From 59be6dc5ff258ca9df174cfd9a91cc4149dc8a40 Mon Sep 17 00:00:00 2001
From: Alex Crichton <alex@alexcrichton.com>
Date: Wed, 11 Nov 2020 11:59:40 -0600
Subject: [PATCH] Fix module initialization with externref element segments
 (#2392)

This commit fixes an issue with reference-types-using-modules where they
panicked on instantiation if any element segments had an externref null
specified.
---
 crates/runtime/src/instance.rs                | 22 +++++++++++--------
 .../reference-types/externref-segment.wast    |  5 +++++
 2 files changed, 18 insertions(+), 9 deletions(-)
 create mode 100644 tests/misc_testsuite/reference-types/externref-segment.wast

diff --git a/crates/runtime/src/instance.rs b/crates/runtime/src/instance.rs
index 87e7c326fe..a95a4b594b 100644
--- a/crates/runtime/src/instance.rs
+++ b/crates/runtime/src/instance.rs
@@ -1270,15 +1270,19 @@ fn initialize_tables(instance: &Instance) -> Result<(), InstantiationError> {
         }
 
         for (i, func_idx) in init.elements.iter().enumerate() {
-            let anyfunc = instance.get_caller_checked_anyfunc(*func_idx).map_or(
-                ptr::null_mut(),
-                |f: &VMCallerCheckedAnyfunc| {
-                    f as *const VMCallerCheckedAnyfunc as *mut VMCallerCheckedAnyfunc
-                },
-            );
-            table
-                .set(u32::try_from(start + i).unwrap(), anyfunc.into())
-                .unwrap();
+            let item = match table.element_type() {
+                TableElementType::Func => instance
+                    .get_caller_checked_anyfunc(*func_idx)
+                    .map_or(ptr::null_mut(), |f: &VMCallerCheckedAnyfunc| {
+                        f as *const VMCallerCheckedAnyfunc as *mut VMCallerCheckedAnyfunc
+                    })
+                    .into(),
+                TableElementType::Val(_) => {
+                    assert!(*func_idx == FuncIndex::reserved_value());
+                    TableElement::ExternRef(None)
+                }
+            };
+            table.set(u32::try_from(start + i).unwrap(), item).unwrap();
         }
     }
 
diff --git a/tests/misc_testsuite/reference-types/externref-segment.wast b/tests/misc_testsuite/reference-types/externref-segment.wast
new file mode 100644
index 0000000000..9da6ff6674
--- /dev/null
+++ b/tests/misc_testsuite/reference-types/externref-segment.wast
@@ -0,0 +1,5 @@
+(module
+  (table 2 externref)
+  (elem (i32.const 0) externref (ref.null extern))
+  (elem (i32.const 1) externref (ref.null extern))
+)