Utility methods for artificial debug types in the generated DWARF

[yurydelendik]

Currently, artificial "wrapper" types are created for pointer types. These wrappers display Wasm pointers as i32. To dereference such wrapper/pointer the function's vmctx is needed. This PR is an attempt to provide overrides for artificial type for operator* and operator->.

The runtime/host has to have set_vmctx_memory and resolve_vmctx_memory_ptr builtins. The former sets current context and the former derefernces the Wasm pointer using current context's memory base.

During LLDB session, a user has to call __vmctx.set() to set the current context before calling any dereference operators. It can be automated via e.g. command regex pp 's/(.+)/p __vmctx->set(),%1/'.

Example of the LLDB session (see also gist):

Process 50041 stopped
* thread #1, queue = 'com.apple.main-thread', stop reason = breakpoint 1.1
    frame #0: 0x00000001042ca05d JIT(0x105000600)`check(i=1) at t.c:13:3
   10   
   11   int check(size_t i) {
   12     struct Test *y = t + i;
-> 13     y->i++;
   14     return y->j;
   15   }
   16   
(lldb) p y
(WebAssemblyPtrWrapper<Test>) $0 = (__ptr = 1032)
(lldb) pp *y
(Test) $1 = (i = 3, j = 4)
(lldb) pp y->j
(int) $2 = 4
(lldb) fr v
(WasmtimeVMContext *) __vmctx = 0x0000000103503ba0
(size_t) i = 1
(WebAssemblyPtrWrapper<Test>) y = (__ptr = 1032)

TODO:

• access to non-exported memory
• find better way to keep _set_vmctx_memory / _resolve_vmctx_memory_ptr exported (Unable to export not mangled symbol in an executable as libraries can do rust-lang/rust#25057 ?)

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[fitzgen]

Clever hack :)

[fitzgen]

If this module is pub (can also be #[doc(hidden)] to avoid "really" making it public) then I think we don't need this hack.

[bjorn3]

Or use #[doc(hidden)] pub use crate::debug_builtins::{set_vmctx_memory, resolve_vmctx_memory_ptr}; in the crate root.

[yurydelendik]

If you are suggesting to remove ensure_exported, then it does not work. See rust-lang/rust#25057