better diagnostics for Tree Borrows + int2ptr casts

Author: RalfJung

src/tools/miri/src/bin/miri.rs

@@ -620,6 +620,14 @@ fn main() {
             "-Zmiri-unique-is-unique only has an effect when -Zmiri-tree-borrows is also used"
         );
     }
+    // Tree Borrows + permissive provenance does not work.
+    if miri_config.provenance_mode == ProvenanceMode::Permissive
+        && matches!(miri_config.borrow_tracker, Some(BorrowTrackerMethod::TreeBorrows))
+    {
+        show_error!(
+            "Tree Borrows does not support integer-to-pointer casts, and is hence not compatible with permissive provenance"
+        );
+    }
 
     debug!("rustc arguments: {:?}", rustc_args);
     debug!("crate arguments: {:?}", miri_config.args);

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

@@ -232,6 +232,10 @@ impl GlobalStateInner {
     pub fn remove_unreachable_allocs(&mut self, allocs: &LiveAllocs<'_, '_>) {
         self.root_ptr_tags.retain(|id, _| allocs.is_live(*id));
     }
+
+    pub fn borrow_tracker_method(&self) -> BorrowTrackerMethod {
+        self.borrow_tracker_method
+    }
 }
 
 /// Which borrow tracking method to use

src/tools/miri/src/diagnostics.rs

@@ -647,8 +647,8 @@ impl<'tcx> MiriMachine<'tcx> {
         };
 
         let helps = match &e {
-            Int2Ptr { details: true } =>
-                vec![
+            Int2Ptr { details: true } => {
+                let mut v = vec![
                     (
                         None,
                         format!(
@@ -673,13 +673,26 @@ impl<'tcx> MiriMachine<'tcx> {
                             "you can then set `MIRIFLAGS=-Zmiri-strict-provenance` to ensure you are not relying on `with_exposed_provenance` semantics"
                         ),
                     ),
-                    (
+                ];
+                if self.borrow_tracker.as_ref().is_some_and(|b| {
+                    matches!(b.borrow().borrow_tracker_method(), BorrowTrackerMethod::TreeBorrows)
+                }) {
+                    v.push((
+                        None,
+                        format!(
+                            "Tree Borrows does not support integer-to-pointer casts, so the program is likely to go wrong when this pointer gets used"
+                        ),
+                    ));
+                } else {
+                    v.push((
                         None,
                         format!(
                             "alternatively, `MIRIFLAGS=-Zmiri-permissive-provenance` disables this warning"
                         ),
-                    ),
-                ],
+                    ));
+                }
+                v
+            }
             ExternTypeReborrow => {
                 vec![
                     (

src/tools/miri/tests/pass/adjacent-allocs.rs

@@ -1,5 +1,3 @@
-//@revisions: stack tree
-//@[tree]compile-flags: -Zmiri-tree-borrows
 //@compile-flags: -Zmiri-permissive-provenance
 
 fn ensure_allocs_can_be_adjacent() {

src/tools/miri/tests/pass/box.rs

@@ -1,5 +1,4 @@
-//@revisions: stack tree
-//@[tree]compile-flags: -Zmiri-tree-borrows -Zmiri-permissive-provenance
+//@compile-flags: -Zmiri-permissive-provenance
 #![feature(ptr_internals)]
 
 fn main() {

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

@@ -1,12 +1,14 @@
 //@revisions: stack tree
-//@[tree]compile-flags: -Zmiri-tree-borrows -Zmiri-permissive-provenance
-#![feature(extern_types)]
+//@[tree]compile-flags: -Zmiri-tree-borrows
+#![feature(extern_types, strict_provenance)]
+
+use std::ptr;
 
 extern "C" {
     type Foo;
 }
 
 fn main() {
-    let x: &Foo = unsafe { &*(16 as *const Foo) };
+    let x: &Foo = unsafe { &*(ptr::without_provenance::<()>(16) as *const Foo) };
     let _y: &Foo = &*x;
 }

src/tools/miri/tests/pass/box.stack.stdout renamed to src/tools/miri/tests/pass/box.stdout

@@ -1,22 +1,8 @@
-warning: integer-to-pointer cast
-  --> $DIR/extern_types.rs:LL:CC
-   |
-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: 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: 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
-   = 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
+LL |     let x: &Foo = unsafe { &*(ptr::without_provenance::<()>(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