optimize the codegen of Vec::clone

[japaric]

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

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)

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

[japaric]

bors r+

[bors]

Build succeeded:

• ci