Remove a use of StructuredDiag, which is incompatible with automatic error tainting and error translations

compiler/rustc_hir_analysis/messages.ftl

@@ -55,8 +55,6 @@ hir_analysis_cannot_capture_late_bound_ty =
     cannot capture late-bound type parameter in {$what}
     .label = parameter defined here
 
-hir_analysis_cast_thin_pointer_to_fat_pointer = cannot cast thin pointer `{$expr_ty}` to fat pointer `{$cast_ty}`
-
 hir_analysis_closure_implicit_hrtb = implicit types in closure signatures are forbidden when `for<...>` is present
     .label = `for<...>` is here
 

compiler/rustc_hir_analysis/src/errors.rs

@@ -707,15 +707,6 @@ pub(crate) struct PassToVariadicFunction<'tcx, 'a> {
     pub help: Option<()>,
 }
 
-#[derive(Diagnostic)]
-#[diag(hir_analysis_cast_thin_pointer_to_fat_pointer, code = E0607)]
-pub(crate) struct CastThinPointerToFatPointer<'tcx> {
-    #[primary_span]
-    pub span: Span,
-    pub expr_ty: Ty<'tcx>,
-    pub cast_ty: String,
-}
-
 #[derive(Diagnostic)]
 #[diag(hir_analysis_invalid_union_field, code = E0740)]
 pub(crate) struct InvalidUnionField {

compiler/rustc_hir_analysis/src/structured_errors.rs

@@ -1,10 +1,7 @@
 mod missing_cast_for_variadic_arg;
-mod sized_unsized_cast;
 mod wrong_number_of_generic_args;
 
-pub use self::{
-    missing_cast_for_variadic_arg::*, sized_unsized_cast::*, wrong_number_of_generic_args::*,
-};
+pub use self::{missing_cast_for_variadic_arg::*, wrong_number_of_generic_args::*};
 
 use rustc_errors::{Diag, ErrCode};
 use rustc_session::Session;

compiler/rustc_hir_typeck/messages.ftl

@@ -23,6 +23,21 @@ hir_typeck_cannot_cast_to_bool = cannot cast `{$expr_ty}` as `bool`
 
 hir_typeck_cast_enum_drop = cannot cast enum `{$expr_ty}` into integer `{$cast_ty}` because it implements `Drop`
 
+hir_typeck_cast_thin_pointer_to_fat_pointer = cannot cast thin pointer `{$expr_ty}` to fat pointer `{$cast_ty}`
+    .teach_help = Thin pointers are "simple" pointers: they are purely a reference to a
+        memory address.
+
+        Fat pointers are pointers referencing "Dynamically Sized Types" (also
+        called DST). DST don't have a statically known size, therefore they can
+        only exist behind some kind of pointers that contain additional
+        information. Slices and trait objects are DSTs. In the case of slices,
+        the additional information the fat pointer holds is their size.
+
+        To fix this error, don't try to cast directly between thin and fat
+        pointers.
+
+        For more information about casts, take a look at The Book:
+        https://doc.rust-lang.org/reference/expressions/operator-expr.html#type-cast-expressions",
 hir_typeck_cast_unknown_pointer = cannot cast {$to ->
     [true] to
     *[false] from

compiler/rustc_hir_typeck/src/cast.rs

@@ -500,16 +500,12 @@ impl<'a, 'tcx> CastCheck<'tcx> {
                 err.emit();
             }
             CastError::SizedUnsizedCast => {
-                use rustc_hir_analysis::structured_errors::{SizedUnsizedCast, StructuredDiag};
-
-                SizedUnsizedCast {
-                    sess: fcx.tcx.sess,
+                fcx.dcx().emit_err(errors::CastThinPointerToFatPointer {
                     span: self.span,
                     expr_ty: self.expr_ty,
                     cast_ty: fcx.ty_to_string(self.cast_ty),
-                }
-                .diagnostic()
-                .emit();
+                    teach: fcx.tcx.sess.teach(E0607).then_some(()),
+                });
             }
             CastError::IntToFatCast(known_metadata) => {
                 let expr_if_nightly = fcx.tcx.sess.is_nightly_build().then_some(self.expr_span);

compiler/rustc_hir_typeck/src/errors.rs

@@ -689,3 +689,14 @@ pub struct ReplaceWithName {
     pub span: Span,
     pub name: String,
 }
+
+#[derive(Diagnostic)]
+#[diag(hir_typeck_cast_thin_pointer_to_fat_pointer, code = E0607)]
+pub(crate) struct CastThinPointerToFatPointer<'tcx> {
+    #[primary_span]
+    pub span: Span,
+    pub expr_ty: Ty<'tcx>,
+    pub cast_ty: String,
+    #[note(hir_typeck_teach_help)]
+    pub(crate) teach: Option<()>,
+}

tests/ui/consts/slice_elem_ty_mismatch_in_unsizing_cast.rs

@@ -0,0 +1,4 @@
+const FOO: &str = unsafe { &*(1_usize as *const [i64; 0] as *const [u8] as *const str) };
+//~^ ERROR: cannot cast
+
+fn main() {}

tests/ui/consts/slice_elem_ty_mismatch_in_unsizing_cast.stderr

@@ -0,0 +1,9 @@
+error[E0607]: cannot cast thin pointer `*const [i64; 0]` to fat pointer `*const [u8]`
+  --> $DIR/slice_elem_ty_mismatch_in_unsizing_cast.rs:1:31
+   |
+LL | const FOO: &str = unsafe { &*(1_usize as *const [i64; 0] as *const [u8] as *const str) };
+   |                               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0607`.