Auto merge of #121786 - RalfJung:interior-mut-value-reasoning, r=<try>

const checking: do not do value-based reasoning for interior mutability This basically means a nicer error for #121610, see the new test in `tests/ui/consts/refs-to-cell-in-final.rs`. Currently we have a mismatch between the dynamic semantics of `&` (things might be mutable when the pointee type is `!Freeze`, no matter the pointee value) and const-qualif (doing value-based reasoning). This removes that mismatch. I don't think this can break anything that builds on current nightly; any such code would already later run into the error that the interner complains about the mutable pointer (that's exactly #121610). So a crater run makes little sense. Overall this peanuts compared to the *actual* problem with value-based reasoning for interior mutability in promotion (rust-lang/unsafe-code-guidelines#493). I have no idea how to resolve that. But at least now the use of the problematic qualif is down to 1... The first commit is independent, I just got worried about promotion of unions and was happy to notice that at least there we don't do value-based reasoning. r? `@oli-obk`
rust-lang · Mar 2, 2024 · ca7d067 · ca7d067
2 parents 4cdd205 + ac1ed22
commit ca7d067
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diff --git a/compiler/rustc_const_eval/src/const_eval/eval_queries.rs b/compiler/rustc_const_eval/src/const_eval/eval_queries.rs
@@ -412,10 +412,10 @@ pub fn const_validate_mplace<'mir, 'tcx>(
             _ if cid.promoted.is_some() => CtfeValidationMode::Promoted,
             Some(mutbl) => CtfeValidationMode::Static { mutbl }, // a `static`
             None => {
-                // In normal `const` (not promoted), the outermost allocation is always only copied,
-                // so having `UnsafeCell` in there is okay despite them being in immutable memory.
-                let allow_immutable_unsafe_cell = cid.promoted.is_none() && !inner;
-                CtfeValidationMode::Const { allow_immutable_unsafe_cell }
+                // This is a normal `const` (not promoted).
+                // The outermost allocation is always only copied, so having `UnsafeCell` in there
+                // is okay despite them being in immutable memory.
+                CtfeValidationMode::Const { allow_immutable_unsafe_cell: !inner }
             }
         };
         ecx.const_validate_operand(&mplace.into(), path, &mut ref_tracking, mode)?;

diff --git a/compiler/rustc_const_eval/src/transform/check_consts/check.rs b/compiler/rustc_const_eval/src/transform/check_consts/check.rs
@@ -30,7 +30,7 @@ type QualifResults<'mir, 'tcx, Q> =
     rustc_mir_dataflow::ResultsCursor<'mir, 'tcx, FlowSensitiveAnalysis<'mir, 'mir, 'tcx, Q>>;
 
 #[derive(Default)]
-pub struct Qualifs<'mir, 'tcx> {
+pub(crate) struct Qualifs<'mir, 'tcx> {
     has_mut_interior: Option<QualifResults<'mir, 'tcx, HasMutInterior>>,
     needs_drop: Option<QualifResults<'mir, 'tcx, NeedsDrop>>,
     needs_non_const_drop: Option<QualifResults<'mir, 'tcx, NeedsNonConstDrop>>,
@@ -373,6 +373,21 @@ impl<'mir, 'tcx> Checker<'mir, 'tcx> {
             }
         }
     }
+
+    fn has_interior_mut(&self, ty: Ty<'tcx>) -> bool {
+        match ty.kind() {
+            // Empty arrays have no interior mutability no matter their element type.
+            ty::Array(_elem, count)
+                if count
+                    .try_eval_target_usize(self.tcx, self.param_env)
+                    .is_some_and(|v| v == 0) =>
+            {
+                false
+            }
+            // Fallback to checking `Freeze`.
+            _ => !ty.is_freeze(self.tcx, self.param_env),
+        }
+    }
 }
 
 impl<'tcx> Visitor<'tcx> for Checker<'_, 'tcx> {
@@ -484,11 +499,10 @@ impl<'tcx> Visitor<'tcx> for Checker<'_, 'tcx> {
 
