Fix UB in documentation example

.github/workflows/ci.yml

@@ -45,3 +45,13 @@ jobs:
         with:
           rust-version: nightly
       - run: cargo test -Zminimal-versions --verbose --all-features
+
+  miri:
+    name: Run tests under `miri` to check for UB
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v3
+      - uses: dtolnay/rust-toolchain@nightly
+        with:
+          components: miri
+      - run: cargo miri test --all-features

src/ascii_string.rs

@@ -64,13 +64,14 @@ impl AsciiString {
     /// This is highly unsafe, due to the number of invariants that aren't checked:
     ///
     /// * The memory at `buf` need to have been previously allocated by the same allocator this
-    ///   library uses.
+    ///   library uses, with an alignment of 1.
     /// * `length` needs to be less than or equal to `capacity`.
     /// * `capacity` needs to be the correct value.
     /// * `buf` must have `length` valid ascii elements and contain a total of `capacity` total,
     ///   possibly, uninitialized, elements.
+    /// * Nothing else must be using the memory `buf` points to.
     ///
-    /// Violating these may cause problems like corrupting the allocator's internal datastructures.
+    /// Violating these may cause problems like corrupting the allocator's internal data structures.
     ///
     /// # Examples
     ///
@@ -81,14 +82,14 @@ impl AsciiString {
     /// use std::mem;
     ///
     /// unsafe {
-    ///    let s = AsciiString::from_ascii("hello").unwrap();
-    ///    let ptr = s.as_ptr();
+    ///    let mut s = AsciiString::from_ascii("hello").unwrap();
+    ///    let ptr = s.as_mut_ptr();
     ///    let len = s.len();
     ///    let capacity = s.capacity();
     ///
     ///    mem::forget(s);
     ///
-    ///    let s = AsciiString::from_raw_parts(ptr as *mut _, len, capacity);
+    ///    let s = AsciiString::from_raw_parts(ptr, len, capacity);
     ///
     ///    assert_eq!(AsciiString::from_ascii("hello").unwrap(), s);
     /// }
@@ -97,9 +98,9 @@ impl AsciiString {
     #[must_use]
     pub unsafe fn from_raw_parts(buf: *mut AsciiChar, length: usize, capacity: usize) -> Self {
         AsciiString {
-            // SAFETY: Caller guarantees `buf` was previously allocated by this library,
-            //         that `buf` contains `length` valid ascii elements and has a total
-            //         capacity of `capacity` elements.
+            // SAFETY: Caller guarantees that `buf` was previously allocated by this library,
+            //         that `buf` contains `length` valid ascii elements and has a total capacity
+            //         of `capacity` elements, and that nothing else is using the momory.
             vec: unsafe { Vec::from_raw_parts(buf, length, capacity) },
         }
     }