Put context object behind a ref rather than mut ref

This commit puts context object, i.e., the implementor of the
WASI snapshot, behind a reference `&self` rather than a mutable
reference `&mut self`. As suggested by @alexcrichton, this gives
the implementor the possibility to determine how it handles its
interior mutability.

crates/generate/src/module_trait.rs

@@ -39,9 +39,9 @@ pub fn define_module_trait(names: &Names, m: &Module) -> TokenStream {
             .unwrap_or(quote!(()));
 
         if is_anonymous {
-            quote!(fn #funcname(&mut self, #(#args),*) -> Result<(#(#rets),*), #err>;)
+            quote!(fn #funcname(&self, #(#args),*) -> Result<(#(#rets),*), #err>;)
         } else {
-            quote!(fn #funcname<#lifetime>(&mut self, #(#args),*) -> Result<(#(#rets),*), #err>;)
+            quote!(fn #funcname<#lifetime>(&self, #(#args),*) -> Result<(#(#rets),*), #err>;)
         }
     });
     quote! {

crates/runtime/src/guest_type.rs

@@ -50,5 +50,5 @@ builtin_type!(u8, i8, u16, i16, u32, i32, u64, i64, f32, f64, usize);
 pub trait GuestErrorType {
     type Context;
     fn success() -> Self;
-    fn from_error(e: GuestError, ctx: &mut Self::Context) -> Self;
+    fn from_error(e: GuestError, ctx: &Self::Context) -> Self;
 }

crates/test/src/lib.rs

@@ -91,16 +91,17 @@ mod test {
     }
 }
 
+use std::cell::RefCell;
 use wiggle_runtime::GuestError;
 
 pub struct WasiCtx {
-    pub guest_errors: Vec<GuestError>,
+    pub guest_errors: RefCell<Vec<GuestError>>,
 }
 
 impl WasiCtx {
     pub fn new() -> Self {
         Self {
-            guest_errors: vec![],
+            guest_errors: RefCell::new(vec![]),
         }
     }
 }
@@ -117,9 +118,9 @@ macro_rules! impl_errno {
             fn success() -> $errno {
                 <$errno>::Ok
             }
-            fn from_error(e: GuestError, ctx: &mut WasiCtx) -> $errno {
+            fn from_error(e: GuestError, ctx: &WasiCtx) -> $errno {
                 eprintln!("GUEST ERROR: {:?}", e);
-                ctx.guest_errors.push(e);
+                ctx.guest_errors.borrow_mut().push(e);
                 types::Errno::InvalidArg
             }
         }