             Rvalue::Ref(_, BorrowKind::Shared | BorrowKind::Fake, place)
             | Rvalue::AddressOf(Mutability::Not, place) => {
-                let borrowed_place_has_mut_interior = qualifs::in_place::<HasMutInterior, _>(
-                    self.ccx,
-                    &mut |local| self.qualifs.has_mut_interior(self.ccx, local, location),
-                    place.as_ref(),
-                );
+                // We don't do value-based reasoning here, since the rules for interior mutability
+                // are not finalized yet and they seem likely to not be full value-based in the end.
+                let borrowed_place_has_mut_interior =
+                    self.has_interior_mut(place.ty(self.body, self.tcx).ty);
 
                 // If the place is indirect, this is basically a reborrow. We have a reborrow
                 // special case above, but for raw pointers and pointers/references to `static` and
@@ -498,6 +512,17 @@ impl<'tcx> Visitor<'tcx> for Checker<'_, 'tcx> {
                 // `TransientMutBorrow`), but such reborrows got accidentally stabilized already and
                 // it is too much of a breaking change to take back.
                 if borrowed_place_has_mut_interior && !place.is_indirect() {
+                    // We used to do a value-based check here, so when we changed to a purely
+                    // type-based check we started rejecting code that used to work on stable. So
+                    // for that reason we already stabilize "transient borrows of interior mutable
+                    // borrows where value-based reasoning says that there actually is no interior
+                    // mutability". We don't do anything like that for non-transient borrows since
+                    // those are and will remain hard errors.
+                    let allow_transient_on_stable = !qualifs::in_place::<HasMutInterior, _>(
+                        self.ccx,
+                        &mut |local| self.qualifs.has_mut_interior(self.ccx, local, location),
+                        place.as_ref(),
+                    );
                     match self.const_kind() {
                         // In a const fn all borrows are transient or point to the places given via
                         // references in the arguments (so we already checked them with
@@ -506,7 +531,11 @@ impl<'tcx> Visitor<'tcx> for Checker<'_, 'tcx> {
                         // NOTE: Once we have heap allocations during CTFE we need to figure out
                         // how to prevent `const fn` to create long-lived allocations that point
                         // to (interior) mutable memory.
-                        hir::ConstContext::ConstFn => self.check_op(ops::TransientCellBorrow),
+                        hir::ConstContext::ConstFn => {
+                            if !allow_transient_on_stable {
+                                self.check_op(ops::TransientCellBorrow)
+                            }
+                        }
                         _ => {
                             // Locals with StorageDead are definitely not part of the final constant value, and
                             // it is thus inherently safe to permit such locals to have their
@@ -517,7 +546,9 @@ impl<'tcx> Visitor<'tcx> for Checker<'_, 'tcx> {
                             // The good news is that interning will detect if any unexpected mutable
                             // pointer slips through.
                             if self.local_has_storage_dead(place.local) {
-                                self.check_op(ops::TransientCellBorrow);
+                                if !allow_transient_on_stable {
+                                    self.check_op(ops::TransientCellBorrow);
+                                }
                             } else {
                                 self.check_op(ops::CellBorrow);
                             }

diff --git a/compiler/rustc_const_eval/src/transform/check_consts/qualifs.rs b/compiler/rustc_const_eval/src/transform/check_consts/qualifs.rs
@@ -284,6 +284,7 @@ where
                 if Q::in_adt_inherently(cx, def, args) {
                     return true;
                 }
+                // Don't do any value-based reasoning for unions.
                 if def.is_union() && Q::in_any_value_of_ty(cx, rvalue.ty(cx.body, cx.tcx)) {
                     return true;
                 }

diff --git a/tests/ui/consts/enclosing-scope-rule.rs b/tests/ui/consts/enclosing-scope-rule.rs
@@ -0,0 +1,16 @@
+//@build-pass
+// Some code that looks like it might be relying on promotion, but actually this is using the
+// enclosing-scope rule, meaning the reference is "extended" to outlive its block and live as long
+// as the surrounding block (which in this case is the entire program). There are multiple
+// allocations being interned at once.
+
+struct Gen<T>(T);
+impl<'a, T> Gen<&'a T> {
+    // Can't be promoted because `T` might not be `'static`.
+    const C: &'a [T] = &[];
+}
+
+// Can't be promoted because of `Drop`.
+const V: &Vec<i32> = &Vec::new();
+
+fn main() {}
diff --git a/tests/ui/consts/promote-not.rs b/tests/ui/consts/promote-not.rs
@@ -3,6 +3,7 @@
 #![allow(unconditional_panic)]
 
 use std::cell::Cell;
+use std::mem::ManuallyDrop;
 
