Forbid object lifetime changing pointer casts

BoxyUwU · BoxyUwU · commit d5ebeac1f43b · 2025-02-09T16:23:41.000Z
diff --git a/compiler/rustc_borrowck/src/type_check/mod.rs b/compiler/rustc_borrowck/src/type_check/mod.rs
@@ -39,7 +39,7 @@ use rustc_mir_dataflow::move_paths::MoveData;
 use rustc_mir_dataflow::points::DenseLocationMap;
 use rustc_span::def_id::CRATE_DEF_ID;
 use rustc_span::source_map::Spanned;
-use rustc_span::{DUMMY_SP, Span, sym};
+use rustc_span::{Span, sym};
 use rustc_trait_selection::traits::query::type_op::custom::scrape_region_constraints;
 use rustc_trait_selection::traits::query::type_op::{TypeOp, TypeOpOutput};
 use tracing::{debug, instrument, trace};
@@ -52,7 +52,6 @@ use crate::polonius::legacy::{PoloniusFacts, PoloniusLocationTable};
 use crate::polonius::{PoloniusContext, PoloniusLivenessContext};
 use crate::region_infer::TypeTest;
 use crate::region_infer::values::{LivenessValues, PlaceholderIndex, PlaceholderIndices};
-use crate::renumber::RegionCtxt;
 use crate::session_diagnostics::{MoveUnsized, SimdIntrinsicArgConst};
 use crate::type_check::free_region_relations::{CreateResult, UniversalRegionRelations};
 use crate::universal_regions::{DefiningTy, UniversalRegions};
@@ -2120,8 +2119,9 @@ impl<'a, 'tcx> TypeChecker<'a, 'tcx> {
                                 //
                                 // Note that other checks (such as denying `dyn Send` -> `dyn
                                 // Debug`) are in `rustc_hir_typeck`.
-                                if let ty::Dynamic(src_tty, ..) = src_tail.kind()
-                                    && let ty::Dynamic(dst_tty, ..) = dst_tail.kind()
+                                if let ty::Dynamic(src_tty, src_region_bound, ..) = src_tail.kind()
+                                    && let ty::Dynamic(dst_tty, dst_region_bound, ..) =
+                                        dst_tail.kind()
                                     && src_tty.principal().is_some()
                                     && dst_tty.principal().is_some()
                                 {
@@ -2131,25 +2131,17 @@ impl<'a, 'tcx> TypeChecker<'a, 'tcx> {
                                         tcx.mk_poly_existential_predicates(
                                             &src_tty.without_auto_traits().collect::<Vec<_>>(),
                                         ),
-                                        tcx.lifetimes.re_static,
+                                        *src_region_bound,
                                         ty::Dyn,
                                     ));
                                     let dst_obj = tcx.mk_ty_from_kind(ty::Dynamic(
                                         tcx.mk_poly_existential_predicates(
                                             &dst_tty.without_auto_traits().collect::<Vec<_>>(),
                                         ),
-                                        tcx.lifetimes.re_static,
+                                        *dst_region_bound,
                                         ty::Dyn,
                                     ));
 
-                                    // Replace trait object lifetimes with fresh vars, to allow
-                                    // casts like
-                                    // `*mut dyn FnOnce() + 'a` -> `*mut dyn FnOnce() + 'static`
-                                    let src_obj =
-                                        freshen_single_trait_object_lifetime(self.infcx, src_obj);
-                                    let dst_obj =
-                                        freshen_single_trait_object_lifetime(self.infcx, dst_obj);
-
                                     debug!(?src_tty, ?dst_tty, ?src_obj, ?dst_obj);
 
                                     self.sub_types(
@@ -2706,16 +2698,3 @@ impl<'tcx> TypeOp<'tcx> for InstantiateOpaqueType<'tcx> {
         Ok(output)
     }
 }
-
-fn freshen_single_trait_object_lifetime<'tcx>(
-    infcx: &BorrowckInferCtxt<'tcx>,
-    ty: Ty<'tcx>,
-) -> Ty<'tcx> {
-    let &ty::Dynamic(tty, _, dyn_kind @ ty::Dyn) = ty.kind() else { bug!("expected trait object") };
-
-    let fresh = infcx
-        .next_region_var(rustc_infer::infer::RegionVariableOrigin::MiscVariable(DUMMY_SP), || {
-            RegionCtxt::Unknown
-        });
-    infcx.tcx.mk_ty_from_kind(ty::Dynamic(tty, fresh, dyn_kind))
-}
diff --git a/library/std/src/thread/mod.rs b/library/std/src/thread/mod.rs
@@ -578,8 +578,11 @@ impl Builder {
         let main = Box::new(main);
         // SAFETY: dynamic size and alignment of the Box remain the same. See below for why the
         // lifetime change is justified.
-        let main =
-            unsafe { Box::from_raw(Box::into_raw(main) as *mut (dyn FnOnce() + Send + 'static)) };
+        let main = unsafe {
+            let ptr = Box::into_raw(main) as *mut (dyn FnOnce() + Send + '_);
+            let ptr: *mut (dyn FnOnce() + Send + 'static) = crate::mem::transmute(ptr);
+            Box::from_raw(ptr)
+        };
 
         Ok(JoinInner {
             // SAFETY:
diff --git a/tests/ui/cast/ptr-to-ptr-different-regions.rs b/tests/ui/cast/ptr-to-ptr-different-regions.rs
@@ -1,10 +1,10 @@
-//@ check-pass
-
 // https://github.com/rust-lang/rust/issues/113257
 
 #![deny(trivial_casts)] // The casts here are not trivial.
 
