for now, do not do fake reads on non-Unpin mutable references

diff --git a/src/tools/miri/src/borrow_tracker/stacked_borrows/mod.rs b/src/tools/miri/src/borrow_tracker/stacked_borrows/mod.rs
index 50c2ad75ca71e6113334dd5b965b86ef6fb06683..bcac873251f587509c766cd4197cb929feb950f2 100644 (file)
--- a/src/tools/miri/src/borrow_tracker/stacked_borrows/mod.rs
+++ b/src/tools/miri/src/borrow_tracker/stacked_borrows/mod.rs
@@ -45,7 +45,9 @@ pub struct Stacks {
 /// new pointer.
 #[derive(Copy, Clone, Hash, PartialEq, Eq)]
 enum RefKind {
-    /// `&mut` and `Box`.
+    /// `Box`.
+    Box,
+    /// `&mut`.
     Unique { two_phase: bool },
     /// `&` with or without interior mutability.
     Shared,
@@ -56,6 +58,7 @@ enum RefKind {
 impl fmt::Display for RefKind {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
         match self {
+            RefKind::Box => write!(f, "Box"),
             RefKind::Unique { two_phase: false } => write!(f, "unique reference"),
             RefKind::Unique { two_phase: true } => write!(f, "unique reference (two-phase)"),
             RefKind::Shared => write!(f, "shared reference"),
@@ -654,15 +657,17 @@ fn sb_reborrow(
         let (perm, access) = match kind {
             RefKind::Unique { two_phase } => {
                 // Permission is Unique only if the type is `Unpin` and this is not twophase
-                let perm = if !two_phase && place.layout.ty.is_unpin(*this.tcx, this.param_env()) {
-                    Permission::Unique
+                if !two_phase && place.layout.ty.is_unpin(*this.tcx, this.param_env()) {
+                    (Permission::Unique, Some(AccessKind::Write))
                 } else {
-                    Permission::SharedReadWrite
-                };
-                // We do an access for all full borrows, even if `!Unpin`.
-                let access = if !two_phase { Some(AccessKind::Write) } else { None };
-                (perm, access)
+                    // FIXME: We emit `dereferenceable` for `!Unpin` mutable references, so we
+                    // should do fake accesses here. But then we run into
+                    // <https://github.com/rust-lang/unsafe-code-guidelines/issues/381>, so for now
+                    // we don't do that.
+                    (Permission::SharedReadWrite, None)
+                }
             }
+            RefKind::Box => (Permission::Unique, Some(AccessKind::Write)),
             RefKind::Raw { mutable: true } => {
                 // Creating a raw ptr does not count as an access
                 (Permission::SharedReadWrite, None)
@@ -853,7 +858,7 @@ fn visit_box(&mut self, place: &PlaceTy<'tcx, Provenance>) -> InterpResult<'tcx>
                 // Boxes get a weak protectors, since they may be deallocated.
                 self.retag_place(
                     place,
-                    RefKind::Unique { two_phase: false },
+                    RefKind::Box,
                     self.retag_cause,
                     /*protector*/
                     (self.kind == RetagKind::FnEntry).then_some(ProtectorKind::WeakProtector),

diff --git a/src/tools/miri/tests/fail/stacked_borrows/notunpin_dereferenceable_fakeread.rs b/src/tools/miri/tests/fail/stacked_borrows/notunpin_dereferenceable_fakeread.rs
deleted file mode 100644 (file)
index d660921..0000000
--- a/src/tools/miri/tests/fail/stacked_borrows/notunpin_dereferenceable_fakeread.rs
+++ /dev/null
@@ -1,17 +0,0 @@
-//! Reborrowing a `&mut !Unpin` must still act like a (fake) read.
-use std::marker::PhantomPinned;
-
-struct NotUnpin(i32, PhantomPinned);
-
-fn main() {
-    unsafe {
-        let mut x = NotUnpin(0, PhantomPinned);
-        // Mutable borrow of `Unpin` field (with lifetime laundering)
-        let fieldref = &mut *(&mut x.0 as *mut i32);
-        // Mutable reborrow of the entire `x`, which is `!Unpin` but should
-        // still count as a read since we would add `dereferenceable`.
-        let _xref = &mut x;
-        // That read should have invalidated `fieldref`.
-        *fieldref = 0; //~ ERROR: /write access .* tag does not exist in the borrow stack/
-    }
-}

diff --git a/src/tools/miri/tests/fail/stacked_borrows/notunpin_dereferenceable_fakeread.stderr b/src/tools/miri/tests/fail/stacked_borrows/notunpin_dereferenceable_fakeread.stderr
deleted file mode 100644 (file)
index 3ef8a8e..0000000
--- a/src/tools/miri/tests/fail/stacked_borrows/notunpin_dereferenceable_fakeread.stderr
+++ /dev/null
@@ -1,28 +0,0 @@
-error: Undefined Behavior: attempting a write access using <TAG> at ALLOC[0x0], but that tag does not exist in the borrow stack for this location
-  --> $DIR/notunpin_dereferenceable_fakeread.rs:LL:CC
-   |
-LL |         *fieldref = 0;
-   |         ^^^^^^^^^^^^^
-   |         |
-   |         attempting a write access using <TAG> at ALLOC[0x0], but that tag does not exist in the borrow stack for this location
-   |         this error occurs as part of an access at ALLOC[0x0..0x4]
-   |
-   = help: this indicates a potential bug in the program: it performed an invalid operation, but the Stacked Borrows rules it violated are still experimental
-   = help: see https://github.com/rust-lang/unsafe-code-guidelines/blob/master/wip/stacked-borrows.md for further information
-help: <TAG> was created by a Unique retag at offsets [0x0..0x4]
-  --> $DIR/notunpin_dereferenceable_fakeread.rs:LL:CC
-   |
-LL |         let fieldref = &mut *(&mut x.0 as *mut i32);
-   |                        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
-help: <TAG> was later invalidated at offsets [0x0..0x4] by a SharedReadWrite retag
-  --> $DIR/notunpin_dereferenceable_fakeread.rs:LL:CC
-   |
-LL |         let _xref = &mut x;
-   |                     ^^^^^^
-   = note: BACKTRACE:
-   = note: inside `main` at $DIR/notunpin_dereferenceable_fakeread.rs:LL:CC
-
-note: some details are omitted, run with `MIRIFLAGS=-Zmiri-backtrace=full` for a verbose backtrace
-
-error: aborting due to previous error
-