From 83e37f933478de23a903a4725e4336df62636ab5 Mon Sep 17 00:00:00 2001
From: Alex Crichton <alex@alexcrichton.com>
Date: Tue, 16 Aug 2022 16:44:16 -0500
Subject: [PATCH] Fix a compile error on nightly Rust (#4724)

It looks like Rust nightly has gotten a bit more strict about
attributes-on-expressions and previously accepted code is no longer
accepted. This commit updates the generated code for a macro to a form
which is accepted by rustc.
---
 crates/wasmtime/src/component/func.rs | 21 +++++++++++++--------
 1 file changed, 13 insertions(+), 8 deletions(-)

diff --git a/crates/wasmtime/src/component/func.rs b/crates/wasmtime/src/component/func.rs
index 396d6d83c1..fceab79a96 100644
--- a/crates/wasmtime/src/component/func.rs
+++ b/crates/wasmtime/src/component/func.rs
@@ -37,15 +37,20 @@ use wasmtime_runtime::{Export, ExportFunction, VMTrampoline};
 #[doc(hidden)]
 #[macro_export]
 macro_rules! map_maybe_uninit {
-    ($maybe_uninit:ident $($field:tt)*) => (#[allow(unused_unsafe)] unsafe {
-        use $crate::component::__internal::MaybeUninitExt;
+    ($maybe_uninit:ident $($field:tt)*) => ({
+        #[allow(unused_unsafe)]
+        {
+            unsafe {
+                use $crate::component::__internal::MaybeUninitExt;
 
-        let m: &mut std::mem::MaybeUninit<_> = $maybe_uninit;
-        // Note the usage of `addr_of_mut!` here which is an attempt to "stay
-        // safe" here where we never accidentally create `&mut T` where `T` is
-        // actually uninitialized, hopefully appeasing the Rust unsafe
-        // guidelines gods.
-        m.map(|p| std::ptr::addr_of_mut!((*p)$($field)*))
+                let m: &mut std::mem::MaybeUninit<_> = $maybe_uninit;
+                // Note the usage of `addr_of_mut!` here which is an attempt to "stay
+                // safe" here where we never accidentally create `&mut T` where `T` is
+                // actually uninitialized, hopefully appeasing the Rust unsafe
+                // guidelines gods.
+                m.map(|p| std::ptr::addr_of_mut!((*p)$($field)*))
+            }
+        }
     })
 }