Auto merge of #104844 - cjgillot:mention-eval-place, r=jackh726,RalfJung

bors · bors · commit 90afd359c59c · 2023-04-22T09:54:21.000Z
Evaluate place expression in `PlaceMention` rust-lang/rust#102256 introduces a `PlaceMention(place)` MIR statement which keep trace of `let _ = place` statements from surface rust, but without semantics. This PR proposes to change the behaviour of `let _ =` patterns with respect to the borrow-checker to verify that the bound place is live. Specifically, consider this code: ```rust let _ = { let a = 5; &a }; ``` This passes borrowck without error on stable. Meanwhile, replacing `_` by `_: _` or `_p` errors with "error[E0597]: `a` does not live long enough", [see playground](https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=c448d25a7c205dc95a0967fe96bccce8). This PR *does not* change how `_` patterns behave with respect to initializedness: it remains ok to bind a moved-from place to `_`. The relevant test is `tests/ui/borrowck/let_underscore_temporary.rs`. Crater check found no regression. For consistency, this PR changes miri to evaluate the place found in `PlaceMention`, and report eventual dangling pointers found within it. r? `@RalfJung`
diff --git a/src/lib.rs b/src/lib.rs
@@ -130,6 +130,7 @@ pub const MIRI_DEFAULT_ARGS: &[&str] = &[
     "-Zalways-encode-mir",
     "-Zextra-const-ub-checks",
     "-Zmir-emit-retag",
+    "-Zmir-keep-place-mention",
     "-Zmir-opt-level=0",
     "-Zmir-enable-passes=-CheckAlignment",
 ];
diff --git a/tests/fail/dangling_pointers/dangling_pointer_deref_underscore.rs b/tests/fail/dangling_pointers/dangling_pointer_deref_underscore.rs
@@ -0,0 +1,13 @@
+// Make sure we find these even with many checks disabled.
+//@compile-flags: -Zmiri-disable-alignment-check -Zmiri-disable-stacked-borrows -Zmiri-disable-validation
+
+fn main() {
+    let p = {
+        let b = Box::new(42);
+        &*b as *const i32
+    };
+    unsafe {
+        let _ = *p; //~ ERROR: dereferenced after this allocation got freed
+    }
+    panic!("this should never print");
+}
diff --git a/tests/fail/dangling_pointers/dangling_pointer_deref_underscore.stderr b/tests/fail/dangling_pointers/dangling_pointer_deref_underscore.stderr
@@ -0,0 +1,15 @@
+error: Undefined Behavior: pointer to ALLOC was dereferenced after this allocation got freed
+  --> $DIR/dangling_pointer_deref_underscore.rs:LL:CC
+   |
+LL |         let _ = *p;
+   |                 ^^ pointer to ALLOC was dereferenced after this allocation got freed
+   |
+   = help: this indicates a bug in the program: it performed an invalid operation, and caused Undefined Behavior
+   = help: see https://doc.rust-lang.org/nightly/reference/behavior-considered-undefined.html for further information
+   = note: BACKTRACE:
+   = note: inside `main` at $DIR/dangling_pointer_deref_underscore.rs:LL:CC
+
+note: some details are omitted, run with `MIRIFLAGS=-Zmiri-backtrace=full` for a verbose backtrace
+
+error: aborting due to previous error
+
