[WIP] check ptr-to-ptr casts via traits

Author: Lukas Markeffsky

compiler/rustc_hir_typeck/src/cast.rs

@@ -32,9 +32,10 @@ use super::FnCtxt;
 
 use crate::errors;
 use crate::type_error_struct;
-use hir::ExprKind;
+use hir::{ExprKind, LangItem};
 use rustc_errors::{Applicability, Diagnostic, DiagnosticBuilder, ErrorGuaranteed};
 use rustc_hir as hir;
+use rustc_infer::traits::Obligation;
 use rustc_macros::{TypeFoldable, TypeVisitable};
 use rustc_middle::mir::Mutability;
 use rustc_middle::ty::adjustment::AllowTwoPhase;
@@ -45,7 +46,8 @@ use rustc_session::lint;
 use rustc_span::def_id::{DefId, LOCAL_CRATE};
 use rustc_span::symbol::sym;
 use rustc_span::Span;
-use rustc_trait_selection::infer::InferCtxtExt;
+use rustc_trait_selection::infer::InferCtxtExt as _;
+use rustc_trait_selection::traits::query::evaluate_obligation::InferCtxtExt as _;
 
 /// Reifies a cast check to be checked once we have full type information for
 /// a function context.
@@ -787,6 +789,23 @@ impl<'a, 'tcx> CastCheck<'tcx> {
     ) -> Result<(), CastError> {
         debug!(?expr, ?cast, "check_ptr_ptr_cast");
 
+        let meta_did = fcx.tcx.require_lang_item(LangItem::Metadata, Some(self.span));
+        let expr_meta = Ty::new_projection(fcx.tcx, meta_did, [expr]);
+        let cast_meta = Ty::new_projection(fcx.tcx, meta_did, [cast]);
+        let expr_meta = fcx.normalize(self.span, expr_meta);
+        let cast_meta = fcx.normalize(self.span, cast_meta);
+
+        let pred = ty::TraitRef::from_lang_item(
+            fcx.tcx,
+            LangItem::MetadataCast,
+            self.span,
+            [expr_meta, cast_meta],
+        );
+        let obligation = Obligation::new(fcx.tcx, fcx.misc(self.span), fcx.param_env, pred);
+        if fcx.predicate_must_hold_modulo_regions(&obligation) {
+            return Ok(());
+        }
+
         let expr_kind = fcx.pointer_kind(expr, self.span)?;
         let cast_kind = fcx.pointer_kind(cast, self.span)?;
 
@@ -797,6 +816,7 @@ impl<'a, 'tcx> CastCheck<'tcx> {
 
         // Cast to thin pointer is OK
         if cast_kind == PointerKind::Thin {
+            fcx.tcx.dcx().span_err(self.span, format!("{pred}"));
             return Ok(());
         }
 
@@ -812,6 +832,7 @@ impl<'a, 'tcx> CastCheck<'tcx> {
 
         // vtable kinds must match
         if fcx.tcx.erase_regions(cast_kind) == fcx.tcx.erase_regions(expr_kind) {
+            fcx.tcx.dcx().span_err(self.span, format!("{pred}"));
             Ok(())
         } else {
             Err(CastError::DifferingKinds)