introduce CoercePointeeWellformed for coherence checks at typeck stage

compiler/rustc_error_codes/src/error_codes/E0802.md

@@ -0,0 +1,80 @@
+The target of `derive(CoercePointee)` macro has inadmissible specification for
+a meaningful use.
+
+Erroneous code examples:
+
+The target data is not a `struct`.
+
+```compile_fail,E0802
+#[derive(CoercePointee)]
+enum NotStruct<'a, T: ?Sized> {
+    Variant(&'a T),
+}
+```
+
+The target data has a layout that is not transparent, or `repr(transparent)`
+in other words.
+
+```compile_fail,E0802
+#[derive(CoercePointee)]
+struct NotTransparent<'a, #[pointee] T: ?Sized> {
+    ptr: &'a T,
+}
+```
+
+The target data has no data field.
+
+```compile_fail,E0802
+#[derive(CoercePointee)]
+#[repr(transparent)]
+struct NoField<'a, #[pointee] T: ?Sized> {}
+```
+
+The target data is not generic over any data, or has no generic type parameter.
+
+```compile_fail,E0802
+#[derive(CoercePointee)]
+#[repr(transparent)]
+struct NoGeneric<'a>(&'a u8);
+```
+
+The target data has multiple generic type parameters, but none is designated as
+a pointee for coercion.
+
+```compile_fail,E0802
+#[derive(CoercePointee)]
+#[repr(transparent)]
+struct AmbiguousPointee<'a, T1: ?Sized, T2: ?Sized> {
+    a: (&'a T1, &'a T2),
+}
+```
+
+The target data has multiple generic type parameters that are designated as
+pointees for coercion.
+
+```compile_fail,E0802
+#[derive(CoercePointee)]
+#[repr(transparent)]
+struct TooManyPointees<
+    'a,
+    #[pointee] A: ?Sized,
+    #[pointee] B: ?Sized>
+((&'a A, &'a B));
+```
+
+The type parameter that is designated as a pointee is not marked `?Sized`.
+
+```compile_fail,E0802
+#[derive(CoercePointee)]
+#[repr(transparent)]
+struct NoMaybeSized<'a, #[pointee] T> {
+    ptr: &'a T,
+}
+```
+
+In summary, the `CoercePointee` macro demands the type to be a `struct` that is
+generic over at least one type or over more types, one of which is marked with
+`#[pointee]`, and has at least one data field and adopts a `repr(transparent)`
+layout.
+The only generic type or the type marked with `#[pointee]` has to be also
+marked as `?Sized`.

compiler/rustc_error_codes/src/lib.rs

@@ -545,6 +545,7 @@ E0798: 0798,
 E0799: 0799,
 E0800: 0800,
 E0801: 0801,
+E0802: 0802,
         );
     )
 }