show warning when Stacked Borrows skips a reborrow due to 'extern type'

src/tools/miri/src/borrow_tracker/stacked_borrows/mod.rs

@@ -5,6 +5,7 @@ pub mod diagnostics;
 mod item;
 mod stack;
 
+use std::cell::RefCell;
 use std::cmp;
 use std::fmt::Write;
 use std::mem;
@@ -820,7 +821,19 @@ trait EvalContextPrivExt<'tcx, 'ecx>: crate::MiriInterpCxExt<'tcx> {
         // See https://github.com/rust-lang/unsafe-code-guidelines/issues/276.
         let size = match size {
             Some(size) => size,
-            None => return Ok(place.clone()),
+            None => {
+                // The first time this happens, show a warning.
+                thread_local! { static WARNING_SHOWN: RefCell<bool> = const { RefCell::new(false) }; }
+                WARNING_SHOWN.with_borrow_mut(|shown| {
+                    if *shown {
+                        return;
+                    }
+                    // Not yet shown. Show it!
+                    *shown = true;
+                    this.emit_diagnostic(NonHaltingDiagnostic::ExternTypeReborrow);
+                });
+                return Ok(place.clone());
+            }
         };
 
         // Compute new borrow.

src/tools/miri/src/diagnostics.rs

@@ -130,6 +130,7 @@ pub enum NonHaltingDiagnostic {
     WeakMemoryOutdatedLoad {
         ptr: Pointer,
     },
+    ExternTypeReborrow,
 }
 
 /// Level of Miri specific diagnostics
@@ -593,6 +594,8 @@ impl<'tcx> MiriMachine<'tcx> {
             RejectedIsolatedOp(_) =>
                 ("operation rejected by isolation".to_string(), DiagLevel::Warning),
             Int2Ptr { .. } => ("integer-to-pointer cast".to_string(), DiagLevel::Warning),
+            ExternTypeReborrow =>
+                ("reborrow of reference to `extern type`".to_string(), DiagLevel::Warning),
             CreatedPointerTag(..)
             | PoppedPointerTag(..)
             | CreatedCallId(..)
@@ -630,6 +633,8 @@ impl<'tcx> MiriMachine<'tcx> {
             Int2Ptr { .. } => format!("integer-to-pointer cast"),
             WeakMemoryOutdatedLoad { ptr } =>
                 format!("weak memory emulation: outdated value returned from load at {ptr}"),
+            ExternTypeReborrow =>
+                format!("reborrow of a reference to `extern type` is not properly supported"),
         };
 
         let notes = match &e {
@@ -647,34 +652,50 @@ impl<'tcx> MiriMachine<'tcx> {
                     (
                         None,
                         format!(
-                            "This program is using integer-to-pointer casts or (equivalently) `ptr::with_exposed_provenance`, which means that Miri might miss pointer bugs in this program."
+                            "this program is using integer-to-pointer casts or (equivalently) `ptr::with_exposed_provenance`, which means that Miri might miss pointer bugs in this program"
                         ),
                     ),
                     (
                         None,
                         format!(
-                            "See https://doc.rust-lang.org/nightly/std/ptr/fn.with_exposed_provenance.html for more details on that operation."
+                            "see https://doc.rust-lang.org/nightly/std/ptr/fn.with_exposed_provenance.html for more details on that operation"
                         ),
                     ),
                     (
                         None,
                         format!(
-                            "To ensure that Miri does not miss bugs in your program, use Strict Provenance APIs (https://doc.rust-lang.org/nightly/std/ptr/index.html#strict-provenance, https://crates.io/crates/sptr) instead."
+                            "to ensure that Miri does not miss bugs in your program, use Strict Provenance APIs (https://doc.rust-lang.org/nightly/std/ptr/index.html#strict-provenance, https://crates.io/crates/sptr) instead"
                         ),
                     ),
                     (
                         None,
                         format!(
-                            "You can then set `MIRIFLAGS=-Zmiri-strict-provenance` to ensure you are not relying on `with_exposed_provenance` semantics."
+                            "you can then set `MIRIFLAGS=-Zmiri-strict-provenance` to ensure you are not relying on `with_exposed_provenance` semantics"
                         ),
                     ),
                     (
                         None,
                         format!(
-                            "Alternatively, `MIRIFLAGS=-Zmiri-permissive-provenance` disables this warning."
+                            "alternatively, `MIRIFLAGS=-Zmiri-permissive-provenance` disables this warning"
                         ),
                     ),
                 ],
+            ExternTypeReborrow => {
+                vec![
+                    (
+                        None,
+                        format!(
+                            "`extern type` are not compatible with the Stacked Borrows aliasing model implemented by Miri; Miri may miss bugs in this code"
+                        ),
+                    ),
+                    (
+                        None,
+                        format!(
+                            "try running with `MIRIFLAGS=-Zmiri-tree-borrows` to use the more permissive but also even more experimental Tree Borrows aliasing checks instead"
+                        ),
+                    ),
+                ]
+            }
             _ => vec![],
         };
 

