use BTreeMap instead of HashMap in async_support::FutureState (bytecodealliance#1339)

As of this writing, initializing the default hasher for `HashMap` requires
calling `wasi_snapshot_preview1:random_get`, which requires initializing the
`wasi_snapshot_preview1` adapter when targeting `wasm32-wasip2` and later, and
that's expensive enough that we'd prefer to avoid it for apps which otherwise
make no use of the adapter.

Signed-off-by: Joel Dice <[email protected]>

Author: dicej

crates/guest-rust/rt/src/async_support.rs

@@ -7,7 +7,7 @@
 extern crate std;
 use core::sync::atomic::{AtomicBool, Ordering};
 use std::boxed::Box;
-use std::collections::HashMap;
+use std::collections::BTreeMap;
 use std::ffi::c_void;
 use std::future::Future;
 use std::mem;
@@ -68,7 +68,14 @@ struct FutureState {
 
     /// State of all waitables in `waitable_set`, and the ptr/callback they're
     /// associated with.
-    waitables: HashMap<u32, (*mut c_void, unsafe extern "C" fn(*mut c_void, u32))>,
+    //
+    // Note that this is a `BTreeMap` rather than a `HashMap` only because, as
+    // of this writing, initializing the default hasher for `HashMap` requires
+    // calling `wasi_snapshot_preview1:random_get`, which requires initializing
+    // the `wasi_snapshot_preview1` adapter when targeting `wasm32-wasip2` and
+    // later, and that's expensive enough that we'd prefer to avoid it for apps
+    // which otherwise make no use of the adapter.
+    waitables: BTreeMap<u32, (*mut c_void, unsafe extern "C" fn(*mut c_void, u32))>,
 
     /// Raw structure used to pass to `cabi::wasip3_task_set`
     wasip3_task: cabi::wasip3_task,
@@ -89,7 +96,7 @@ impl FutureState {
             waker,
             tasks: [future].into_iter().collect(),
             waitable_set: None,
-            waitables: HashMap::new(),
+            waitables: BTreeMap::new(),
             wasip3_task: cabi::wasip3_task {
                 // This pointer is filled in before calling `wasip3_task_set`.
                 ptr: ptr::null_mut(),