Make heap-allocated alternatives or replacements for MemoryDump and MemoryShim

[gipsond]

what

lc3_isa::util::MemoryDump and lc3_shims::memory::MemoryShim contain full LC-3 memory images (128 KiB). Currently, they allocate this large amount of memory on the stack. This has led to the assembler and TUI increasing their stack sizes. Due to the memory overhead, we already avoid using these structs on non-hosted platforms, so we could change the structs to use heap-allocated memory, or add a heap-allocated alternative. We expect this would allow the assembler and TUI to return to using default stack sizes.

steps

• Make heap-allocated MemoryDump alternative or replacement
• Make heap-allocated MemoryShim alternative or replacement
• Replace usages of MemoryDump and MemoryShim (as necessary)

where

branch: imp-heap-memorydump

lc3_isa::util::MemoryDump
lc3_shims::memory::MemoryShim

relevant code:

• lc3_isa::util::MemoryDump
• lc3_shims::memory::MemoryShim

open questions

• Replace MemoryDump entirely? Or just provide alternative?
• Replace MemoryShim entirely? Or just provide alternative?

[rrbutani]

Sorry for the delay! I'm going to go work on this now.

---

Just for posterity, here's some context from Slack:

an example

I think we can just fix MemoryShim and MemoryDump to do the right thing (internally house a Vec<u16> that's private and guaranteed to be the right size; implement Deref<[u16; ADDR_SPACE_SIZE]> using unwrap_unchecked, etc.)

I looked through all the usages and I think we're fine doing this
we do use MemoryDump in some APIs that are exposed in crates that need to compile with #![no_std] but we already never expected to be able to create a MemoryDump on non-hosted platforms (256K is too much to put on the stack, etc.)
I think I'll probably do:

// a separate struct so that we can make the inner field private
struct OwnedMemoryDump(Vec<Word>);

pub enum MemoryDump {
    #[cfg(feature = "std")]
    Owned(OwnedMemoryDump),
    Borrowed(&'static [u16; ADDR_SPACE_SIZE]), 
}

or something similar just so we don't have to go gate those APIs on std