src/tools/miri/tests/fail/extern-type-field-offset.stderr

@@ -1,3 +1,14 @@
+warning: reborrow of reference to `extern type`
+  --> $DIR/extern-type-field-offset.rs:LL:CC
+   |
+LL |     let x: &Newtype = unsafe { &*(&buf as *const _ as *const Newtype) };
+   |                                ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ reborrow of a reference to `extern type` is not properly supported
+   |
+   = help: `extern type` are not compatible with the Stacked Borrows aliasing model implemented by Miri; Miri may miss bugs in this code
+   = help: try running with `MIRIFLAGS=-Zmiri-tree-borrows` to use the more permissive but also even more experimental Tree Borrows aliasing checks instead
+   = note: BACKTRACE:
+   = note: inside `main` at $DIR/extern-type-field-offset.rs:LL:CC
+
 error: unsupported operation: `extern type` field does not have a known offset
   --> $DIR/extern-type-field-offset.rs:LL:CC
    |
@@ -10,5 +21,5 @@ LL |     let _field = &x.a;
 
 note: some details are omitted, run with `MIRIFLAGS=-Zmiri-backtrace=full` for a verbose backtrace
 
-error: aborting due to 1 previous error
+error: aborting due to 1 previous error; 1 warning emitted
 

src/tools/miri/tests/fail/intrinsics/ptr_metadata_uninit_slice_len.stderr

@@ -4,11 +4,11 @@ warning: integer-to-pointer cast
 LL |         (*p.as_mut_ptr().cast::<[*const i32; 2]>())[0] = 4 as *const i32;
    |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ integer-to-pointer cast
    |
-   = help: This program is using integer-to-pointer casts or (equivalently) `ptr::with_exposed_provenance`, which means that Miri might miss pointer bugs in this program.
+   = help: this program is using integer-to-pointer casts or (equivalently) `ptr::with_exposed_provenance`, which means that Miri might miss pointer bugs in this program
-   = help: See https://doc.rust-lang.org/nightly/std/ptr/fn.with_exposed_provenance.html for more details on that operation.
+   = help: see https://doc.rust-lang.org/nightly/std/ptr/fn.with_exposed_provenance.html for more details on that operation
-   = help: To ensure that Miri does not miss bugs in your program, use Strict Provenance APIs (https://doc.rust-lang.org/nightly/std/ptr/index.html#strict-provenance, https://crates.io/crates/sptr) instead.
+   = help: to ensure that Miri does not miss bugs in your program, use Strict Provenance APIs (https://doc.rust-lang.org/nightly/std/ptr/index.html#strict-provenance, https://crates.io/crates/sptr) instead
-   = help: You can then set `MIRIFLAGS=-Zmiri-strict-provenance` to ensure you are not relying on `with_exposed_provenance` semantics.
+   = help: you can then set `MIRIFLAGS=-Zmiri-strict-provenance` to ensure you are not relying on `with_exposed_provenance` semantics
-   = help: Alternatively, `MIRIFLAGS=-Zmiri-permissive-provenance` disables this warning.
+   = help: alternatively, `MIRIFLAGS=-Zmiri-permissive-provenance` disables this warning
    = note: BACKTRACE:
    = note: inside `main` at $DIR/ptr_metadata_uninit_slice_len.rs:LL:CC
 

src/tools/miri/tests/pass/box.stack.stderr

@@ -4,11 +4,11 @@ warning: integer-to-pointer cast
 LL |         let r2 = ((r as usize) + 0) as *mut i32;
    |                  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ integer-to-pointer cast
    |
-   = help: This program is using integer-to-pointer casts or (equivalently) `ptr::with_exposed_provenance`, which means that Miri might miss pointer bugs in this program.
+   = help: this program is using integer-to-pointer casts or (equivalently) `ptr::with_exposed_provenance`, which means that Miri might miss pointer bugs in this program
-   = help: See https://doc.rust-lang.org/nightly/std/ptr/fn.with_exposed_provenance.html for more details on that operation.
+   = help: see https://doc.rust-lang.org/nightly/std/ptr/fn.with_exposed_provenance.html for more details on that operation
-   = help: To ensure that Miri does not miss bugs in your program, use Strict Provenance APIs (https://doc.rust-lang.org/nightly/std/ptr/index.html#strict-provenance, https://crates.io/crates/sptr) instead.
+   = help: to ensure that Miri does not miss bugs in your program, use Strict Provenance APIs (https://doc.rust-lang.org/nightly/std/ptr/index.html#strict-provenance, https://crates.io/crates/sptr) instead
-   = help: You can then set `MIRIFLAGS=-Zmiri-strict-provenance` to ensure you are not relying on `with_exposed_provenance` semantics.
+   = help: you can then set `MIRIFLAGS=-Zmiri-strict-provenance` to ensure you are not relying on `with_exposed_provenance` semantics
-   = help: Alternatively, `MIRIFLAGS=-Zmiri-permissive-provenance` disables this warning.
+   = help: alternatively, `MIRIFLAGS=-Zmiri-permissive-provenance` disables this warning
    = note: BACKTRACE:
    = note: inside `into_raw` at $DIR/box.rs:LL:CC
 note: inside `main`

