Never consider raw pointer casts to be trival

compiler/rustc_hir_typeck/src/cast.rs

@@ -660,9 +660,21 @@ impl<'a, 'tcx> CastCheck<'tcx> {
         } else {
             match self.try_coercion_cast(fcx) {
                 Ok(()) => {
-                    self.trivial_cast_lint(fcx);
-                    debug!(" -> CoercionCast");
-                    fcx.typeck_results.borrow_mut().set_coercion_cast(self.expr.hir_id.local_id);
+                    if self.expr_ty.is_unsafe_ptr() && self.cast_ty.is_unsafe_ptr() {
+                        // When casting a raw pointer to another raw pointer, we cannot convert the cast into
+                        // a coercion because the pointee types might only differ in regions, which HIR typeck
+                        // cannot distinguish. This would cause us to erroneously discard a cast which will
+                        // lead to a borrowck error like #113257.
+                        // We still did a coercion above to unify inference variables for `ptr as _` casts.
+                        // This does cause us to miss some trivial casts in the trival cast lint.
+                        debug!(" -> PointerCast");
+                    } else {
+                        self.trivial_cast_lint(fcx);
+                        debug!(" -> CoercionCast");
+                        fcx.typeck_results
+                            .borrow_mut()
+                            .set_coercion_cast(self.expr.hir_id.local_id);
+                    }
                 }
                 Err(_) => {
                     match self.do_check(fcx) {

compiler/rustc_mir_build/src/thir/cx/expr.rs

@@ -191,11 +191,16 @@ impl<'tcx> Cx<'tcx> {
                 source: self.mirror_expr(source),
                 cast: PointerCoercion::ArrayToPointer,
             }
-        } else {
-            // check whether this is casting an enum variant discriminant
-            // to prevent cycles, we refer to the discriminant initializer
+        } else if let hir::ExprKind::Path(ref qpath) = source.kind
+           && let res = self.typeck_results().qpath_res(qpath, source.hir_id)
+           && let ty = self.typeck_results().node_type(source.hir_id)
+           && let ty::Adt(adt_def, args) = ty.kind()
+           && let Res::Def(DefKind::Ctor(CtorOf::Variant, CtorKind::Const), variant_ctor_id) = res
+        {
+            // Check whether this is casting an enum variant discriminant.
+            // To prevent cycles, we refer to the discriminant initializer,
             // which is always an integer and thus doesn't need to know the
-            // enum's layout (or its tag type) to compute it during const eval
+            // enum's layout (or its tag type) to compute it during const eval.
             // Example:
             // enum Foo {
             //     A,
@@ -204,21 +209,6 @@ impl<'tcx> Cx<'tcx> {
             // The correct solution would be to add symbolic computations to miri,
             // so we wouldn't have to compute and store the actual value
 
-            let hir::ExprKind::Path(ref qpath) = source.kind else {
-                return ExprKind::Cast { source: self.mirror_expr(source) };
-            };
-
-            let res = self.typeck_results().qpath_res(qpath, source.hir_id);
-            let ty = self.typeck_results().node_type(source.hir_id);
-            let ty::Adt(adt_def, args) = ty.kind() else {
-                return ExprKind::Cast { source: self.mirror_expr(source) };
-            };
-
-            let Res::Def(DefKind::Ctor(CtorOf::Variant, CtorKind::Const), variant_ctor_id) = res
-            else {
-                return ExprKind::Cast { source: self.mirror_expr(source) };
-            };
-
             let idx = adt_def.variant_index_with_ctor_id(variant_ctor_id);
             let (discr_did, discr_offset) = adt_def.discriminant_def_for_variant(idx);
 
@@ -255,6 +245,10 @@ impl<'tcx> Cx<'tcx> {
             };
 
