optimize the codegen of Vec::clone

these changes optimize `Vec<u8, 1024>::clone` down to these operations

1. reserve the stack space (1028 bytes on 32-bit ARM) and leave it uninitialized
2. zero the `len` field
3. memcpy `len` bytes of data from the parent

analyzed source code

``` rust
use heapless::Vec;

fn clone(vec: &Vec<u8, 1024>) {
    let mut vec = vec.clone();
    black_box(&mut vec);
}

fn black_box<T>(val: &mut T) {
    unsafe { asm!("// {0}", in(reg) val) }
}
```

machine code with `lto = fat`, `codegen-units = 1` and `opt-level = 'z'` ('z' instead of 3 to avoid loop unrolling and keep the machine code readable)

``` armasm
00020100 <clone>:
   20100:              b5d0             push    {r4, r6, r7, lr}
   20102:              af02             add     r7, sp, #8
   20104:              f5ad 6d81        sub.w   sp, sp, #1032   ; 0x408
   20108:              2300             movs    r3, #0
   2010a:              c802             ldmia   r0!, {r1}
   2010c:              9301             str     r3, [sp, #4]
   2010e:              aa01             add     r2, sp, #4
   20110:       /--/-X b141             cbz     r1, 20124 <clone+0x24>
   20112:       |  |   4413             add     r3, r2
   20114:       |  |   f810 4b01        ldrb.w  r4, [r0], #1
   20118:       |  |   3901             subs    r1, #1
   2011a:       |  |   711c             strb    r4, [r3, #4]
   2011c:       |  |   9b01             ldr     r3, [sp, #4]
   2011e:       |  |   3301             adds    r3, #1
   20120:       |  |   9301             str     r3, [sp, #4]
   20122:       |  \-- e7f5             b.n     20110 <clone+0x10>
   20124:       \----> a801             add     r0, sp, #4
   20126:              f50d 6d81        add.w   sp, sp, #1032   ; 0x408
   2012a:              bdd0             pop     {r4, r6, r7, pc}
```

note that it's not optimizing step (3) to an actual `memcpy` because we lack the 'trait specialization' code that libstd uses

---

before `clone` was optimized to

1. reserve and zero (`memclr`) 1028 (!?) bytes of stack space
2. (unnecessarily) runtime check if `len` is equal or less than 1024 (capacity) -- this included a panicking branch
3. memcpy `len` bytes of data from the parent

Author: japaric

src/vec.rs

@@ -34,12 +34,18 @@ use hash32;
 /// assert_eq!(*vec, [7, 1, 2, 3]);
 /// ```
 pub struct Vec<T, const N: usize> {
-    buffer: [MaybeUninit<T>; N],
+    // NOTE order is important for optimizations. the `len` first layout lets the compiler optimize
+    // `new` to: reserve stack space and zero the first word. With the fields in the reverse order
+    // the compiler optimizes `new` to `memclr`-ing the *entire* stack space, including the `buffer`
+    // field which should be left uninitialized. Optimizations were last checked with Rust 1.60
     len: usize,
+
+    buffer: [MaybeUninit<T>; N],
 }
 
 impl<T, const N: usize> Vec<T, N> {
-    const INIT: MaybeUninit<T> = MaybeUninit::uninit();
+    const ELEM: MaybeUninit<T> = MaybeUninit::uninit();
+    const INIT: [MaybeUninit<T>; N] = [Self::ELEM; N]; // important for optimization of `new`
 
     /// Constructs a new, empty vector with a fixed capacity of `N`
     ///
@@ -60,8 +66,8 @@ impl<T, const N: usize> Vec<T, N> {
         crate::sealed::greater_than_eq_0::<N>();
 
         Self {
-            buffer: [Self::INIT; N],
             len: 0,
+            buffer: Self::INIT,
         }
     }
 
@@ -92,7 +98,12 @@ impl<T, const N: usize> Vec<T, N> {
         T: Clone,
     {
         let mut new = Self::new();
-        new.extend_from_slice(self.as_slice()).unwrap();
+        // avoid `extend_from_slice` as that introduces a runtime check / panicking branch
+        for elem in self {
+            unsafe {
+                new.push_unchecked(elem.clone());
+            }
+        }
         new
     }