src/tools/miri/tests/pass/extern_types.stack.stderr

@@ -4,11 +4,22 @@ warning: integer-to-pointer cast
 LL |     let x: &Foo = unsafe { &*(16 as *const Foo) };
    |                              ^^^^^^^^^^^^^^^^^^ integer-to-pointer cast
    |
-   = help: This program is using integer-to-pointer casts or (equivalently) `ptr::with_exposed_provenance`, which means that Miri might miss pointer bugs in this program.
+   = help: this program is using integer-to-pointer casts or (equivalently) `ptr::with_exposed_provenance`, which means that Miri might miss pointer bugs in this program
-   = help: See https://doc.rust-lang.org/nightly/std/ptr/fn.with_exposed_provenance.html for more details on that operation.
+   = help: see https://doc.rust-lang.org/nightly/std/ptr/fn.with_exposed_provenance.html for more details on that operation
-   = help: To ensure that Miri does not miss bugs in your program, use Strict Provenance APIs (https://doc.rust-lang.org/nightly/std/ptr/index.html#strict-provenance, https://crates.io/crates/sptr) instead.
+   = help: to ensure that Miri does not miss bugs in your program, use Strict Provenance APIs (https://doc.rust-lang.org/nightly/std/ptr/index.html#strict-provenance, https://crates.io/crates/sptr) instead
-   = help: You can then set `MIRIFLAGS=-Zmiri-strict-provenance` to ensure you are not relying on `with_exposed_provenance` semantics.
+   = help: you can then set `MIRIFLAGS=-Zmiri-strict-provenance` to ensure you are not relying on `with_exposed_provenance` semantics
-   = help: Alternatively, `MIRIFLAGS=-Zmiri-permissive-provenance` disables this warning.
+   = help: alternatively, `MIRIFLAGS=-Zmiri-permissive-provenance` disables this warning
+   = note: BACKTRACE:
+   = note: inside `main` at $DIR/extern_types.rs:LL:CC
+
+warning: reborrow of reference to `extern type`
+  --> $DIR/extern_types.rs:LL:CC
+   |
+LL |     let x: &Foo = unsafe { &*(16 as *const Foo) };
+   |                            ^^^^^^^^^^^^^^^^^^^^ reborrow of a reference to `extern type` is not properly supported
+   |
+   = help: `extern type` are not compatible with the Stacked Borrows aliasing model implemented by Miri; Miri may miss bugs in this code
+   = help: try running with `MIRIFLAGS=-Zmiri-tree-borrows` to use the more permissive but also even more experimental Tree Borrows aliasing checks instead
    = note: BACKTRACE:
    = note: inside `main` at $DIR/extern_types.rs:LL:CC
 

src/tools/miri/tests/pass/stacked-borrows/issue-miri-2389.stderr

@@ -4,11 +4,11 @@ warning: integer-to-pointer cast
 LL |         let wildcard = &root0 as *const Cell<i32> as usize as *const Cell<i32>;
    |                        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ integer-to-pointer cast
    |
-   = help: This program is using integer-to-pointer casts or (equivalently) `ptr::with_exposed_provenance`, which means that Miri might miss pointer bugs in this program.
+   = help: this program is using integer-to-pointer casts or (equivalently) `ptr::with_exposed_provenance`, which means that Miri might miss pointer bugs in this program
-   = help: See https://doc.rust-lang.org/nightly/std/ptr/fn.with_exposed_provenance.html for more details on that operation.
+   = help: see https://doc.rust-lang.org/nightly/std/ptr/fn.with_exposed_provenance.html for more details on that operation
-   = help: To ensure that Miri does not miss bugs in your program, use Strict Provenance APIs (https://doc.rust-lang.org/nightly/std/ptr/index.html#strict-provenance, https://crates.io/crates/sptr) instead.
+   = help: to ensure that Miri does not miss bugs in your program, use Strict Provenance APIs (https://doc.rust-lang.org/nightly/std/ptr/index.html#strict-provenance, https://crates.io/crates/sptr) instead
-   = help: You can then set `MIRIFLAGS=-Zmiri-strict-provenance` to ensure you are not relying on `with_exposed_provenance` semantics.
+   = help: you can then set `MIRIFLAGS=-Zmiri-strict-provenance` to ensure you are not relying on `with_exposed_provenance` semantics
-   = help: Alternatively, `MIRIFLAGS=-Zmiri-permissive-provenance` disables this warning.
+   = help: alternatively, `MIRIFLAGS=-Zmiri-permissive-provenance` disables this warning
    = note: BACKTRACE:
    = note: inside `main` at $DIR/issue-miri-2389.rs:LL:CC