Add comments about vmctx pointers in various datastructures. (#2925)

This forward-ports the relevant parts of #1396.

crates/runtime/src/export.rs

@@ -89,7 +89,8 @@ impl From<ExportMemory> for Export {
 pub struct ExportGlobal {
     /// The address of the global storage.
     pub definition: *mut VMGlobalDefinition,
-    /// Pointer to the containing `VMContext`.
+    /// Pointer to a `VMContext` which has a lifetime at least as long as the
+    /// global. This may not be the `VMContext` which defines the global.
     pub vmctx: *mut VMContext,
     /// The global declaration, used for compatibilty checking.
     pub global: Global,

crates/runtime/src/vmcontext.rs

@@ -159,6 +159,10 @@ mod test_vmmemory_import {
 
 /// The fields compiled code needs to access to utilize a WebAssembly global
 /// variable imported from another instance.
+///
+/// Note that unlike with functions, tables, and memories, `VMGlobalImport`
+/// doesn't include a `vmctx` pointer. Globals are never resized, and don't
+/// require a `vmctx` pointer to access.
 #[derive(Debug, Copy, Clone)]
 #[repr(C)]
 pub struct VMGlobalImport {
@@ -744,8 +748,6 @@ mod test_vminterrupts {
 /// The struct here is empty, as the sizes of these fields are dynamic, and
 /// we can't describe them in Rust's type system. Sufficient memory is
 /// allocated at runtime.
-///
-/// TODO: We could move the globals into the `vmctx` allocation too.
 #[derive(Debug)]
 #[repr(C, align(16))] // align 16 since globals are aligned to that and contained inside
 pub struct VMContext {