(); // error: unrecognized intrinsic function: `foo`
@@ -22,6 +23,7 @@ functions are defined in `compiler/rustc_codegen_llvm/src/intrinsic.rs` and in
 
 ```
 #![feature(intrinsics)]
+#![allow(internal_features)]
 
 extern "rust-intrinsic" {
     fn atomic_fence_seqcst(); // ok!
diff --git a/compiler/rustc_error_codes/src/error_codes/E0094.md b/compiler/rustc_error_codes/src/error_codes/E0094.md
index cc546bdbb3b..67a8c3678c5 100644
--- a/compiler/rustc_error_codes/src/error_codes/E0094.md
+++ b/compiler/rustc_error_codes/src/error_codes/E0094.md
@@ -4,6 +4,7 @@ Erroneous code example:
 
 ```compile_fail,E0094
 #![feature(intrinsics)]
+#![allow(internal_features)]
 
 extern "rust-intrinsic" {
     #[rustc_safe_intrinsic]
@@ -18,6 +19,7 @@ Example:
 
 ```
 #![feature(intrinsics)]
+#![allow(internal_features)]
 
 extern "rust-intrinsic" {
     #[rustc_safe_intrinsic]
diff --git a/compiler/rustc_error_codes/src/error_codes/E0208.md b/compiler/rustc_error_codes/src/error_codes/E0208.md
index c6db9b5d61b..2b811b4b850 100644
--- a/compiler/rustc_error_codes/src/error_codes/E0208.md
+++ b/compiler/rustc_error_codes/src/error_codes/E0208.md
@@ -8,6 +8,7 @@ Erroneous code example:
 ```compile_fail
 // NOTE: this feature is perma-unstable and should *only* be used for
 //       testing purposes.
+#![allow(internal_features)]
 #![feature(rustc_attrs)]
 
 #[rustc_variance]
diff --git a/compiler/rustc_error_codes/src/error_codes/E0211.md b/compiler/rustc_error_codes/src/error_codes/E0211.md
index 8c2462ebd9b..70f14fffae6 100644
--- a/compiler/rustc_error_codes/src/error_codes/E0211.md
+++ b/compiler/rustc_error_codes/src/error_codes/E0211.md
@@ -5,6 +5,7 @@ used. Erroneous code examples:
 
 ```compile_fail
 #![feature(intrinsics)]
+#![allow(internal_features)]
 
 extern "rust-intrinsic" {
     #[rustc_safe_intrinsic]
@@ -41,6 +42,7 @@ For the first code example, please check the function definition. Example:
 
 ```
 #![feature(intrinsics)]
+#![allow(internal_features)]
 
 extern "rust-intrinsic" {
     #[rustc_safe_intrinsic]
diff --git a/compiler/rustc_error_codes/src/error_codes/E0230.md b/compiler/rustc_error_codes/src/error_codes/E0230.md
index cfb72e74319..87ea90e73c9 100644
--- a/compiler/rustc_error_codes/src/error_codes/E0230.md
+++ b/compiler/rustc_error_codes/src/error_codes/E0230.md
@@ -5,6 +5,7 @@ compiled:
 
 ```compile_fail,E0230
 #![feature(rustc_attrs)]
+#![allow(internal_features)]
 
 #[rustc_on_unimplemented = "error on `{Self}` with params `<{A},{B}>`"] // error
 trait BadAnnotation<A> {}
diff --git a/compiler/rustc_error_codes/src/error_codes/E0231.md b/compiler/rustc_error_codes/src/error_codes/E0231.md
index 23a0a88ecdd..a1aaf90df49 100644
--- a/compiler/rustc_error_codes/src/error_codes/E0231.md
+++ b/compiler/rustc_error_codes/src/error_codes/E0231.md
@@ -5,6 +5,7 @@ compiled:
 
 ```compile_fail,E0231
 #![feature(rustc_attrs)]
+#![allow(internal_features)]
 
 #[rustc_on_unimplemented = "error on `{Self}` with params `<{A},{}>`"] // error!
 trait BadAnnotation<A> {}
diff --git a/compiler/rustc_error_codes/src/error_codes/E0232.md b/compiler/rustc_error_codes/src/error_codes/E0232.md
index b310caefa6e..0e50cf589ee 100644
--- a/compiler/rustc_error_codes/src/error_codes/E0232.md
+++ b/compiler/rustc_error_codes/src/error_codes/E0232.md
@@ -5,6 +5,7 @@ compiled:
 
 ```compile_fail,E0232
 #![feature(rustc_attrs)]
+#![allow(internal_features)]
 
 #[rustc_on_unimplemented(lorem="")] // error!
 trait BadAnnotation {}
diff --git a/compiler/rustc_error_codes/src/error_codes/E0264.md b/compiler/rustc_error_codes/src/error_codes/E0264.md
index e2a27f7b106..d7906076229 100644
--- a/compiler/rustc_error_codes/src/error_codes/E0264.md
+++ b/compiler/rustc_error_codes/src/error_codes/E0264.md
@@ -4,6 +4,7 @@ Erroneous code example:
 
