Use BikeshedGuaranteedNoDrop to defend against union DerefMut coercions

Author: eggyal

compiler/rustc_hir_typeck/src/place_op.rs

@@ -1,4 +1,5 @@
 use rustc_errors::Applicability;
+use rustc_hir::LangItem;
 use rustc_hir_analysis::autoderef::Autoderef;
 use rustc_infer::infer::InferOk;
 use rustc_infer::traits::{Obligation, ObligationCauseCode};
@@ -8,7 +9,8 @@ use rustc_middle::ty::adjustment::{
     PointerCoercion,
 };
 use rustc_middle::ty::{self, Ty};
-use rustc_span::{Span, sym};
+use rustc_span::{DUMMY_SP, Span, sym};
+use rustc_trait_selection::infer::InferCtxtExt;
 use tracing::debug;
 use {rustc_ast as ast, rustc_hir as hir};
 
@@ -312,14 +314,24 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                         };
                         *deref = OverloadedDeref { mutbl, span: deref.span };
                         self.enforce_context_effects(None, expr.span, method.def_id, method.args);
-                        // If this is a union field, also throw an error for `DerefMut` of `ManuallyDrop` (see RFC 2514).
+                        // If this is a union field, also throw an error for `DerefMut` of non `BikeshedGuaranteedNoDrop` (see RFC 2514).
                         // This helps avoid accidental drops.
                         if inside_union
-                            && source.ty_adt_def().is_some_and(|adt| adt.is_manually_drop())
+                            && !self
+                                .infcx
+                                .type_implements_trait(
+                                    self.tcx.require_lang_item(
+                                        LangItem::BikeshedGuaranteedNoDrop,
+                                        DUMMY_SP,
+                                    ),
+                                    [adjustment.target],
+                                    self.param_env,
+                                )
+                                .must_apply_considering_regions()
                         {
                             self.dcx().struct_span_err(
                                 expr.span,
-                                "not automatically applying `DerefMut` on `ManuallyDrop` union field",
+                                "not automatically applying `DerefMut` through union field to target that might have `Drop` glue",
                             )
                             .with_help(
                                 "writing to this reference calls the destructor for the old value",

tests/ui/union/union-deref.rs

@@ -10,32 +10,32 @@ union U2<T> { x:(), f: (ManuallyDrop<(T,)>,) }
 fn main() {
     let mut u : U1<Vec<i32>> = U1 { x: () };
     unsafe { (*u.f).0 = Vec::new() }; // explicit deref, this compiles
-    unsafe { u.f.0 = Vec::new() }; //~ERROR not automatically applying `DerefMut` on `ManuallyDrop` union field
+    unsafe { u.f.0 = Vec::new() }; //~ERROR not automatically applying `DerefMut` through union field to target that might have `Drop` glue
     unsafe { &mut (*u.f).0 }; // explicit deref, this compiles
-    unsafe { &mut u.f.0 }; //~ERROR not automatically applying `DerefMut` on `ManuallyDrop` union field
+    unsafe { &mut u.f.0 }; //~ERROR not automatically applying `DerefMut` through union field to target that might have `Drop` glue
     unsafe { (*u.f).0.push(0) }; // explicit deref, this compiles
-    unsafe { u.f.0.push(0) }; //~ERROR not automatically applying `DerefMut` on `ManuallyDrop` union field
+    unsafe { u.f.0.push(0) }; //~ERROR not automatically applying `DerefMut` through union field to target that might have `Drop` glue
 
     unsafe { (*(&mut u).f).0 = Vec::new() }; // explicit deref, this compiles
-    unsafe { (&mut u).f.0 = Vec::new() }; //~ERROR not automatically applying `DerefMut` on `ManuallyDrop` union field
+    unsafe { (&mut u).f.0 = Vec::new() }; //~ERROR not automatically applying `DerefMut` through union field to target that might have `Drop` glue
     unsafe { &mut (*(&mut u).f).0 }; // explicit deref, this compiles
-    unsafe { &mut (&mut u).f.0 }; //~ERROR not automatically applying `DerefMut` on `ManuallyDrop` union field
+    unsafe { &mut (&mut u).f.0 }; //~ERROR not automatically applying `DerefMut` through union field to target that might have `Drop` glue
     unsafe { (*(&mut u).f).0.push(0) }; // explicit deref, this compiles
-    unsafe { (&mut u).f.0.push(0) }; //~ERROR not automatically applying `DerefMut` on `ManuallyDrop` union field
+    unsafe { (&mut u).f.0.push(0) }; //~ERROR not automatically applying `DerefMut` through union field to target that might have `Drop` glue
 
