Rollup merge of #66461 - clemencetbk:master, r=GuillaumeGomez

Add explanation message for E0641

Part of #61137

src/librustc_error_codes/error_codes/E0641.md

@@ -0,0 +1,19 @@
+Attempted to cast to/from a pointer with an unknown kind.
+
+Erroneous code examples:
+
+```compile_fail,E0641
+let b = 0 as *const _; // error
+```
+
+Must give information for type of pointer that is being cast from/to if the
+type cannot be inferred.
+
+```
+// Creating a pointer from reference: type can be inferred
+let a = &(String::from("Hello world!")) as *const _; // Ok
+
+let b = 0 as *const i32; // Ok
+
+let c: *const i32 = 0 as *const _; // Ok
+```