 ```compile_fail,E0264
 #![feature(lang_items)]
+#![allow(internal_features)]
 
 extern "C" {
     #[lang = "cake"] // error: unknown external lang item: `cake`
@@ -16,6 +17,7 @@ A list of available external lang items is available in
 
 ```
 #![feature(lang_items)]
+#![allow(internal_features)]
 
 extern "C" {
     #[lang = "panic_impl"] // ok!
diff --git a/compiler/rustc_error_codes/src/error_codes/E0539.md b/compiler/rustc_error_codes/src/error_codes/E0539.md
index c53d60a5f47..cd28afbc48d 100644
--- a/compiler/rustc_error_codes/src/error_codes/E0539.md
+++ b/compiler/rustc_error_codes/src/error_codes/E0539.md
@@ -4,6 +4,7 @@ Erroneous code example:
 
 ```compile_fail,E0539
 #![feature(staged_api)]
+#![allow(internal_features)]
 #![stable(since = "1.0.0", feature = "test")]
 
 #[deprecated(note)] // error!
@@ -28,6 +29,7 @@ To fix these issues you need to give required key-value pairs.
 
 ```
 #![feature(staged_api)]
+#![allow(internal_features)]
 #![stable(since = "1.0.0", feature = "test")]
 
 #[deprecated(since = "1.39.0", note = "reason")] // ok!
diff --git a/compiler/rustc_error_codes/src/error_codes/E0542.md b/compiler/rustc_error_codes/src/error_codes/E0542.md
index c69e574179b..be186dbd2cc 100644
--- a/compiler/rustc_error_codes/src/error_codes/E0542.md
+++ b/compiler/rustc_error_codes/src/error_codes/E0542.md
@@ -4,6 +4,7 @@ Erroneous code example:
 
 ```compile_fail,E0542
 #![feature(staged_api)]
+#![allow(internal_features)]
 #![stable(since = "1.0.0", feature = "test")]
 
 #[stable(feature = "_stable_fn")] // invalid
@@ -23,6 +24,7 @@ To fix this issue, you need to provide the `since` field. Example:
 
 ```
 #![feature(staged_api)]
+#![allow(internal_features)]
 #![stable(since = "1.0.0", feature = "test")]
 
 #[stable(feature = "_stable_fn", since = "1.0.0")] // ok!
diff --git a/compiler/rustc_error_codes/src/error_codes/E0543.md b/compiler/rustc_error_codes/src/error_codes/E0543.md
index d0b2e2f7a7d..5051c72a151 100644
--- a/compiler/rustc_error_codes/src/error_codes/E0543.md
+++ b/compiler/rustc_error_codes/src/error_codes/E0543.md
@@ -4,6 +4,7 @@ Erroneous code example:
 
 ```compile_fail,E0543
 #![feature(staged_api)]
+#![allow(internal_features)]
 #![stable(since = "1.0.0", feature = "test")]
 
 #[stable(since = "0.1.0", feature = "_deprecated_fn")]
@@ -17,6 +18,7 @@ To fix this issue, you need to provide the `note` field. Example:
 
 ```
 #![feature(staged_api)]
+#![allow(internal_features)]
 #![stable(since = "1.0.0", feature = "test")]
 
 #[stable(since = "0.1.0", feature = "_deprecated_fn")]
diff --git a/compiler/rustc_error_codes/src/error_codes/E0544.md b/compiler/rustc_error_codes/src/error_codes/E0544.md
index 2227e2a06bf..202401f9d45 100644
--- a/compiler/rustc_error_codes/src/error_codes/E0544.md
+++ b/compiler/rustc_error_codes/src/error_codes/E0544.md
@@ -4,6 +4,7 @@ Erroneous code example:
 
 ```compile_fail,E0544
 #![feature(staged_api)]
+#![allow(internal_features)]
 #![stable(since = "1.0.0", feature = "rust1")]
 
 #[stable(feature = "rust1", since = "1.0.0")]
@@ -15,6 +16,7 @@ To fix this issue, ensure that each item has at most one stability attribute.
 
 ```
 #![feature(staged_api)]
+#![allow(internal_features)]
 #![stable(since = "1.0.0", feature = "rust1")]
 
 #[stable(feature = "test", since = "2.0.0")] // ok!
diff --git a/compiler/rustc_error_codes/src/error_codes/E0545.md b/compiler/rustc_error_codes/src/error_codes/E0545.md
index 7aba084f4d3..880378ebd3a 100644
--- a/compiler/rustc_error_codes/src/error_codes/E0545.md
+++ b/compiler/rustc_error_codes/src/error_codes/E0545.md
@@ -4,6 +4,7 @@ Erroneous code example:
 
 ```compile_fail,E0545
 #![feature(staged_api)]
+#![allow(internal_features)]
 #![stable(since = "1.0.0", feature = "test")]
 
 #[unstable(feature = "_unstable_fn", issue = "0")] // invalid
@@ -18,6 +19,7 @@ Example:
 
 ```
 #![feature(staged_api)]
+#![allow(internal_features)]
 #![stable(since = "1.0.0", feature = "test")]
 
 #[unstable(feature = "_unstable_fn", issue = "none")] // ok!
diff --git a/compiler/rustc_error_codes/src/error_codes/E0546.md b/compiler/rustc_error_codes/src/error_codes/E0546.md
index a33dcb7a9ac..8c98eaa07bb 100644
--- a/compiler/rustc_error_codes/src/error_codes/E0546.md
+++ b/compiler/rustc_error_codes/src/error_codes/E0546.md
@@ -4,6 +4,7 @@ Erroneous code example:
 
