Rollup merge of #101101 - RalfJung:read-pointer-as-bytes, r=oli-obk

interpret: make read-pointer-as-bytes a CTFE-only error with extra information

Next step in the reaction to https://github.com/rust-lang/rust/issues/99923. Also teaches Miri to implicitly strip provenance in more situations when transmuting pointers to integers, which fixes https://github.com/rust-lang/miri/issues/2456.

Pointer-to-int transmutation during CTFE now produces a message like this:
```
   = help: this code performed an operation that depends on the underlying bytes representing a pointer
   = help: the absolute address of a pointer is not known at compile-time, so such operations are not supported
```

r? ``@oli-obk``

compiler/rustc_const_eval/src/const_eval/error.rs

@@ -10,6 +10,7 @@ use rustc_span::{Span, Symbol};
 use super::InterpCx;
 use crate::interpret::{
     struct_error, ErrorHandled, FrameInfo, InterpError, InterpErrorInfo, Machine, MachineStopType,
+    UnsupportedOpInfo,
 };
 
 /// The CTFE machine has some custom error kinds.
@@ -149,6 +150,18 @@ impl<'tcx> ConstEvalErr<'tcx> {
             if let Some(span_msg) = span_msg {
                 err.span_label(self.span, span_msg);
             }
+            // Add some more context for select error types.
+            match self.error {
+                InterpError::Unsupported(
+                    UnsupportedOpInfo::ReadPointerAsBytes
+                    | UnsupportedOpInfo::PartialPointerOverwrite(_)
+                    | UnsupportedOpInfo::PartialPointerCopy(_),
+                ) => {
+                    err.help("this code performed an operation that depends on the underlying bytes representing a pointer");
+                    err.help("the absolute address of a pointer is not known at compile-time, so such operations are not supported");
+                }
+                _ => {}
+            }
             // Add spans for the stacktrace. Don't print a single-line backtrace though.
             if self.stacktrace.len() > 1 {
                 // Helper closure to print duplicated lines.