Rollup merge of #69967 - mark-i-m:rinfctx, r=matthewjasper

Remove a few `Rc`s from RegionInferenceCtxt

fixes https://github.com/rust-lang/rust/issues/55853

r? @matthewjasper

src/librustc_data_structures/frozen.rs

@@ -0,0 +1,63 @@
+//! An immutable, owned value (except for interior mutability).
+//!
+//! The purpose of `Frozen` is to make a value immutable for the sake of defensive programming. For example,
+//! suppose we have the following:
+//!
+//! ```rust
+//! struct Bar { /* some data */ }
+//!
+//! struct Foo {
+//!     /// Some computed data that should never change after construction.
+//!     pub computed: Bar,
+//!
+//!     /* some other fields */
+//! }
+//!
+//! impl Bar {
+//!     /// Mutate the `Bar`.
+//!     pub fn mutate(&mut self) { }
+//! }
+//! ```
+//!
+//! Now suppose we want to pass around a mutable `Foo` instance but, we want to make sure that
+//! `computed` does not change accidentally (e.g. somebody might accidentally call
+//! `foo.computed.mutate()`). This is what `Frozen` is for. We can do the following:
+//!
+//! ```rust
+//! use rustc_data_structures::frozen::Frozen;
+//!
+//! struct Foo {
+//!     /// Some computed data that should never change after construction.
+//!     pub computed: Frozen<Bar>,
+//!
+//!     /* some other fields */
+//! }
+//! ```
+//!
+//! `Frozen` impls `Deref`, so we can ergonomically call methods on `Bar`, but it doesn't `impl
+//! DerefMut`.  Now calling `foo.compute.mutate()` will result in a compile-time error stating that
+//! `mutate` requires a mutable reference but we don't have one.
+//!
+//! # Caveats
+//!
+//! - `Frozen` doesn't try to defend against interior mutability (e.g. `Frozen<RefCell<Bar>>`).
+//! - `Frozen` doesn't pin it's contents (e.g. one could still do `foo.computed =
+//!    Frozen::freeze(new_bar)`).
+
+/// An owned immutable value.
+#[derive(Debug)]
+pub struct Frozen<T>(T);
+
+impl<T> Frozen<T> {
+    pub fn freeze(val: T) -> Self {
+        Frozen(val)
+    }
+}
+
+impl<T> std::ops::Deref for Frozen<T> {
+    type Target = T;
+
+    fn deref(&self) -> &T {
+        &self.0
+    }
+}

src/librustc_data_structures/lib.rs

@@ -94,6 +94,7 @@ pub mod profiling;
 pub mod vec_linked_list;
 pub mod work_queue;
 pub use atomic_ref::AtomicRef;
+pub mod frozen;
 
 pub struct OnDrop<F: Fn()>(pub F);