 // We do not promote mutable references.
 static mut TEST1: Option<&mut [i32]> = Some(&mut [1, 2, 3]); //~ ERROR temporary value dropped while borrowed
@@ -69,4 +70,12 @@ fn main() {
     let _val: &'static _ = &[&TEST_DROP; 1];
     //~^ ERROR temporary value dropped while borrowed
     //~| ERROR temporary value dropped while borrowed
+
+    // Make sure there is no value-based reasoning for unions.
+    union UnionWithCell {
+        f1: i32,
+        f2: ManuallyDrop<Cell<i32>>,
+    }
+    let x: &'static _ = &UnionWithCell { f1: 0 };
+    //~^ ERROR temporary value dropped while borrowed
 }
diff --git a/tests/ui/consts/promote-not.stderr b/tests/ui/consts/promote-not.stderr
@@ -1,5 +1,5 @@
 error[E0716]: temporary value dropped while borrowed
-  --> $DIR/promote-not.rs:8:50
+  --> $DIR/promote-not.rs:9:50
    |
 LL | static mut TEST1: Option<&mut [i32]> = Some(&mut [1, 2, 3]);
    |                                        ----------^^^^^^^^^-
@@ -9,7 +9,7 @@ LL | static mut TEST1: Option<&mut [i32]> = Some(&mut [1, 2, 3]);
    |                                        using this value as a static requires that borrow lasts for `'static`
 
 error[E0716]: temporary value dropped while borrowed
-  --> $DIR/promote-not.rs:11:18
+  --> $DIR/promote-not.rs:12:18
    |
 LL |     let x = &mut [1,2,3];
    |                  ^^^^^^^ creates a temporary value which is freed while still in use
@@ -19,7 +19,7 @@ LL | };
    | - temporary value is freed at the end of this statement
 
 error[E0716]: temporary value dropped while borrowed
-  --> $DIR/promote-not.rs:33:29
+  --> $DIR/promote-not.rs:34:29
    |
 LL |     let _x: &'static i32 = &unsafe { U { x: 0 }.x };
    |             ------------    ^^^^^^^^^^^^^^^^^^^^^^^ creates a temporary value which is freed while still in use
@@ -29,7 +29,7 @@ LL | };
    | - temporary value is freed at the end of this statement
 
 error[E0716]: temporary value dropped while borrowed
-  --> $DIR/promote-not.rs:39:29
+  --> $DIR/promote-not.rs:40:29
    |
 LL |     let _val: &'static _ = &(Cell::new(1), 2).1;
    |               ----------    ^^^^^^^^^^^^^^^^^ creates a temporary value which is freed while still in use
@@ -39,7 +39,7 @@ LL | };
    | - temporary value is freed at the end of this statement
 
 error[E0716]: temporary value dropped while borrowed
-  --> $DIR/promote-not.rs:20:32
+  --> $DIR/promote-not.rs:21:32
    |
 LL |         let _x: &'static () = &foo();
    |                 -----------    ^^^^^ creates a temporary value which is freed while still in use
@@ -49,7 +49,7 @@ LL |     }
    |     - temporary value is freed at the end of this statement
 
 error[E0716]: temporary value dropped while borrowed
-  --> $DIR/promote-not.rs:28:29
+  --> $DIR/promote-not.rs:29:29
    |
 LL |     let _x: &'static i32 = &unsafe { U { x: 0 }.x };
    |             ------------    ^^^^^^^^^^^^^^^^^^^^^^^ creates a temporary value which is freed while still in use
@@ -59,7 +59,7 @@ LL | }
    | - temporary value is freed at the end of this statement
 
