Auto merge of #78363 - RalfJung:promotion, r=oli-obk

remove this weird special case from promotion

Promotion has a special case to ignore interior mutability under some specific circumstances. The purpose of this PR is to figure out what changes if we remove that. Since `Cell::new` and friends only get promoted inside `const`/`static` initializers these days, it actually is not easy to exploit this case: you need something like
```rust
const TEST_INTERIOR_MUT: () = {
    // The "0." case is already ruled out by not permitting any interior mutability in `const`.
    let _val: &'static _ = &(Cell::new(1), 2).1;
};
```

I assume something like `&Some(&(Cell::new(1), 2).1)` would hit the nested case inside `validate_rvalue`... though I am not sure why that would not just trigger nested promotion, first promoting the inner reference and then the outer one?

Fixes https://github.com/rust-lang/rust/issues/67534 (by simply rejecting that code^^)

r? `@oli-obk` (but for now this is not meant to be merged!)
Cc `@rust-lang/wg-const-eval`

4 files changed, 56 insertions, 78 deletions

diff --git a/compiler/rustc_mir/src/transform/promote_consts.rs b/compiler/rustc_mir/src/transform/promote_consts.rs
index 927aae82a36..8d5ed747c3f 100644
--- a/compiler/rustc_mir/src/transform/promote_consts.rs
+++ b/compiler/rustc_mir/src/transform/promote_consts.rs
@@ -22,7 +22,7 @@ use rustc_middle::ty::cast::CastTy;
 use rustc_middle::ty::subst::InternalSubsts;
 use rustc_middle::ty::{self, List, TyCtxt, TypeFoldable};
 use rustc_span::symbol::sym;
-use rustc_span::{Span, DUMMY_SP};
+use rustc_span::Span;
 
 use rustc_index::vec::{Idx, IndexVec};
 use rustc_target::spec::abi::Abi;
@@ -326,41 +326,16 @@ impl<'tcx> Validator<'_, 'tcx> {
                         if place.projection.contains(&ProjectionElem::Deref) {
                             return Err(Unpromotable);
                         }
-
-                        let mut has_mut_interior =
-                            self.qualif_local::<qualifs::HasMutInterior>(place.local);
-                        // HACK(eddyb) this should compute the same thing as
-                        // `<HasMutInterior as Qualif>::in_projection` from
-                        // `check_consts::qualifs` but without recursion.
-                        if has_mut_interior {
-                            // This allows borrowing fields which don't have
-                            // `HasMutInterior`, from a type that does, e.g.:
-                            // `let _: &'static _ = &(Cell::new(1), 2).1;`
-                            let mut place_projection = &place.projection[..];
-                            // FIXME(eddyb) use a forward loop instead of a reverse one.
-                            while let &[ref proj_base @ .., elem] = place_projection {
-                                // FIXME(eddyb) this is probably excessive, with
-                                // the exception of `union` member accesses.
-                                let ty =
-                                    Place::ty_from(place.local, proj_base, self.body, self.tcx)
-                                        .projection_ty(self.tcx, elem)
-                                        .ty;
-                                if ty.is_freeze(self.tcx.at(DUMMY_SP), self.param_env) {
-                                    has_mut_interior = false;
-                                    break;
-                                }
-
-                                place_projection = proj_base;
-                            }
+                        if self.qualif_local::<qualifs::NeedsDrop>(place.local) {
+                            return Err(Unpromotable);
                         }
 
                         // FIXME(eddyb) this duplicates part of `validate_rvalue`.
+                        let has_mut_interior =
+                            self.qualif_local::<qualifs::HasMutInterior>(place.local);
                         if has_mut_interior {
                             return Err(Unpromotable);
                         }
-                        if self.qualif_local::<qualifs::NeedsDrop>(place.local) {
-                            return Err(Unpromotable);
-                        }
 
                         if let BorrowKind::Mut { .. } = kind {
                             let ty = place.ty(self.body, self.tcx).ty;
@@ -692,28 +667,7 @@ impl<'tcx> Validator<'_, 'tcx> {
 
                 self.validate_place(place)?;
 
-                // HACK(eddyb) this should compute the same thing as
-                // `<HasMutInterior as Qualif>::in_projection` from
-                // `check_consts::qualifs` but without recursion.
-                let mut has_mut_interior =
-                    self.qualif_local::<qualifs::HasMutInterior>(place.local);
-                if has_mut_interior {
-                    let mut place_projection = place.projection;
-                    // FIXME(eddyb) use a forward loop instead of a reverse one.
-                    while let &[ref proj_base @ .., elem] = place_projection {
-                        // FIXME(eddyb) this is probably excessive, with
-                        // the exception of `union` member accesses.
-                        let ty = Place::ty_from(place.local, proj_base, self.body, self.tcx)
-                            .projection_ty(self.tcx, elem)
-                            .ty;
-                        if ty.is_freeze(self.tcx.at(DUMMY_SP), self.param_env) {
-                            has_mut_interior = false;
-                            break;
-                        }
-
-                        place_projection = proj_base;
-                    }
-                }
+                let has_mut_interior = self.qualif_local::<qualifs::HasMutInterior>(place.local);
                 if has_mut_interior {
                     return Err(Unpromotable);
                 }
diff --git a/src/test/ui/consts/promote-not.rs b/src/test/ui/consts/promote-not.rs
index 30bb9917bf7..1e4d8586b87 100644
--- a/src/test/ui/consts/promote-not.rs
+++ b/src/test/ui/consts/promote-not.rs
@@ -3,6 +3,8 @@
 #![allow(unconditional_panic, const_err)]
 #![feature(const_fn, const_fn_union)]
 