 
     let mut u : U2<Vec<i32>> = U2 { x: () };
     unsafe { (*u.f.0).0 = Vec::new() }; // explicit deref, this compiles
-    unsafe { u.f.0.0 = Vec::new() }; //~ERROR not automatically applying `DerefMut` on `ManuallyDrop` union field
+    unsafe { u.f.0.0 = Vec::new() }; //~ERROR not automatically applying `DerefMut` through union field to target that might have `Drop` glue
     unsafe { &mut (*u.f.0).0 }; // explicit deref, this compiles
-    unsafe { &mut u.f.0.0 }; //~ERROR not automatically applying `DerefMut` on `ManuallyDrop` union field
+    unsafe { &mut u.f.0.0 }; //~ERROR not automatically applying `DerefMut` through union field to target that might have `Drop` glue
     unsafe { (*u.f.0).0.push(0) }; // explicit deref, this compiles
-    unsafe { u.f.0.0.push(0) }; //~ERROR not automatically applying `DerefMut` on `ManuallyDrop` union field
+    unsafe { u.f.0.0.push(0) }; //~ERROR not automatically applying `DerefMut` through union field to target that might have `Drop` glue
 
     unsafe { (*(&mut u).f.0).0 = Vec::new() }; // explicit deref, this compiles
-    unsafe { (&mut u).f.0.0 = Vec::new() }; //~ERROR not automatically applying `DerefMut` on `ManuallyDrop` union field
+    unsafe { (&mut u).f.0.0 = Vec::new() }; //~ERROR not automatically applying `DerefMut` through union field to target that might have `Drop` glue
     unsafe { &mut (*(&mut u).f.0).0 }; // explicit deref, this compiles
-    unsafe { &mut (&mut u).f.0.0 }; //~ERROR not automatically applying `DerefMut` on `ManuallyDrop` union field
+    unsafe { &mut (&mut u).f.0.0 }; //~ERROR not automatically applying `DerefMut` through union field to target that might have `Drop` glue
     unsafe { (*(&mut u).f.0).0.push(0) }; // explicit deref, this compiles
-    unsafe { (&mut u).f.0.0.push(0) }; //~ERROR not automatically applying `DerefMut` on `ManuallyDrop` union field
+    unsafe { (&mut u).f.0.0.push(0) }; //~ERROR not automatically applying `DerefMut` through union field to target that might have `Drop` glue
 }

tests/ui/union/union-deref.stderr

@@ -1,4 +1,4 @@
-error: not automatically applying `DerefMut` on `ManuallyDrop` union field
+error: not automatically applying `DerefMut` through union field to target that might have `Drop` glue
   --> $DIR/union-deref.rs:13:14
    |
 LL |     unsafe { u.f.0 = Vec::new() };
@@ -7,7 +7,7 @@ LL |     unsafe { u.f.0 = Vec::new() };
    = help: writing to this reference calls the destructor for the old value
    = help: add an explicit `*` if that is desired, or call `ptr::write` to not run the destructor
 
-error: not automatically applying `DerefMut` on `ManuallyDrop` union field
+error: not automatically applying `DerefMut` through union field to target that might have `Drop` glue
   --> $DIR/union-deref.rs:15:19
    |
 LL |     unsafe { &mut u.f.0 };
@@ -16,7 +16,7 @@ LL |     unsafe { &mut u.f.0 };
    = help: writing to this reference calls the destructor for the old value
    = help: add an explicit `*` if that is desired, or call `ptr::write` to not run the destructor
 
-error: not automatically applying `DerefMut` on `ManuallyDrop` union field
+error: not automatically applying `DerefMut` through union field to target that might have `Drop` glue
   --> $DIR/union-deref.rs:17:14
    |
 LL |     unsafe { u.f.0.push(0) };
@@ -25,7 +25,7 @@ LL |     unsafe { u.f.0.push(0) };
    = help: writing to this reference calls the destructor for the old value
    = help: add an explicit `*` if that is desired, or call `ptr::write` to not run the destructor
 
