Add try_push_uninit and test

Author: phughk

.gitignore

@@ -10,3 +10,5 @@
 # Generated by Cargo
 /Cargo.lock
 /target
+.idea
+

src/arrayvec.rs

@@ -206,6 +206,22 @@ impl<T, const CAP: usize> ArrayVec<T, CAP> {
         ArrayVecImpl::try_push(self, element)
     }
 
+    /// Push a new uninitialised value to the end of the vector and return
+    /// a mutable reference to it.
+    ///
+    /// This is useful if the backed value is large and won't fit on stack.
+    ///
+    /// Capacity error carries the data - since we don't have data then we need a different error type.
+    pub unsafe fn try_push_uninit(&mut self) -> Result<*mut T, ()> {
+        let len = self.len();
+        if len >= Self::CAPACITY {
+            return Err(())
+        }
+        let new_ptr = self.as_mut_ptr().add(len);
+        self.set_len(len + 1);
+        Ok(new_ptr)
+    }
+
     /// Push `element` to the end of the vector without checking the capacity.
     ///
     /// It is up to the caller to ensure the capacity of the vector is

tests/tests.rs

@@ -7,7 +7,7 @@ use std::mem;
 use arrayvec::CapacityError;
 
 use std::collections::HashMap;
-
+use std::sync::RwLock;
 
 #[test]
 fn test_simple() {
@@ -777,3 +777,33 @@ fn test_arraystring_zero_filled_has_some_sanity_checks() {
     assert_eq!(string.as_str(), "\0\0\0\0");
     assert_eq!(string.len(), 4);
 }
+
+/// 3 MB struct
+struct ReallyBigStruct {
+    pub(crate) field_one: [u8; 1_000_000],
+    pub(crate) field_two: [u8; 1_000_000],
+    pub(crate) field_three: [u8; 1_000_000],
+}
+
+/// Const initialised struct outside of stack
+/// We need to initialise this outside of the stack, since otherwise there is a memory copy from
+/// the stack into the heap.
+/// With a static initialisation, we do not have a stack copy.
+static large_struct: RwLock<ArrayVec<ReallyBigStruct, 100>> = RwLock::new(ArrayVec::new_const());
+
+#[test]
+fn test_push_uninit() {
+    let mut lock = large_struct.write().unwrap();
+    let ptr = unsafe { lock.try_push_uninit().unwrap() };
+    let field_one = unsafe {&mut (*ptr).field_one};
+    *field_one = [1; 1_000_000];
+    let field_two = unsafe {&mut (*ptr).field_two};
+    *field_two = [2; 1_000_000];
+    let field_three = unsafe {&mut (*ptr).field_three};
+    *field_three = [3; 1_000_000];
+
+    assert_eq!(lock.len(), 1);
+    assert_eq!(lock[0].field_one[3], 1);
+    assert_eq!(lock[0].field_two[999], 2);
+    assert_eq!(lock[0].field_three[999_999], 3);
+}