1 files changed, 143 insertions, 0 deletions
diff --git a/third_party/rust/safemem/src/lib.rs b/third_party/rust/safemem/src/lib.rs
new file mode 100644
index 0000000000..a30c933906
--- /dev/null
+++ b/third_party/rust/safemem/src/lib.rs
@@ -0,0 +1,143 @@
+//! Safe wrappers for memory-accessing functions like `std::ptr::copy()`.
+
+#![cfg_attr(not(feature = "std"), no_std)]
+
+#[cfg(not(feature = "std"))]
+extern crate core as std;
+use std::ptr;
+
+macro_rules! idx_check (
+    ($slice:expr, $idx:expr) => {
+        assert!($idx < $slice.len(),
+            concat!("`", stringify!($idx), "` ({}) out of bounds. Length: {}"),
+            $idx, $slice.len());
+    }
+);
+
+macro_rules! len_check (
+    ($slice:expr, $start:ident, $len:ident) => {
+        assert!(
+            $start.checked_add($len)
+                .expect(concat!("Overflow evaluating ", stringify!($start + $len)))
+                <= $slice.len(),
+            "Length {} starting at {} is out of bounds (slice len {}).", $len, $start, $slice.len()
+        )
+    }
+);
+
+/// Copy `len` elements from `src_idx` to `dest_idx`. Ranges may overlap.
+///
+/// Safe wrapper for `memmove()`/`std::ptr::copy()`.
+///
+/// ###Panics
+/// * If either `src_idx` or `dest_idx` are out of bounds, or if either of these plus `len` is out of
+/// bounds.
+/// * If `src_idx + len` or `dest_idx + len` overflows.
+pub fn copy_over<T: Copy>(slice: &mut [T], src_idx: usize, dest_idx: usize, len: usize) {
+    if slice.len() == 0 { return; }
+
+    idx_check!(slice, src_idx);
+    idx_check!(slice, dest_idx);
+    len_check!(slice, src_idx, len);
+    len_check!(slice, dest_idx, len);
+
+    // At any point a Rust reference exists, the compiler is free to do this.
+    // So we explicitely add it to be caught by miri.
+    #[cfg(miri)]
+    slice.iter().copied().for_each(drop);
+
+    let ptr = slice.as_mut_ptr();
+
+    unsafe {
+        ptr::copy(ptr.offset(src_idx as isize), ptr.offset(dest_idx as isize), len);
+    }
+}
+
+/// Safe wrapper for `std::ptr::write_bytes()`/`memset()`.
+pub fn write_bytes(slice: &mut [u8], byte: u8) {
+    unsafe {
+        ptr::write_bytes(slice.as_mut_ptr(), byte, slice.len());
+    }
+}
+
+/// Prepend `elems` to `vec`, resizing if necessary.
+///
+/// ### Panics
+///
+/// If `vec.len() + elems.len()` overflows.
+#[cfg(feature = "std")]
+pub fn prepend<T: Copy>(elems: &[T], vec: &mut Vec<T>) {
+    let elems_len = elems.len(); // `<= isize::MAX as usize`
+    if elems_len == 0 { return; }
+
+    let old_len = vec.len(); // `<= isize::MAX as usize`
+    if old_len == 0 {
+        // Prepend = append: delegate to Rust's stdlib implementation.
+        vec.extend_from_slice(elems);
+    } else {
+        // Our overflow check occurs here, no need to do it ourselves.
+        vec.reserve(elems_len);
+        let ptr = vec.as_mut_ptr();
+        unsafe {
+            // Move the old elements down to the end.
+            ptr::copy(
+                ptr,
+                ptr.offset(elems_len as isize),
+                old_len,
+            );
+            // Copy the input elements to the start
+            ptr::copy_nonoverlapping(
+                elems.as_ptr(),
+                ptr,
+                elems_len,
+            );
+            // Set the len *after* having initialized the elements.
+            vec.set_len(old_len + elems_len);
+        }
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    #[should_panic]
+    fn bounds_check() {
+        let mut arr = [0i32, 1, 2, 3, 4, 5];
+
+        copy_over(&mut arr, 2, 1, 7);
+    }
+
+    #[test]
+    fn copy_empty() {
+        let mut arr: [i32; 0] = [];
+
+        copy_over(&mut arr, 0, 0, 0);
+    }
+
+    #[test]
+    #[cfg(feature = "std")]
+    fn prepend_empty() {
+        let mut vec: Vec<i32> = vec![];
+        prepend(&[1, 2, 3], &mut vec);
+    }
+
+    #[test]
+    #[cfg(feature = "std")]
+    fn prepend_i32() {
+        let mut vec = vec![3, 4, 5];
+        prepend(&[1, 2], &mut vec);
+        assert_eq!(vec, &[1, 2, 3, 4, 5]);
+    }
+
+    /// Detect potential uninit values when running miri
+    #[test]
+    #[cfg(all(
+        feature = "std",
+        miri,
+    ))]
+    fn prepend_bool() {
+        prepend(&[true], &mut vec![false]);
+    }
+}