 ```compile_fail,E0546
 #![feature(staged_api)]
+#![allow(internal_features)]
 #![stable(since = "1.0.0", feature = "test")]
 
 #[unstable(issue = "none")] // invalid
@@ -17,6 +18,7 @@ To fix this issue, you need to provide the `feature` field. Example:
 
 ```
 #![feature(staged_api)]
+#![allow(internal_features)]
 #![stable(since = "1.0.0", feature = "test")]
 
 #[unstable(feature = "unstable_fn", issue = "none")] // ok!
diff --git a/compiler/rustc_error_codes/src/error_codes/E0547.md b/compiler/rustc_error_codes/src/error_codes/E0547.md
index 4950325df64..5b0f7cd44cf 100644
--- a/compiler/rustc_error_codes/src/error_codes/E0547.md
+++ b/compiler/rustc_error_codes/src/error_codes/E0547.md
@@ -4,6 +4,7 @@ Erroneous code example:
 
 ```compile_fail,E0547
 #![feature(staged_api)]
+#![allow(internal_features)]
 #![stable(since = "1.0.0", feature = "test")]
 
 #[unstable(feature = "_unstable_fn")] // invalid
@@ -17,6 +18,7 @@ To fix this issue, you need to provide the `issue` field. Example:
 
 ```
 #![feature(staged_api)]
+#![allow(internal_features)]
 #![stable(since = "1.0.0", feature = "test")]
 
 #[unstable(feature = "_unstable_fn", issue = "none")] // ok!
diff --git a/compiler/rustc_error_codes/src/error_codes/E0549.md b/compiler/rustc_error_codes/src/error_codes/E0549.md
index 70e458a9867..cc6a47fe253 100644
--- a/compiler/rustc_error_codes/src/error_codes/E0549.md
+++ b/compiler/rustc_error_codes/src/error_codes/E0549.md
@@ -5,6 +5,7 @@ Erroneous code example:
 
 ```compile_fail,E0549
 #![feature(staged_api)]
+#![allow(internal_features)]
 #![stable(since = "1.0.0", feature = "test")]
 
 #[deprecated(
@@ -19,6 +20,7 @@ Example:
 
 ```
 #![feature(staged_api)]
+#![allow(internal_features)]
 #![stable(since = "1.0.0", feature = "test")]
 
 #[stable(since = "1.0.0", feature = "test")]
diff --git a/compiler/rustc_error_codes/src/error_codes/E0622.md b/compiler/rustc_error_codes/src/error_codes/E0622.md
index 3ba3ed10e5c..5d71ee9949d 100644
--- a/compiler/rustc_error_codes/src/error_codes/E0622.md
+++ b/compiler/rustc_error_codes/src/error_codes/E0622.md
@@ -4,6 +4,8 @@ Erroneous code example:
 
 ```compile_fail,E0622
 #![feature(intrinsics)]
+#![allow(internal_features)]
+
 extern "rust-intrinsic" {
     pub static breakpoint: fn(); // error: intrinsic must be a function
 }
@@ -17,6 +19,8 @@ error, just declare a function. Example:
 
 ```no_run
 #![feature(intrinsics)]
+#![allow(internal_features)]
+
 extern "rust-intrinsic" {
     pub fn breakpoint(); // ok!
 }
diff --git a/compiler/rustc_error_codes/src/error_codes/E0773.md b/compiler/rustc_error_codes/src/error_codes/E0773.md
index b19a58bf33d..aa65a475a4f 100644
--- a/compiler/rustc_error_codes/src/error_codes/E0773.md
+++ b/compiler/rustc_error_codes/src/error_codes/E0773.md
@@ -5,6 +5,7 @@ Erroneous code example:
 ```compile_fail,E0773
 #![feature(decl_macro)]
 #![feature(rustc_attrs)]
+#![allow(internal_features)]
 