+use std::cell::Cell;
+
 // We do not promote mutable references.
 static mut TEST1: Option<&mut [i32]> = Some(&mut [1, 2, 3]); //~ ERROR temporary value dropped while borrowed
 
@@ -32,4 +34,14 @@ const TEST_UNION: () = {
     let _x: &'static i32 = &unsafe { U { x: 0 }.x }; //~ ERROR temporary value dropped while borrowed
 };
 
-fn main() {}
+// In a `const`, we do not promote things with interior mutability. Not even if we "project it away".
+const TEST_INTERIOR_MUT: () = {
+    // The "0." case is already ruled out by not permitting any interior mutability in `const`.
+    let _val: &'static _ = &(Cell::new(1), 2).1; //~ ERROR temporary value dropped while borrowed
+};
+
+fn main() {
+    // We must not promote things with interior mutability. Not even if we "project it away".
+    let _val: &'static _ = &(Cell::new(1), 2).0; //~ ERROR temporary value dropped while borrowed
+    let _val: &'static _ = &(Cell::new(1), 2).1; //~ ERROR temporary value dropped while borrowed
+}
diff --git a/src/test/ui/consts/promote-not.stderr b/src/test/ui/consts/promote-not.stderr
index 6ca7a4c273e..6e76d9ee6c1 100644
--- a/src/test/ui/consts/promote-not.stderr
+++ b/src/test/ui/consts/promote-not.stderr
@@ -1,5 +1,5 @@
 error[E0716]: temporary value dropped while borrowed
-  --> $DIR/promote-not.rs:7:50
+  --> $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:10:18
+  --> $DIR/promote-not.rs:12:18
    |
 LL |     let x = &mut [1,2,3];
    |                  ^^^^^^^ creates a temporary 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:19:32
+  --> $DIR/promote-not.rs:21:32
    |
 LL |         let _x: &'static () = &foo();
    |                 -----------    ^^^^^ creates a temporary 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:27:29
+  --> $DIR/promote-not.rs:29:29
    |
 LL |     let _x: &'static i32 = &unsafe { U { x: 0 }.x };
    |             ------------    ^^^^^^^^^^^^^^^^^^^^^^^ creates a temporary 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:32:29
+  --> $DIR/promote-not.rs:34:29
    |
 LL |     let _x: &'static i32 = &unsafe { U { x: 0 }.x };
    |             ------------    ^^^^^^^^^^^^^^^^^^^^^^^ creates a temporary which is freed while still in use
@@ -48,6 +48,37 @@ LL |     let _x: &'static i32 = &unsafe { U { x: 0 }.x };
 LL | };
    | - temporary value is freed at the end of this statement
 
-error: aborting due to 5 previous errors
+error[E0716]: temporary value dropped while borrowed
+  --> $DIR/promote-not.rs:40:29
+   |
+LL |     let _val: &'static _ = &(Cell::new(1), 2).1;
+   |               ----------    ^^^^^^^^^^^^^^^^^ creates a temporary which is freed while still in use
+   |               |
+   |               type annotation requires that borrow lasts for `'static`
+LL | };
+   | - temporary value is freed at the end of this statement
+
+error[E0716]: temporary value dropped while borrowed
+  --> $DIR/promote-not.rs:45:29
+   |
+LL |     let _val: &'static _ = &(Cell::new(1), 2).0;
+   |               ----------    ^^^^^^^^^^^^^^^^^ creates a temporary which is freed while still in use
+   |               |
+   |               type annotation requires that borrow lasts for `'static`
+LL |     let _val: &'static _ = &(Cell::new(1), 2).1;
+LL | }
+   | - temporary value is freed at the end of this statement
+
+error[E0716]: temporary value dropped while borrowed
+  --> $DIR/promote-not.rs:46:29
+   |
+LL |     let _val: &'static _ = &(Cell::new(1), 2).1;
+   |               ----------    ^^^^^^^^^^^^^^^^^ creates a temporary which is freed while still in use
+   |               |
+   |               type annotation requires that borrow lasts for `'static`
+LL | }
+   | - temporary value is freed at the end of this statement
+
+error: aborting due to 8 previous errors
 
 For more information about this error, try `rustc --explain E0716`.
diff --git a/src/test/ui/issues/issue-49955-2.rs b/src/test/ui/issues/issue-49955-2.rs
deleted file mode 100644
index 267ed746322..00000000000
--- a/src/test/ui/issues/issue-49955-2.rs
+++ /dev/null
@@ -1,19 +0,0 @@
-// run-pass
-// compile-flags: -Z borrowck=mir
-
-use std::cell::Cell;
-
-const FIVE: Cell<i32> = Cell::new(5);
-
-#[inline(never)]
-fn tuple_field() -> &'static u32 {
-    // This test is MIR-borrowck-only because the old borrowck
-    // doesn't agree that borrows of "frozen" (i.e., without any
-    // interior mutability) fields of non-frozen temporaries,
-    // should be promoted, while MIR promotion does promote them.
-    &(FIVE, 42).1
-}
-
-fn main() {
-    assert_eq!(tuple_field().to_string(), "42");
-}