-error: not automatically applying `DerefMut` on `ManuallyDrop` union field
+error: not automatically applying `DerefMut` through union field to target that might have `Drop` glue
   --> $DIR/union-deref.rs:20:14
    |
 LL |     unsafe { (&mut u).f.0 = Vec::new() };
@@ -34,7 +34,7 @@ LL |     unsafe { (&mut u).f.0 = Vec::new() };
    = help: writing to this reference calls the destructor for the old value
    = help: add an explicit `*` if that is desired, or call `ptr::write` to not run the destructor
 
-error: not automatically applying `DerefMut` on `ManuallyDrop` union field
+error: not automatically applying `DerefMut` through union field to target that might have `Drop` glue
   --> $DIR/union-deref.rs:22:19
    |
 LL |     unsafe { &mut (&mut u).f.0 };
@@ -43,7 +43,7 @@ LL |     unsafe { &mut (&mut u).f.0 };
    = help: writing to this reference calls the destructor for the old value
    = help: add an explicit `*` if that is desired, or call `ptr::write` to not run the destructor
 
-error: not automatically applying `DerefMut` on `ManuallyDrop` union field
+error: not automatically applying `DerefMut` through union field to target that might have `Drop` glue
   --> $DIR/union-deref.rs:24:14
    |
 LL |     unsafe { (&mut u).f.0.push(0) };
@@ -52,7 +52,7 @@ LL |     unsafe { (&mut u).f.0.push(0) };
    = help: writing to this reference calls the destructor for the old value
    = help: add an explicit `*` if that is desired, or call `ptr::write` to not run the destructor
 
-error: not automatically applying `DerefMut` on `ManuallyDrop` union field
+error: not automatically applying `DerefMut` through union field to target that might have `Drop` glue
   --> $DIR/union-deref.rs:29:14
    |
 LL |     unsafe { u.f.0.0 = Vec::new() };
@@ -61,7 +61,7 @@ LL |     unsafe { u.f.0.0 = Vec::new() };
    = help: writing to this reference calls the destructor for the old value
    = help: add an explicit `*` if that is desired, or call `ptr::write` to not run the destructor
 
-error: not automatically applying `DerefMut` on `ManuallyDrop` union field
+error: not automatically applying `DerefMut` through union field to target that might have `Drop` glue
   --> $DIR/union-deref.rs:31:19
    |
 LL |     unsafe { &mut u.f.0.0 };
@@ -70,7 +70,7 @@ LL |     unsafe { &mut u.f.0.0 };
    = help: writing to this reference calls the destructor for the old value
    = help: add an explicit `*` if that is desired, or call `ptr::write` to not run the destructor
 
-error: not automatically applying `DerefMut` on `ManuallyDrop` union field
+error: not automatically applying `DerefMut` through union field to target that might have `Drop` glue
   --> $DIR/union-deref.rs:33:14
    |
 LL |     unsafe { u.f.0.0.push(0) };
@@ -79,7 +79,7 @@ LL |     unsafe { u.f.0.0.push(0) };
    = help: writing to this reference calls the destructor for the old value
    = help: add an explicit `*` if that is desired, or call `ptr::write` to not run the destructor
 
-error: not automatically applying `DerefMut` on `ManuallyDrop` union field
+error: not automatically applying `DerefMut` through union field to target that might have `Drop` glue
   --> $DIR/union-deref.rs:36:14
    |
 LL |     unsafe { (&mut u).f.0.0 = Vec::new() };
@@ -88,7 +88,7 @@ LL |     unsafe { (&mut u).f.0.0 = Vec::new() };
    = help: writing to this reference calls the destructor for the old value
    = help: add an explicit `*` if that is desired, or call `ptr::write` to not run the destructor
 
-error: not automatically applying `DerefMut` on `ManuallyDrop` union field
+error: not automatically applying `DerefMut` through union field to target that might have `Drop` glue
   --> $DIR/union-deref.rs:38:19
    |
 LL |     unsafe { &mut (&mut u).f.0.0 };
@@ -97,7 +97,7 @@ LL |     unsafe { &mut (&mut u).f.0.0 };
    = help: writing to this reference calls the destructor for the old value
    = help: add an explicit `*` if that is desired, or call `ptr::write` to not run the destructor
 
-error: not automatically applying `DerefMut` on `ManuallyDrop` union field
+error: not automatically applying `DerefMut` through union field to target that might have `Drop` glue
   --> $DIR/union-deref.rs:40:14
    |
 LL |     unsafe { (&mut u).f.0.0.push(0) };