-struct Foo<'a> { a: &'a () }
+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.
@@ -15,6 +15,7 @@ trait Trait {}
 
 fn assert_static<'a>(ptr: *mut (dyn Trait + 'a)) -> *mut (dyn Trait + 'static) {
     ptr as _
+    //~^ ERROR: lifetime may not live long enough
 }
 
 fn main() {
diff --git a/tests/ui/cast/ptr-to-ptr-different-regions.stderr b/tests/ui/cast/ptr-to-ptr-different-regions.stderr
@@ -0,0 +1,22 @@
+error: lifetime may not live long enough
+  --> $DIR/ptr-to-ptr-different-regions.rs:17:5
+   |
+LL | fn assert_static<'a>(ptr: *mut (dyn Trait + 'a)) -> *mut (dyn Trait + 'static) {
+   |                  -- lifetime `'a` defined here
+LL |     ptr as _
+   |     ^^^^^^^^ returning this value requires that `'a` must outlive `'static`
+   |
+   = note: requirement occurs because of a mutable pointer to `dyn Trait`
+   = note: mutable pointers are invariant over their type parameter
+   = help: see <https://doc.rust-lang.org/nomicon/subtyping.html> for more information about variance
+help: consider changing the trait object's explicit `'static` bound to the lifetime of argument `ptr`
+   |
+LL | fn assert_static<'a>(ptr: *mut (dyn Trait + 'a)) -> *mut (dyn Trait + 'a) {
+   |                                                                       ~~
+help: alternatively, add an explicit `'static` bound to this reference
+   |
+LL | fn assert_static<'a>(ptr: *mut (dyn Trait + 'static)) -> *mut (dyn Trait + 'static) {
+   |                           ~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+error: aborting due to 1 previous error
+
diff --git a/tests/ui/cast/ptr-to-trait-obj-ok.rs b/tests/ui/cast/ptr-to-trait-obj-ok.rs
@@ -1,5 +1,3 @@
-//@ check-pass
-
 trait Trait<'a> {}
 
 fn remove_auto<'a>(x: *mut (dyn Trait<'a> + Send)) -> *mut dyn Trait<'a> {
@@ -8,6 +6,7 @@ fn remove_auto<'a>(x: *mut (dyn Trait<'a> + Send)) -> *mut dyn Trait<'a> {
 
 fn cast_inherent_lt<'a, 'b>(x: *mut (dyn Trait<'static> + 'a)) -> *mut (dyn Trait<'static> + 'b) {
     x as _
+    //~^ ERROR: lifetime may not live long enough
 }
 
 fn cast_away_higher_ranked<'a>(x: *mut dyn for<'b> Trait<'b>) -> *mut dyn Trait<'a> {
@@ -33,6 +32,7 @@ fn cast_inherent_lt_wrap<'a, 'b>(
     x: *mut (dyn Trait<'static> + 'a),
 ) -> *mut Wrapper<dyn Trait<'static> + 'b> {
     x as _
+    //~^ ERROR: lifetime may not live long enough
 }
 
 fn cast_away_higher_ranked_wrap<'a>(x: *mut dyn for<'b> Trait<'b>) -> *mut Wrapper<dyn Trait<'a>> {
diff --git a/tests/ui/cast/ptr-to-trait-obj-ok.stderr b/tests/ui/cast/ptr-to-trait-obj-ok.stderr
@@ -0,0 +1,33 @@
+error: lifetime may not live long enough
+  --> $DIR/ptr-to-trait-obj-ok.rs:8:5
+   |
+LL | fn cast_inherent_lt<'a, 'b>(x: *mut (dyn Trait<'static> + 'a)) -> *mut (dyn Trait<'static> + 'b) {
+   |                     --  -- lifetime `'b` defined here
+   |                     |
+   |                     lifetime `'a` defined here
+LL |     x as _
+   |     ^^^^^^ function was supposed to return data with lifetime `'b` but it is returning data with lifetime `'a`
+   |
+   = help: consider adding the following bound: `'a: 'b`
+   = note: requirement occurs because of a mutable pointer to `dyn Trait<'_>`
+   = note: mutable pointers are invariant over their type parameter
+   = help: see <https://doc.rust-lang.org/nomicon/subtyping.html> for more information about variance
+
+error: lifetime may not live long enough
+  --> $DIR/ptr-to-trait-obj-ok.rs:34:5
+   |
+LL | fn cast_inherent_lt_wrap<'a, 'b>(
+   |                          --  -- lifetime `'b` defined here
+   |                          |
+   |                          lifetime `'a` defined here
+...
+LL |     x as _
+   |     ^^^^^^ function was supposed to return data with lifetime `'b` but it is returning data with lifetime `'a`
+   |
+   = help: consider adding the following bound: `'a: 'b`
+   = note: requirement occurs because of a mutable pointer to `Wrapper<dyn Trait<'_>>`
+   = note: mutable pointers are invariant over their type parameter
+   = help: see <https://doc.rust-lang.org/nomicon/subtyping.html> for more information about variance
+
+error: aborting due to 2 previous errors
+
