Document why get_export requires a mutable context

crates/wasmtime/src/instance.rs

@@ -299,6 +299,23 @@ impl Instance {
     /// # Panics
     ///
     /// Panics if `store` does not own this instance.
+    ///
+    /// # Why does `get_export` take a mutable context?
+    ///
+    /// This method requires a mutable context because an instance's exports are
+    /// lazily populated. While `wasmtime` could use interior mutability to
+    /// paper over this, since getting an export is "logically" a non-mutating
+    /// operation, this would bring with it a new problem. `wasmtime` would have
+    /// to choose whether to use a mutex or `RefCell` for the interior
+    /// mutability; the former implies unnecessasry overhead for single-threaded
+    /// usage, while the latter would prohibit `Store` from implementing `Send`
+    /// and `Sync`, making multi-threaded usage impossible.
+    ///
+    /// Given these trade offs -- and because `wasmtime` will not stop lazily
+    /// populating exports, because this makes instantiation faster -- we
+    /// decided that the best option is to avoid interior mutability and expose
+    /// what's happening under the covers to users via requiring a mutable
+    /// context.
     pub fn get_export(&self, mut store: impl AsContextMut, name: &str) -> Option<Extern> {
         self._get_export(&mut store.as_context_mut().opaque(), name)
     }