Resolve projections during internal mutability analysis

tests/ui/borrow_interior_mutable_const/projections.rs

@@ -0,0 +1,42 @@
+#![deny(clippy::borrow_interior_mutable_const)]
+#![deny(clippy::declare_interior_mutable_const)]
+
+// Inspired by https://github.com/rust-lang/rust/pull/130543#issuecomment-2364828139
+
+use std::cell::UnsafeCell;
+
+trait Trait {
+    type Assoc;
+}
+
+type Assoc<T> = <T as Trait>::Assoc;
+
+impl Trait for u8 {
+    type Assoc = UnsafeCell<u8>;
+}
+
+impl Trait for () {
+    type Assoc = ();
+}
+
+enum MaybeMutable {
+    Mutable(Assoc<u8>),
+    Immutable(Assoc<()>),
+}
+
+const CELL: Assoc<u8> = UnsafeCell::new(0); //~ ERROR: interior mutable
+const UNIT: Assoc<()> = ();
+const MUTABLE: MaybeMutable = MaybeMutable::Mutable(CELL); //~ ERROR: interior mutable
+const IMMUTABLE: MaybeMutable = MaybeMutable::Immutable(UNIT);
+
+fn print_ref<T>(t: &T) {
+    let p: *const T = t;
+    println!("{p:p}")
+}
+
+fn main() {
+    print_ref(&CELL); //~ ERROR: interior mutability
+    print_ref(&UNIT);
+    print_ref(&MUTABLE); //~ ERROR: interior mutability
+    print_ref(&IMMUTABLE);
+}

tests/ui/borrow_interior_mutable_const/projections.stderr

@@ -0,0 +1,44 @@
+error: a `const` item should not be interior mutable
+  --> tests/ui/borrow_interior_mutable_const/projections.rs:27:1
+   |
+LL | const CELL: Assoc<u8> = UnsafeCell::new(0);
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = help: consider making this `Sync` so that it can go in a static item or using a `thread_local`
+note: the lint level is defined here
+  --> tests/ui/borrow_interior_mutable_const/projections.rs:2:9
+   |
+LL | #![deny(clippy::declare_interior_mutable_const)]
+   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: a `const` item should not be interior mutable
+  --> tests/ui/borrow_interior_mutable_const/projections.rs:29:1
+   |
+LL | const MUTABLE: MaybeMutable = MaybeMutable::Mutable(CELL);
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = help: consider making this `Sync` so that it can go in a static item or using a `thread_local`
+
+error: a `const` item with interior mutability should not be borrowed
+  --> tests/ui/borrow_interior_mutable_const/projections.rs:38:16
+   |
+LL |     print_ref(&CELL);
+   |                ^^^^
+   |
+   = help: assign this const to a local or static variable, and use the variable here
+note: the lint level is defined here
+  --> tests/ui/borrow_interior_mutable_const/projections.rs:1:9
+   |
+LL | #![deny(clippy::borrow_interior_mutable_const)]
+   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: a `const` item with interior mutability should not be borrowed
+  --> tests/ui/borrow_interior_mutable_const/projections.rs:40:16
+   |
+LL |     print_ref(&MUTABLE);
+   |                ^^^^^^^
+   |
+   = help: assign this const to a local or static variable, and use the variable here
+
+error: aborting due to 4 previous errors
+