Rollup merge of rust-lang#136107 - dingxiangfei2009:coerce-pointee-wellformed, r=compiler-errors

Introduce CoercePointeeWellformed for coherence checks at typeck stage

Fix rust-lang#135206

This is the first PR to introduce the "wellformedness" check for `derive(CoercePointee)`.

This patch introduces a new error code to cover all the prerequisites of the said macro. The checks that is enforced with this patch is whether the data is indeed `struct` and whether the layout is set to `repr(transparent)`.

A following series of patch will arrive later to address the following concern.
1. rust-lang#135217 so that we would only admit one single coercion on one type parameter, and leave the rest for future consideration in tandem of development of other coercion rules.
1. Enforcement of data field requirements.

**An open question** is whether there is a good schema to encode the `#[pointee]` as well, so that we could also check if the `#[pointee]` type parameter is indeed `?Sized`.

``@rustbot`` label F-derive_coerce_pointee

Author: matthiaskrgr

Diff for: core/src/marker.rs

@@ -1284,8 +1284,22 @@ pub trait FnPtr: Copy + Clone {
 /// }
 /// ```
 #[rustc_builtin_macro(CoercePointee, attributes(pointee))]
-#[allow_internal_unstable(dispatch_from_dyn, coerce_unsized, unsize)]
+#[allow_internal_unstable(dispatch_from_dyn, coerce_unsized, unsize, coerce_pointee_validated)]
 #[unstable(feature = "derive_coerce_pointee", issue = "123430")]
 pub macro CoercePointee($item:item) {
     /* compiler built-in */
 }
+
+/// A trait that is implemented for ADTs with `derive(CoercePointee)` so that
+/// the compiler can enforce the derive impls are valid post-expansion, since
+/// the derive has stricter requirements than if the impls were written by hand.
+///
+/// This trait is not intended to be implemented by users or used other than
+/// validation, so it should never be stabilized.
+#[cfg(not(bootstrap))]
+#[lang = "coerce_pointee_validated"]
+#[unstable(feature = "coerce_pointee_validated", issue = "none")]
+#[doc(hidden)]
+pub trait CoercePointeeValidated {
+    /* compiler built-in */
+}