Auto merge of #113262 - Nilstrieb:rawr-casting, r=lcnr

Never consider raw pointer casts to be trival

HIR typeck tries to figure out which casts are trivial by doing them as
coercions and seeing whether this works. Since HIR typeck is oblivious
of lifetimes, this doesn't work for pointer casts that only change the
lifetime of the pointee, which are, as borrowck will tell you, not
trivial.

This change makes it so that raw pointer casts are never considered
trivial.

This also incidentally fixes the "trivial cast" lint false positive on
the same code. Unfortunately, "trivial cast" lints are now never emitted
on raw pointer casts, even if they truly are trivial. This could be
fixed by also doing the lint in borrowck for raw pointers specifically.

fixes #113257

Author: bors

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);
+}