 #[rustc_builtin_macro]
 pub macro test($item:item) {
@@ -24,6 +25,7 @@ To fix the issue, remove the duplicate declaration:
 ```
 #![feature(decl_macro)]
 #![feature(rustc_attrs)]
+#![allow(internal_features)]
 
 #[rustc_builtin_macro]
 pub macro test($item:item) {
diff --git a/compiler/rustc_error_codes/src/error_codes/E0789.md b/compiler/rustc_error_codes/src/error_codes/E0789.md
index 89b7cd422fe..2c0018cc799 100644
--- a/compiler/rustc_error_codes/src/error_codes/E0789.md
+++ b/compiler/rustc_error_codes/src/error_codes/E0789.md
@@ -10,6 +10,7 @@ Erroneous code example:
 //       used outside of the compiler and standard library.
 #![feature(rustc_attrs)]
 #![feature(staged_api)]
+#![allow(internal_features)]
 
 #![unstable(feature = "foo_module", reason = "...", issue = "123")]
 
diff --git a/compiler/rustc_error_messages/src/lib.rs b/compiler/rustc_error_messages/src/lib.rs
index 77e2c900c0d..3bf15505090 100644
--- a/compiler/rustc_error_messages/src/lib.rs
+++ b/compiler/rustc_error_messages/src/lib.rs
@@ -4,6 +4,7 @@
 #![feature(type_alias_impl_trait)]
 #![deny(rustc::untranslatable_diagnostic)]
 #![deny(rustc::diagnostic_outside_of_impl)]
+#![cfg_attr(not(bootstrap), allow(internal_features))]
 
 #[macro_use]
 extern crate tracing;
diff --git a/compiler/rustc_errors/src/lib.rs b/compiler/rustc_errors/src/lib.rs
index 3d1639db4af..3b753629128 100644
--- a/compiler/rustc_errors/src/lib.rs
+++ b/compiler/rustc_errors/src/lib.rs
@@ -15,6 +15,7 @@
 #![feature(box_patterns)]
 #![feature(error_reporter)]
 #![allow(incomplete_features)]
+#![cfg_attr(not(bootstrap), allow(internal_features))]
 
 #[macro_use]
 extern crate rustc_macros;
diff --git a/compiler/rustc_expand/src/lib.rs b/compiler/rustc_expand/src/lib.rs
index 83a5043b0aa..c4a9b2ace9a 100644
--- a/compiler/rustc_expand/src/lib.rs
+++ b/compiler/rustc_expand/src/lib.rs
@@ -11,6 +11,7 @@
 #![feature(try_blocks)]
 #![recursion_limit = "256"]
 #![deny(rustc::untranslatable_diagnostic)]
+#![cfg_attr(not(bootstrap), allow(internal_features))]
 
 #[macro_use]
 extern crate rustc_macros;
diff --git a/compiler/rustc_feature/src/active.rs b/compiler/rustc_feature/src/active.rs
index bbc3d291e20..ff25f744ded 100644
--- a/compiler/rustc_feature/src/active.rs
+++ b/compiler/rustc_feature/src/active.rs
@@ -16,12 +16,22 @@ macro_rules! set {
     }};
 }
 
+#[derive(PartialEq)]
+enum FeatureStatus {
+    Default,
+    Incomplete,
+    Internal,
+}
+
 macro_rules! declare_features {
-    (__status_to_bool active) => {
-        false
+    (__status_to_enum active) => {
+        FeatureStatus::Default
     };
-    (__status_to_bool incomplete) => {
-        true
+    (__status_to_enum incomplete) => {
+        FeatureStatus::Incomplete
+    };
+    (__status_to_enum internal) => {
+        FeatureStatus::Internal
     };
     ($(
         $(#[doc = $doc:tt])* ($status:ident, $feature:ident, $ver:expr, $issue:expr, $edition:expr),
@@ -83,7 +93,7 @@ macro_rules! declare_features {
             pub fn incomplete(&self, feature: Symbol) -> bool {
                 match feature {
                     $(
-                        sym::$feature => declare_features!(__status_to_bool $status),
+                        sym::$feature => declare_features!(__status_to_enum $status) == FeatureStatus::Incomplete,
                     )*
                     // accepted and removed features aren't in this file but are never incomplete
                     _ if self.declared_lang_features.iter().any(|f| f.0 == feature) => false,
@@ -91,6 +101,22 @@ macro_rules! declare_features {
                     _ => panic!("`{}` was not listed in `declare_features`", feature),
                 }
             }
+
+            /// Some features are internal to the compiler and standard library and should not
+            /// be used in normal projects. We warn the user about these
+            /// to alert them.
+            pub fn internal(&self, feature: Symbol) -> bool {
+                match feature {
+                    $(
+                        sym::$feature => declare_features!(__status_to_enum $status) == FeatureStatus::Internal,
+                    )*
+                    // accepted and removed features aren't in this file but are never internal
+                    // (a removed feature might have been internal, but it doesn't matter anymore)
+                    _ if self.declared_lang_features.iter().any(|f| f.0 == feature) => false,
+                    _ if self.declared_lib_features.iter().any(|f| f.0 == feature) => false,
