diff --git a/lib/jit/src/instantiate.rs b/lib/jit/src/instantiate.rs
index 2c5bf8c2b5..43ffdf9e19 100644
--- a/lib/jit/src/instantiate.rs
+++ b/lib/jit/src/instantiate.rs
@@ -174,6 +174,7 @@ impl CompiledModule {
             self.imports.clone(),
             &data_initializers,
             self.signatures.clone(),
+            Box::new(()),
         )
     }
 }
@@ -214,6 +215,7 @@ pub fn instantiate(
         raw.imports,
         &*raw.data_initializers,
         raw.signatures,
+        Box::new(()),
     )
     .map_err(SetupError::Instantiate)
 }
diff --git a/lib/runtime/src/instance.rs b/lib/runtime/src/instance.rs
index a55399a932..c08544d605 100644
--- a/lib/runtime/src/instance.rs
+++ b/lib/runtime/src/instance.rs
@@ -13,6 +13,7 @@ use crate::vmcontext::{
     VMGlobalImport, VMMemoryDefinition, VMMemoryImport, VMSharedSignatureIndex, VMTableDefinition,
     VMTableImport,
 };
+use core::any::Any;
 use core::slice;
 use core::{mem, ptr};
 use cranelift_entity::EntityRef;
@@ -22,6 +23,7 @@ use cranelift_wasm::{
     GlobalIndex, GlobalInit, MemoryIndex, SignatureIndex, TableIndex,
 };
 use std::borrow::ToOwned;
+use std::boxed::Box;
 use std::rc::Rc;
 use std::string::String;
 use wasmtime_environ::{DataInitializer, Module, TableElements, VMOffsets};
@@ -85,7 +87,12 @@ pub(crate) struct InstanceContents {
     /// Pointers to functions in executable memory.
     finished_functions: BoxedSlice<DefinedFuncIndex, *const VMFunctionBody>,
 
-    /// Context pointer used by compiled wasm code.
+    /// Hosts can store arbitrary per-instance information here.
+    host_state: Box<dyn Any>,
+
+    /// Context pointer used by compiled wasm code. This field is last, and
+    /// represents a dynamically-sized array that extends beyond the nominal
+    /// end of the struct (similar to a flexible array member).
     vmctx: VMContext,
 }
 
@@ -453,6 +460,7 @@ impl Instance {
         imports: Imports,
         data_initializers: &[DataInitializer<'_>],
         vmshared_signatures: BoxedSlice<SignatureIndex, VMSharedSignatureIndex>,
+        host_state: Box<dyn Any>,
     ) -> Result<Self, InstantiationError> {
         let mut tables = create_tables(&module);
         let mut memories = create_memories(&module)?;
@@ -498,6 +506,7 @@ impl Instance {
                 memories,
                 tables,
                 finished_functions,
+                host_state,
                 vmctx: VMContext {},
             };
             unsafe {
@@ -674,6 +683,11 @@ impl Instance {
         let temporary_mut = &mut *(self as *const Self as *mut Self);
         temporary_mut.lookup(field)
     }
+
+    /// Return a reference to the custom state attached to this instance.
+    pub fn host_state(&mut self) -> &mut Any {
+        return &mut *self.mmap_field.contents_mut().host_state;
+    }
 }
 
 fn check_table_init_bounds(
diff --git a/lib/wast/src/spectest.rs b/lib/wast/src/spectest.rs
index d31d8dc9de..6f74877cda 100644
--- a/lib/wast/src/spectest.rs
+++ b/lib/wast/src/spectest.rs
@@ -2,9 +2,10 @@ use cranelift_codegen::ir::types;
 use cranelift_codegen::{ir, isa};
 use cranelift_entity::PrimaryMap;
 use cranelift_wasm::{DefinedFuncIndex, Global, GlobalInit, Memory, Table, TableElementType};
+use std::rc::Rc;
 use target_lexicon::HOST;
 use wasmtime_environ::{translate_signature, Export, MemoryPlan, Module, TablePlan};
-use wasmtime_jit::{target_tunables, CompiledModule};
+use wasmtime_jit::target_tunables;
 use wasmtime_runtime::{Imports, Instance, InstantiationError, VMFunctionBody};
 
 extern "C" fn spectest_print() {}
@@ -213,12 +214,12 @@ pub fn instantiate_spectest() -> Result<Instance, InstantiationError> {
     let data_initializers = Vec::new();
     let signatures = PrimaryMap::new();
 
-    CompiledModule::from_parts(
-        module,
+    Instance::new(
+        Rc::new(module),
         finished_functions.into_boxed_slice(),
         imports,
-        data_initializers.into_boxed_slice(),
+        &data_initializers,
         signatures.into_boxed_slice(),
+        Box::new(()),
     )
-    .instantiate()
 }