tree borrows: test String::as_mut_ptr

Author: RalfJung

src/tools/miri/tests/pass/tree-borrows/read-only-from-mut.rs

@@ -0,0 +1,41 @@
+//@compile-flags: -Zmiri-tree-borrows
+
+use std::mem;
+
+fn main() {
+    aliasing_read_only_mutable_refs();
+    string_as_mut_ptr();
+}
+
+// Tree Borrows has no issue with several mutable references existing
+// at the same time, as long as they are used only immutably.
+// I.e. multiple Reserved can coexist.
+pub fn aliasing_read_only_mutable_refs() {
+    unsafe {
+        let base = &mut 42u64;
+        let r1 = &mut *(base as *mut u64);
+        let r2 = &mut *(base as *mut u64);
+        let _l = *r1;
+        let _l = *r2;
+    }
+}
+
+pub fn string_as_mut_ptr() {
+    // This errors in Stacked Borrows since as_mut_ptr restricts the provenance,
+    // but with Tree Borrows it should work.
+    unsafe {
+        let mut s = String::from("hello");
+        s.reserve(1); // make the `str` that `s` derefs to not cover the entire `s`.
+
+        // Prevent automatically dropping the String's data
+        let mut s = mem::ManuallyDrop::new(s);
+
+        let ptr = s.as_mut_ptr();
+        let len = s.len();
+        let capacity = s.capacity();
+
+        let s = String::from_raw_parts(ptr, len, capacity);
+
+        assert_eq!(String::from("hello"), s);
+    }
+}