+                    _ => panic!("`{}` was not listed in `declare_features`", feature),
+                }
+            }
         }
     };
 }
@@ -137,29 +163,29 @@ declare_features! (
     /// Allows using the `vectorcall` ABI.
     (active, abi_vectorcall, "1.7.0", None, None),
     /// Allows using `#![needs_allocator]`, an implementation detail of `#[global_allocator]`.
-    (active, allocator_internals, "1.20.0", None, None),
+    (internal, allocator_internals, "1.20.0", None, None),
     /// Allows using `#[allow_internal_unsafe]`. This is an
     /// attribute on `macro_rules!` and can't use the attribute handling
     /// below (it has to be checked before expansion possibly makes
     /// macros disappear).
-    (active, allow_internal_unsafe, "1.0.0", None, None),
+    (internal, allow_internal_unsafe, "1.0.0", None, None),
     /// Allows using `#[allow_internal_unstable]`. This is an
     /// attribute on `macro_rules!` and can't use the attribute handling
     /// below (it has to be checked before expansion possibly makes
     /// macros disappear).
-    (active, allow_internal_unstable, "1.0.0", None, None),
+    (internal, allow_internal_unstable, "1.0.0", None, None),
     /// Allows using anonymous lifetimes in argument-position impl-trait.
     (active, anonymous_lifetime_in_impl_trait, "1.63.0", None, None),
     /// Allows identifying the `compiler_builtins` crate.
-    (active, compiler_builtins, "1.13.0", None, None),
+    (internal, compiler_builtins, "1.13.0", None, None),
     /// Allows writing custom MIR
-    (active, custom_mir, "1.65.0", None, None),
+    (internal, custom_mir, "1.65.0", None, None),
     /// Outputs useful `assert!` messages
     (active, generic_assert, "1.63.0", None, None),
     /// Allows using the `rust-intrinsic`'s "ABI".
-    (active, intrinsics, "1.0.0", None, None),
+    (internal, intrinsics, "1.0.0", None, None),
     /// Allows using `#[lang = ".."]` attribute for linking items to special compiler logic.
-    (active, lang_items, "1.0.0", None, None),
+    (internal, lang_items, "1.0.0", None, None),
     /// Allows `#[link(..., cfg(..))]`; perma-unstable per #37406
     (active, link_cfg, "1.14.0", None, None),
     /// Allows the `multiple_supertrait_upcastable` lint.
@@ -167,22 +193,22 @@ declare_features! (
     /// Allow negative trait bounds. This is an internal-only feature for testing the trait solver!
     (incomplete, negative_bounds, "1.71.0", None, None),
     /// Allows using `#[omit_gdb_pretty_printer_section]`.
-    (active, omit_gdb_pretty_printer_section, "1.5.0", None, None),
+    (internal, omit_gdb_pretty_printer_section, "1.5.0", None, None),
     /// Allows using `#[prelude_import]` on glob `use` items.
-    (active, prelude_import, "1.2.0", None, None),
+    (internal, prelude_import, "1.2.0", None, None),
     /// Used to identify crates that contain the profiler runtime.
-    (active, profiler_runtime, "1.18.0", None, None),
+    (internal, profiler_runtime, "1.18.0", None, None),
     /// Allows using `rustc_*` attributes (RFC 572).
-    (active, rustc_attrs, "1.0.0", None, None),
+    (internal, rustc_attrs, "1.0.0", None, None),
     /// Allows using the `#[stable]` and `#[unstable]` attributes.
-    (active, staged_api, "1.0.0", None, None),
+    (internal, staged_api, "1.0.0", None, None),
     /// Added for testing E0705; perma-unstable.
-    (active, test_2018_feature, "1.31.0", None, Some(Edition::Edition2018)),
+    (internal, test_2018_feature, "1.31.0", None, Some(Edition::Edition2018)),
     /// Added for testing unstable lints; perma-unstable.
-    (active, test_unstable_lint, "1.60.0", None, None),
+    (internal, test_unstable_lint, "1.60.0", None, None),
     /// Allows non-`unsafe` —and thus, unsound— access to `Pin` constructions.
-    /// Marked `incomplete` since perma-unstable and unsound.
-    (incomplete, unsafe_pin_internals, "1.60.0", None, None),
+    /// Marked `internal` since perma-unstable and unsound.
+    (internal, unsafe_pin_internals, "1.60.0", None, None),
     /// Use for stable + negative coherence and strict coherence depending on trait's
     /// rustc_strict_coherence value.
     (active, with_negative_coherence, "1.60.0", None, None),
@@ -216,19 +242,19 @@ declare_features! (
     /// Allows using the `#[linkage = ".."]` attribute.
     (active, linkage, "1.0.0", Some(29603), None),
     /// Allows declaring with `#![needs_panic_runtime]` that a panic runtime is needed.
-    (active, needs_panic_runtime, "1.10.0", Some(32837), None),