 error[E0716]: temporary value dropped while borrowed
-  --> $DIR/promote-not.rs:46:29
+  --> $DIR/promote-not.rs:47:29
    |
 LL |     let _val: &'static _ = &(Cell::new(1), 2).0;
    |               ----------    ^^^^^^^^^^^^^^^^^ creates a temporary value which is freed while still in use
@@ -70,7 +70,7 @@ LL | }
    | - temporary value is freed at the end of this statement
 
 error[E0716]: temporary value dropped while borrowed
-  --> $DIR/promote-not.rs:47:29
+  --> $DIR/promote-not.rs:48:29
    |
 LL |     let _val: &'static _ = &(Cell::new(1), 2).1;
    |               ----------    ^^^^^^^^^^^^^^^^^ creates a temporary value which is freed while still in use
@@ -81,7 +81,7 @@ LL | }
    | - temporary value is freed at the end of this statement
 
 error[E0716]: temporary value dropped while borrowed
-  --> $DIR/promote-not.rs:50:29
+  --> $DIR/promote-not.rs:51:29
    |
 LL |     let _val: &'static _ = &(1/0);
    |               ----------    ^^^^^ creates a temporary value which is freed while still in use
@@ -92,7 +92,7 @@ LL | }
    | - temporary value is freed at the end of this statement
 
 error[E0716]: temporary value dropped while borrowed
-  --> $DIR/promote-not.rs:51:29
+  --> $DIR/promote-not.rs:52:29
    |
 LL |     let _val: &'static _ = &(1/(1-1));
    |               ----------    ^^^^^^^^^ creates a temporary value which is freed while still in use
@@ -103,7 +103,7 @@ LL | }
    | - temporary value is freed at the end of this statement
 
 error[E0716]: temporary value dropped while borrowed
-  --> $DIR/promote-not.rs:52:29
+  --> $DIR/promote-not.rs:53:29
    |
 LL |     let _val: &'static _ = &((1+1)/(1-1));
    |               ----------    ^^^^^^^^^^^^^ creates a temporary value which is freed while still in use
@@ -114,7 +114,7 @@ LL | }
    | - temporary value is freed at the end of this statement
 
 error[E0716]: temporary value dropped while borrowed
-  --> $DIR/promote-not.rs:53:29
+  --> $DIR/promote-not.rs:54:29
    |
 LL |     let _val: &'static _ = &(i32::MIN/-1);
    |               ----------    ^^^^^^^^^^^^^ creates a temporary value which is freed while still in use
@@ -125,7 +125,7 @@ LL | }
    | - temporary value is freed at the end of this statement
 
 error[E0716]: temporary value dropped while borrowed
-  --> $DIR/promote-not.rs:54:29
+  --> $DIR/promote-not.rs:55:29
    |
 LL |     let _val: &'static _ = &(i32::MIN/(0-1));
    |               ----------    ^^^^^^^^^^^^^^^^ creates a temporary value which is freed while still in use
@@ -136,7 +136,7 @@ LL | }
    | - temporary value is freed at the end of this statement
 
 error[E0716]: temporary value dropped while borrowed
-  --> $DIR/promote-not.rs:55:29
+  --> $DIR/promote-not.rs:56:29
    |
 LL |     let _val: &'static _ = &(-128i8/-1);
    |               ----------    ^^^^^^^^^^^ creates a temporary value which is freed while still in use
@@ -147,7 +147,7 @@ LL | }
    | - temporary value is freed at the end of this statement
 
 error[E0716]: temporary value dropped while borrowed
-  --> $DIR/promote-not.rs:56:29
+  --> $DIR/promote-not.rs:57:29
    |
 LL |     let _val: &'static _ = &(1%0);
    |               ----------    ^^^^^ creates a temporary value which is freed while still in use
@@ -158,7 +158,7 @@ LL | }
    | - temporary value is freed at the end of this statement
 
 error[E0716]: temporary value dropped while borrowed
-  --> $DIR/promote-not.rs:57:29
+  --> $DIR/promote-not.rs:58:29
    |
 LL |     let _val: &'static _ = &(1%(1-1));
    |               ----------    ^^^^^^^^^ creates a temporary value which is freed while still in use
@@ -169,7 +169,7 @@ LL | }
    | - temporary value is freed at the end of this statement
 
