LWDR (Light Weight D Runtime) v0.3.0

[Dylan Graham]

[Github](https://github.com/0dyl/LWDR)
[DUB](https://code.dlang.org/packages/lwdr)
[Previous 
announcement](https://forum.dlang.org/post/giigcnoyxfoxxaevjmxy@forum.dlang.org)

LWDR (Light Weight D Runtime) is a ground-up implementation of a 
D runtime targeting the ARM Cortex-M microcontrollers and other 
barebones environments. It works by providing a series of basic 
API hooks (as defined in 
[rtoslink.d](https://github.com/0dyl/LWDR/blob/master/source/rtoslink.d) that you must implement and/or point to your RTOS implementation.

This is V0.3.0 of LWDR. Since V0.2.3, the following has been 
worked on:
1. Thread Local Storage support
2. Primitive memory tracking for Phobos allocations that would 
normally rely on a GC
3. Transition to an opt-in system
4. Replacement of `delete` with `LWDR.free(..)` due to deprecation
5. Source code documentation improvements
6. `RefCount!T` and `Unique!T` LWDR-specific implementations

**Thread Local Storage**
This feature is rather abstract, and it is an opt-in with version 
`LWDR_TLS`. You must provide support in your linker script for 
`tdata` and `tbss` sections. It works by utilising the underlying 
RTOS's TLS implementation 
([example](https://www.freertos.org/pvTaskGetThreadLocalStoragePointer.html)). When `LWDR.registerCurrentThread()` is called a block of D memory is allocated containing the TLS variables for the current thread, and the pointer to the block is stored in the thread's TCB (Thread Control Block). When a TLS variable (ie, a static variable) is accessed, `__aeabi_read_tp` is called, yielding the pointer.

**Memory Tracking**
This is very primitive. It's *only* meant to assist with stopping 
GC-reliant stdlib allocations from leaking. It pretty much 
behaves as defined 
[here](https://forum.dlang.org/post/gfyhdqecjxszrgutlhmi@forum.dlang.org).

**Opt In**
To be able to keep the size of `TypeInfo` vtables down and such, 
LWDR has adopted an opt-in system, which relies on [D's version 
feature](https://dlang.org/spec/version.html). The current 
opt-ins are:
1. `LWDR_TLS` - Enables TLS support
2. `LWDR_DynamicArray` - Enables dynamic arrays
3. `LWDR_TrackMem` - Enables the mess above.

**Replacement of `delete`**
`delete` has been 
[deprecated](https://dlang.org/deprecate.html#delete). 
`LWDR.free` has been implemented in its place to prevent compiler 
warnings.

**Source Code Documentation**
Runtimes are hairy and scary - so I'm beginning to put more 
effort into documenting how things work. So far, it's only 
[ddoc](https://dlang.org/spec/ddoc.html) comments.

**`RefCount!T` and `Unique!T`**
To alleviate the lack of GC, I have implemented an LWDR-specific 
solution inspired by 
[automem](https://code.dlang.org/packages/automem).

**What works?**
1. Class allocations and deallocations (via `new` and `LWDR.free`)
2. Struct heap allocations and deallocations (via `new` and 
`LWDR.free`)
3. Invariants
4. Asserts
5. Contract programming
6. Basic RTTI (via `TypeInfo` stubs)
7. Interfaces
8. Static Arrays
9. Virtual functions and overrides
10. Abstract classes
11. Static classes
12. Allocation and deallocation of dynamic arrays (opt in by 
version `LWDR_DynamicArray`)
13. Concatenate an item to a dynamic array (opt in by version 
`LWDR_DynamicArray`)
14. Concatenate two dynamic arrays together (opt in by version 
`LWDR_DynamicArray`)
15. Dynamic array resizing (opt in by version `LWDR_DynamicArray`)
16. Thread local storage (opt in by version `LWDR_TLS`)

**What doesn't work?**
1. Exceptions and Throwables (experimental implementation was 
removed)
2. Module constructors and destructors
3. Static constructors and destructors
4. Shared static constructors and destructors
5. Module info
6. There is no GC implementation (primitive memory tracking is 
now available with `LWDR_TrackMem`, `RefCount!T` and `Unique!T` 
are now available)
7. Delegates/closures
8. Associative arrays
9. Object monitors
10. `shared`/`synchronised`
11. Object hashing
12. Other things I can't remember off the top of my head.

It's still a beta - so expect bugs and warts. Some bits have been 
thoroughly tested, others not so much.

Because the runtime has ballooned so quickly, I want to pause on 
development for a little bit so that I can begin using LWDR in a 
proper project and find and squash bugs. The project is pretty 
much the successor to my [Driving with D 
article](https://dlang.org/blog/2021/06/01/driving-with-d/) (it's 
an automotive project). I'm also thinking of applying with LWDR 
to the Autumn of Code thingo.