+    (internal, needs_panic_runtime, "1.10.0", Some(32837), None),
     /// Allows using `+bundled,+whole-archive` native libs.
     (active, packed_bundled_libs, "1.69.0", Some(108081), None),
     /// Allows using the `#![panic_runtime]` attribute.
-    (active, panic_runtime, "1.10.0", Some(32837), None),
+    (internal, panic_runtime, "1.10.0", Some(32837), None),
     /// Allows using `#[rustc_allow_const_fn_unstable]`.
     /// This is an attribute on `const fn` for the same
     /// purpose as `#[allow_internal_unstable]`.
-    (active, rustc_allow_const_fn_unstable, "1.49.0", Some(69399), None),
+    (internal, rustc_allow_const_fn_unstable, "1.49.0", Some(69399), None),
     /// Allows using compiler's own crates.
     (active, rustc_private, "1.0.0", Some(27812), None),
     /// Allows using internal rustdoc features like `doc(keyword)`.
-    (active, rustdoc_internals, "1.58.0", Some(90418), None),
+    (internal, rustdoc_internals, "1.58.0", Some(90418), None),
     /// Allows using the `rustdoc::missing_doc_code_examples` lint
     (active, rustdoc_missing_doc_code_examples, "1.31.0", Some(101730), None),
     /// Allows using `#[start]` on a function indicating that it is the program entrypoint.
diff --git a/compiler/rustc_hir/src/lib.rs b/compiler/rustc_hir/src/lib.rs
index 616de57dc63..34214931a08 100644
--- a/compiler/rustc_hir/src/lib.rs
+++ b/compiler/rustc_hir/src/lib.rs
@@ -13,6 +13,7 @@
 #![recursion_limit = "256"]
 #![deny(rustc::untranslatable_diagnostic)]
 #![deny(rustc::diagnostic_outside_of_impl)]
+#![cfg_attr(not(bootstrap), allow(internal_features))]
 
 #[macro_use]
 extern crate rustc_macros;
diff --git a/compiler/rustc_incremental/src/assert_module_sources.rs b/compiler/rustc_incremental/src/assert_module_sources.rs
index 0111a6d302d..8e22ab4083e 100644
--- a/compiler/rustc_incremental/src/assert_module_sources.rs
+++ b/compiler/rustc_incremental/src/assert_module_sources.rs
@@ -6,6 +6,7 @@
 //!
 //! ```
 //! # #![feature(rustc_attrs)]
+//! # #![allow(internal_features)]
 //! #![rustc_partition_reused(module="spike", cfg="rpass2")]
 //! #![rustc_partition_codegened(module="spike-x", cfg="rpass2")]
 //! ```
diff --git a/compiler/rustc_index/src/lib.rs b/compiler/rustc_index/src/lib.rs
index 6fd9f34b29e..9942c70c4ae 100644
--- a/compiler/rustc_index/src/lib.rs
+++ b/compiler/rustc_index/src/lib.rs
@@ -12,6 +12,7 @@
         test
     )
 )]
+#![cfg_attr(all(not(bootstrap), feature = "nightly"), allow(internal_features))]
 
 #[cfg(feature = "nightly")]
 pub mod bit_set;
diff --git a/compiler/rustc_lint/messages.ftl b/compiler/rustc_lint/messages.ftl
index f482e3d7c12..e696a18207b 100644
--- a/compiler/rustc_lint/messages.ftl
+++ b/compiler/rustc_lint/messages.ftl
@@ -72,6 +72,9 @@ lint_builtin_incomplete_features = the feature `{$name}` is incomplete and may n
     .note = see issue #{$n} <https://github.com/rust-lang/rust/issues/{$n}> for more information
     .help = consider using `min_{$name}` instead, which is more stable and complete
 
+lint_builtin_internal_features = the feature `{$name}` is internal to the compiler or standard library
+    .note = using it is strongly discouraged
+
 lint_builtin_keyword_idents = `{$kw}` is a keyword in the {$next} edition
     .suggestion = you can use a raw identifier to stay compatible
 
diff --git a/compiler/rustc_lint/src/builtin.rs b/compiler/rustc_lint/src/builtin.rs
index e6917f4b2d3..cc6d5330c33 100644
--- a/compiler/rustc_lint/src/builtin.rs
+++ b/compiler/rustc_lint/src/builtin.rs
@@ -28,8 +28,8 @@ use crate::{
         BuiltinClashingExternSub, BuiltinConstNoMangle, BuiltinDeprecatedAttrLink,
         BuiltinDeprecatedAttrLinkSuggestion, BuiltinDeprecatedAttrUsed, BuiltinDerefNullptr,
         BuiltinEllipsisInclusiveRangePatternsLint, BuiltinExplicitOutlives,
-        BuiltinExplicitOutlivesSuggestion, BuiltinIncompleteFeatures,
-        BuiltinIncompleteFeaturesHelp, BuiltinIncompleteFeaturesNote, BuiltinKeywordIdents,
+        BuiltinExplicitOutlivesSuggestion, BuiltinFeatureIssueNote, BuiltinIncompleteFeatures,
+        BuiltinIncompleteFeaturesHelp, BuiltinInternalFeatures, BuiltinKeywordIdents,
         BuiltinMissingCopyImpl, BuiltinMissingDebugImpl, BuiltinMissingDoc,
         BuiltinMutablesTransmutes, BuiltinNoMangleGeneric, BuiltinNonShorthandFieldPatterns,
         BuiltinSpecialModuleNameUsed, BuiltinTrivialBounds, BuiltinTypeAliasGenericBounds,
@@ -2301,12 +2301,36 @@ declare_lint! {
     "incomplete features that may function improperly in some or all cases"
 }
 
+declare_lint! {