 error[E0716]: temporary value dropped while borrowed
-  --> $DIR/promote-not.rs:58:29
+  --> $DIR/promote-not.rs:59:29
    |
 LL |     let _val: &'static _ = &([1,2,3][4]+1);
    |               ----------    ^^^^^^^^^^^^^^ creates a temporary value which is freed while still in use
@@ -180,7 +180,7 @@ LL | }
    | - temporary value is freed at the end of this statement
 
 error[E0716]: temporary value dropped while borrowed
-  --> $DIR/promote-not.rs:61:29
+  --> $DIR/promote-not.rs:62:29
    |
 LL |     let _val: &'static _ = &TEST_DROP;
    |               ----------    ^^^^^^^^^ creates a temporary value which is freed while still in use
@@ -191,7 +191,7 @@ LL | }
    | - temporary value is freed at the end of this statement
 
 error[E0716]: temporary value dropped while borrowed
-  --> $DIR/promote-not.rs:63:29
+  --> $DIR/promote-not.rs:64:29
    |
 LL |     let _val: &'static _ = &&TEST_DROP;
    |               ----------    ^^^^^^^^^^ creates a temporary value which is freed while still in use
@@ -202,7 +202,7 @@ LL | }
    | - temporary value is freed at the end of this statement
 
 error[E0716]: temporary value dropped while borrowed
-  --> $DIR/promote-not.rs:63:30
+  --> $DIR/promote-not.rs:64:30
    |
 LL |     let _val: &'static _ = &&TEST_DROP;
    |               ----------     ^^^^^^^^^ creates a temporary value which is freed while still in use
@@ -213,7 +213,7 @@ LL | }
    | - temporary value is freed at the end of this statement
 
 error[E0716]: temporary value dropped while borrowed
-  --> $DIR/promote-not.rs:66:29
+  --> $DIR/promote-not.rs:67:29
    |
 LL |     let _val: &'static _ = &(&TEST_DROP,);
    |               ----------    ^^^^^^^^^^^^^ creates a temporary value which is freed while still in use
@@ -224,7 +224,7 @@ LL | }
    | - temporary value is freed at the end of this statement
 
 error[E0716]: temporary value dropped while borrowed
-  --> $DIR/promote-not.rs:66:31
+  --> $DIR/promote-not.rs:67:31
    |
 LL |     let _val: &'static _ = &(&TEST_DROP,);
    |               ----------      ^^^^^^^^^ creates a temporary value which is freed while still in use
@@ -235,7 +235,7 @@ LL | }
    | - temporary value is freed at the end of this statement
 
 error[E0716]: temporary value dropped while borrowed
-  --> $DIR/promote-not.rs:69:29
+  --> $DIR/promote-not.rs:70:29
    |
 LL |     let _val: &'static _ = &[&TEST_DROP; 1];
    |               ----------    ^^^^^^^^^^^^^^^ creates a temporary value which is freed while still in use
@@ -246,14 +246,25 @@ LL | }
    | - temporary value is freed at the end of this statement
 
 error[E0716]: temporary value dropped while borrowed
-  --> $DIR/promote-not.rs:69:31
+  --> $DIR/promote-not.rs:70:31
    |
 LL |     let _val: &'static _ = &[&TEST_DROP; 1];
    |               ----------      ^^^^^^^^^    - temporary value is freed at the end of this statement
    |               |               |
    |               |               creates a temporary value which is freed while still in use
    |               type annotation requires that borrow lasts for `'static`
 
-error: aborting due to 24 previous errors
+error[E0716]: temporary value dropped while borrowed
+  --> $DIR/promote-not.rs:79:26
+   |
+LL |     let x: &'static _ = &UnionWithCell { f1: 0 };
+   |            ----------    ^^^^^^^^^^^^^^^^^^^^^^^ creates a temporary value which is freed while still in use
+   |            |
+   |            type annotation requires that borrow lasts for `'static`
+LL |
+LL | }
+   | - temporary value is freed at the end of this statement
+
+error: aborting due to 25 previous errors
 
 For more information about this error, try `rustc --explain E0716`.