             ExprKind::Cast { source }
+        } else {
+            // Default to `ExprKind::Cast` for all explicit casts.
+            // MIR building then picks the right MIR casts based on the types.
+            ExprKind::Cast { source: self.mirror_expr(source) }
         }
     }
 

tests/mir-opt/instsimplify/casts.redundant.InstSimplify.diff

@@ -6,22 +6,27 @@
       let mut _0: *const &u8;
       let mut _2: *const &u8;
       let mut _3: *const &u8;
+      let mut _4: *const &u8;
       scope 1 (inlined generic_cast::<&u8, &u8>) {
-          debug x => _3;
-          let mut _4: *const &u8;
+          debug x => _4;
+          let mut _5: *const &u8;
       }
 
       bb0: {
           StorageLive(_2);
           StorageLive(_3);
-          _3 = _1;
           StorageLive(_4);
-          _4 = _3;
--         _2 = move _4 as *const &u8 (PtrToPtr);
-+         _2 = move _4;
+          _4 = _1;
+          StorageLive(_5);
+          _5 = _4;
+-         _3 = move _5 as *const &u8 (PtrToPtr);
++         _3 = move _5;
+          StorageDead(_5);
           StorageDead(_4);
-          StorageDead(_3);
+-         _2 = move _3 as *const &u8 (PtrToPtr);
++         _2 = move _3;
           _0 = _2;
+          StorageDead(_3);
           StorageDead(_2);
           return;
       }

tests/mir-opt/instsimplify/casts.roundtrip.InstSimplify.diff

@@ -4,16 +4,21 @@
   fn roundtrip(_1: *const u8) -> *const u8 {
       debug x => _1;
       let mut _0: *const u8;
-      let mut _2: *mut u8;
-      let mut _3: *const u8;
+      let mut _2: *const u8;
+      let mut _3: *mut u8;
+      let mut _4: *const u8;
 
       bb0: {
           StorageLive(_2);
           StorageLive(_3);
-          _3 = _1;
-          _2 = move _3 as *mut u8 (PtrToPtr);
-          _0 = move _2 as *const u8 (PointerCoercion(MutToConstPointer));
+          StorageLive(_4);
+          _4 = _1;
+          _3 = move _4 as *mut u8 (PtrToPtr);
+          _2 = move _3 as *const u8 (PointerCoercion(MutToConstPointer));
+          StorageDead(_4);
           StorageDead(_3);
+-         _0 = move _2 as *const u8 (PtrToPtr);
++         _0 = move _2;
           StorageDead(_2);
           return;
       }

tests/mir-opt/instsimplify/casts.rs

@@ -18,8 +18,8 @@ pub fn redundant<'a, 'b: 'a>(x: *const &'a u8) -> *const &'a u8 {
 // EMIT_MIR casts.roundtrip.InstSimplify.diff
 pub fn roundtrip(x: *const u8) -> *const u8 {
     // CHECK-LABEL: fn roundtrip(
-    // CHECK: _3 = _1;
-    // CHECK: _2 = move _3 as *mut u8 (PtrToPtr);
-    // CHECK: _0 = move _2 as *const u8 (PointerCoercion(MutToConstPointer));
+    // CHECK: _4 = _1;
+    // CHECK: _3 = move _4 as *mut u8 (PtrToPtr);
+    // CHECK: _2 = move _3 as *const u8 (PointerCoercion(MutToConstPointer));
     x as *mut u8 as *const u8
 }

tests/ui/cast/ptr-to-ptr-different-regions.rs

@@ -0,0 +1,24 @@
+// check-pass
+
+// https://github.com/rust-lang/rust/issues/113257
+
+#![deny(trivial_casts)] // The casts here are not trivial.
+
+struct Foo<'a> { a: &'a () }
+
+fn extend_lifetime_very_very_safely<'a>(v: *const Foo<'a>) -> *const Foo<'static> {
+    // This should pass because raw pointer casts can do anything they want.
+    v as *const Foo<'static>
+}
+
+trait Trait {}
+
+fn assert_static<'a>(ptr: *mut (dyn Trait + 'a)) -> *mut (dyn Trait + 'static) {
+    ptr as _
+}
+
+fn main() {
+    let unit = ();
+    let foo = Foo { a: &unit };
+    let _long: *const Foo<'static> = extend_lifetime_very_very_safely(&foo);
+}