+    /// The `internal_features` lint detects unstable features enabled with
+    /// the [`feature` attribute] that are internal to the compiler or standard
+    /// library.
+    ///
+    /// [`feature` attribute]: https://doc.rust-lang.org/nightly/unstable-book/
+    ///
+    /// ### Example
+    ///
+    /// ```rust,compile_fail
+    /// #![feature(rustc_attrs)]
+    /// ```
+    ///
+    /// {{produces}}
+    ///
+    /// ### Explanation
+    ///
+    /// These features are an implementation detail of the compiler and standard
+    /// library and are not supposed to be used in user code.
+    pub INTERNAL_FEATURES,
+    Deny,
+    "internal features are not supposed to be used"
+}
+
 declare_lint_pass!(
     /// Check for used feature gates in `INCOMPLETE_FEATURES` in `rustc_feature/src/active.rs`.
-    IncompleteFeatures => [INCOMPLETE_FEATURES]
+    IncompleteInternalFeatures => [INCOMPLETE_FEATURES, INTERNAL_FEATURES]
 );
 
-impl EarlyLintPass for IncompleteFeatures {
+impl EarlyLintPass for IncompleteInternalFeatures {
     fn check_crate(&mut self, cx: &EarlyContext<'_>, _: &ast::Crate) {
         let features = cx.sess().features_untracked();
         features
@@ -2314,17 +2338,26 @@ impl EarlyLintPass for IncompleteFeatures {
             .iter()
             .map(|(name, span, _)| (name, span))
             .chain(features.declared_lib_features.iter().map(|(name, span)| (name, span)))
-            .filter(|(&name, _)| features.incomplete(name))
+            .filter(|(&name, _)| features.incomplete(name) || features.internal(name))
             .for_each(|(&name, &span)| {
                 let note = rustc_feature::find_feature_issue(name, GateIssue::Language)
-                    .map(|n| BuiltinIncompleteFeaturesNote { n });
-                let help =
-                    HAS_MIN_FEATURES.contains(&name).then_some(BuiltinIncompleteFeaturesHelp);
-                cx.emit_spanned_lint(
-                    INCOMPLETE_FEATURES,
-                    span,
-                    BuiltinIncompleteFeatures { name, note, help },
-                );
+                    .map(|n| BuiltinFeatureIssueNote { n });
+
+                if features.incomplete(name) {
+                    let help =
+                        HAS_MIN_FEATURES.contains(&name).then_some(BuiltinIncompleteFeaturesHelp);
+                    cx.emit_spanned_lint(
+                        INCOMPLETE_FEATURES,
+                        span,
+                        BuiltinIncompleteFeatures { name, note, help },
+                    );
+                } else {
+                    cx.emit_spanned_lint(
+                        INTERNAL_FEATURES,
+                        span,
+                        BuiltinInternalFeatures { name, note },
+                    );
+                }
             });
     }
 }
diff --git a/compiler/rustc_lint/src/lib.rs b/compiler/rustc_lint/src/lib.rs
index 96fd3ccf774..2c8b15de040 100644
--- a/compiler/rustc_lint/src/lib.rs
+++ b/compiler/rustc_lint/src/lib.rs
@@ -40,6 +40,7 @@
 #![recursion_limit = "256"]
 #![deny(rustc::untranslatable_diagnostic)]
 #![deny(rustc::diagnostic_outside_of_impl)]
+#![cfg_attr(not(bootstrap), allow(internal_features))]
 
 #[macro_use]
 extern crate rustc_middle;
@@ -173,7 +174,7 @@ early_lint_methods!(
             WhileTrue: WhileTrue,
             NonAsciiIdents: NonAsciiIdents,
             HiddenUnicodeCodepoints: HiddenUnicodeCodepoints,
-            IncompleteFeatures: IncompleteFeatures,
+            IncompleteInternalFeatures: IncompleteInternalFeatures,
             RedundantSemicolons: RedundantSemicolons,
             UnusedDocComment: UnusedDocComment,
             UnexpectedCfgs: UnexpectedCfgs,
diff --git a/compiler/rustc_lint/src/lints.rs b/compiler/rustc_lint/src/lints.rs
index a6a48bf4ffa..a23eb8b6d47 100644
--- a/compiler/rustc_lint/src/lints.rs
+++ b/compiler/rustc_lint/src/lints.rs
@@ -405,18 +405,27 @@ pub struct BuiltinExplicitOutlivesSuggestion {
 pub struct BuiltinIncompleteFeatures {
     pub name: Symbol,
     #[subdiagnostic]
-    pub note: Option<BuiltinIncompleteFeaturesNote>,
+    pub note: Option<BuiltinFeatureIssueNote>,
     #[subdiagnostic]
     pub help: Option<BuiltinIncompleteFeaturesHelp>,
 }
 
+#[derive(LintDiagnostic)]
+#[diag(lint_builtin_internal_features)]
+#[note]
+pub struct BuiltinInternalFeatures {
+    pub name: Symbol,
+    #[subdiagnostic]
+    pub note: Option<BuiltinFeatureIssueNote>,
+}
+
 #[derive(Subdiagnostic)]
 #[help(lint_help)]
 pub struct BuiltinIncompleteFeaturesHelp;
 
