Utility methods for artificial debug types in the generated DWARF (#1482)

* add operator*
* add operator->
* add ptr() unwrap method
* comments/refactor
* macro_rules
* external symbols workaround

crates/runtime/src/debug_builtins.rs

@@ -0,0 +1,41 @@
+#![doc(hidden)]
+
+use crate::instance::InstanceHandle;
+use crate::vmcontext::VMContext;
+use wasmtime_environ::entity::EntityRef;
+use wasmtime_environ::wasm::MemoryIndex;
+
+static mut VMCTX_AND_MEMORY: (*mut VMContext, usize) = (std::ptr::null_mut(), 0);
+
+#[no_mangle]
+pub unsafe extern "C" fn resolve_vmctx_memory_ptr(p: *const u32) -> *const u8 {
+    let ptr = std::ptr::read(p);
+    assert!(
+        !VMCTX_AND_MEMORY.0.is_null(),
+        "must call `__vmctx->set()` before resolving Wasm pointers"
+    );
+    let handle = InstanceHandle::from_vmctx(VMCTX_AND_MEMORY.0);
+    assert!(
+        VMCTX_AND_MEMORY.1 < handle.instance().module().local.memory_plans.len(),
+        "memory index for debugger is out of bounds"
+    );
+    let index = MemoryIndex::new(VMCTX_AND_MEMORY.1);
+    let mem = handle.instance().get_memory(index);
+    mem.base.add(ptr as usize)
+}
+
+#[no_mangle]
+pub unsafe extern "C" fn set_vmctx_memory(vmctx_ptr: *mut VMContext) {
+    // TODO multi-memory
+    VMCTX_AND_MEMORY = (vmctx_ptr, 0);
+}
+
+// Ensures that set_vmctx_memory and resolve_vmctx_memory_ptr are linked and
+// exported as symbols. It is a workaround: the executable normally ignores
+// `pub extern "C"`, see rust-lang/rust#25057.
+pub fn ensure_exported() {
+    unsafe {
+        std::ptr::read_volatile(resolve_vmctx_memory_ptr as *const u8);
+        std::ptr::read_volatile(set_vmctx_memory as *const u8);
+    }
+}