 #[derive(Subdiagnostic)]
 #[note(lint_note)]
-pub struct BuiltinIncompleteFeaturesNote {
+pub struct BuiltinFeatureIssueNote {
     pub n: NonZeroU32,
 }
 
diff --git a/compiler/rustc_lint_defs/src/builtin.rs b/compiler/rustc_lint_defs/src/builtin.rs
index 7e3b6e9e218..1a612731808 100644
--- a/compiler/rustc_lint_defs/src/builtin.rs
+++ b/compiler/rustc_lint_defs/src/builtin.rs
@@ -3925,7 +3925,6 @@ declare_lint! {
     ///
     /// // in crate B
     /// #![feature(non_exhaustive_omitted_patterns_lint)]
-    ///
     /// match Bar::A {
     ///     Bar::A => {},
     ///     #[warn(non_exhaustive_omitted_patterns)]
diff --git a/compiler/rustc_macros/src/lib.rs b/compiler/rustc_macros/src/lib.rs
index 904f8eb5731..f4593d0fe73 100644
--- a/compiler/rustc_macros/src/lib.rs
+++ b/compiler/rustc_macros/src/lib.rs
@@ -7,6 +7,7 @@
 #![allow(rustc::default_hash_types)]
 #![deny(rustc::untranslatable_diagnostic)]
 #![deny(rustc::diagnostic_outside_of_impl)]
+#![cfg_attr(not(bootstrap), allow(internal_features))]
 #![recursion_limit = "128"]
 
 use synstructure::decl_derive;
diff --git a/compiler/rustc_middle/src/lib.rs b/compiler/rustc_middle/src/lib.rs
index 1b125e8e26d..f5576b59571 100644
--- a/compiler/rustc_middle/src/lib.rs
+++ b/compiler/rustc_middle/src/lib.rs
@@ -64,6 +64,7 @@
 #![feature(macro_metavar_expr)]
 #![recursion_limit = "512"]
 #![allow(rustc::potential_query_instability)]
+#![cfg_attr(not(bootstrap), allow(internal_features))]
 
 #[macro_use]
 extern crate bitflags;
diff --git a/compiler/rustc_parse/src/lib.rs b/compiler/rustc_parse/src/lib.rs
index 6712db26693..892be36aae7 100644
--- a/compiler/rustc_parse/src/lib.rs
+++ b/compiler/rustc_parse/src/lib.rs
@@ -8,6 +8,7 @@
 #![feature(never_type)]
 #![feature(rustc_attrs)]
 #![recursion_limit = "256"]
+#![cfg_attr(not(bootstrap), allow(internal_features))]
 
 #[macro_use]
 extern crate tracing;
diff --git a/compiler/rustc_query_impl/src/lib.rs b/compiler/rustc_query_impl/src/lib.rs
index 4cf0f1305a7..53005ede843 100644
--- a/compiler/rustc_query_impl/src/lib.rs
+++ b/compiler/rustc_query_impl/src/lib.rs
@@ -11,6 +11,7 @@
 #![allow(rustc::potential_query_instability, unused_parens)]
 #![deny(rustc::untranslatable_diagnostic)]
 #![deny(rustc::diagnostic_outside_of_impl)]
+#![cfg_attr(not(bootstrap), allow(internal_features))]
 
 #[macro_use]
 extern crate rustc_middle;
diff --git a/compiler/rustc_resolve/src/lib.rs b/compiler/rustc_resolve/src/lib.rs
index da5e92a075a..e403386e60c 100644
--- a/compiler/rustc_resolve/src/lib.rs
+++ b/compiler/rustc_resolve/src/lib.rs
@@ -18,6 +18,7 @@
 #![recursion_limit = "256"]
 #![allow(rustdoc::private_intra_doc_links)]
 #![allow(rustc::potential_query_instability)]
+#![cfg_attr(not(bootstrap), allow(internal_features))]
 
 #[macro_use]
 extern crate tracing;
diff --git a/compiler/rustc_session/src/lib.rs b/compiler/rustc_session/src/lib.rs
index d57aa820fcb..a270817f310 100644
--- a/compiler/rustc_session/src/lib.rs
+++ b/compiler/rustc_session/src/lib.rs
@@ -10,6 +10,7 @@
 #![allow(rustc::potential_query_instability)]
 #![deny(rustc::untranslatable_diagnostic)]
 #![deny(rustc::diagnostic_outside_of_impl)]
+#![cfg_attr(not(bootstrap), allow(internal_features))]
 
 #[macro_use]
 extern crate rustc_macros;
diff --git a/compiler/rustc_span/src/lib.rs b/compiler/rustc_span/src/lib.rs
index fbd9bf05528..afee5c0fe2b 100644
--- a/compiler/rustc_span/src/lib.rs
+++ b/compiler/rustc_span/src/lib.rs
@@ -23,6 +23,7 @@
 #![feature(round_char_boundary)]
 #![deny(rustc::untranslatable_diagnostic)]
 #![deny(rustc::diagnostic_outside_of_impl)]
+#![cfg_attr(not(bootstrap), allow(internal_features))]
 
 #[macro_use]
 extern crate rustc_macros;
diff --git a/compiler/rustc_target/src/lib.rs b/compiler/rustc_target/src/lib.rs
index 3307244a217..b52002b1239 100644
--- a/compiler/rustc_target/src/lib.rs
+++ b/compiler/rustc_target/src/lib.rs
@@ -19,6 +19,7 @@
 #![feature(step_trait)]
 #![deny(rustc::untranslatable_diagnostic)]
 #![deny(rustc::diagnostic_outside_of_impl)]
+#![cfg_attr(not(bootstrap), allow(internal_features))]
 
 use std::path::{Path, PathBuf};
 
diff --git a/compiler/rustc_type_ir/src/lib.rs b/compiler/rustc_type_ir/src/lib.rs
index c2655d68b79..1a6a253f360 100644
--- a/compiler/rustc_type_ir/src/lib.rs
+++ b/compiler/rustc_type_ir/src/lib.rs
@@ -6,6 +6,7 @@
 #![feature(unwrap_infallible)]
 #![deny(rustc::untranslatable_diagnostic)]
 #![deny(rustc::diagnostic_outside_of_impl)]
+#![cfg_attr(not(bootstrap), allow(internal_features))]
 
 #[macro_use]
 extern crate bitflags;