diff --git a/syntax/check.rs b/syntax/check.rs
index f5ccba91..5a3590ff 100644
--- a/syntax/check.rs
+++ b/syntax/check.rs
@@ -4,139 +4,144 @@ use proc_macro2::{Delimiter, Group, Ident, TokenStream};
 use quote::quote;
 use syn::{Error, Result};
 
+struct Check<'a> {
+    apis: &'a [Api],
+    types: &'a Types<'a>,
+    errors: &'a mut Vec<Error>,
+}
+
 pub(crate) fn typecheck(apis: &[Api], types: &Types) -> Result<()> {
-    let ref mut errors = Vec::new();
+    let mut errors = Vec::new();
+    let mut cx = Check {
+        apis,
+        types,
+        errors: &mut errors,
+    };
+    do_typecheck(&mut cx);
+    combine_errors(errors)
+}
 
-    for ty in types {
+fn do_typecheck(cx: &mut Check) {
+    for ty in cx.types {
         match ty {
-            Type::Ident(ident) => check_type_ident(errors, types, ident),
-            Type::RustBox(ptr) => check_type_box(errors, types, ptr),
-            Type::UniquePtr(ptr) => check_type_unique_ptr(errors, types, ptr),
-            Type::Ref(ty) => check_type_ref(errors, ty),
-            Type::Fn(_) => errors.push(unimplemented_fn_type(ty)),
+            Type::Ident(ident) => check_type_ident(cx, ident),
+            Type::RustBox(ptr) => check_type_box(cx, ptr),
+            Type::UniquePtr(ptr) => check_type_unique_ptr(cx, ptr),
+            Type::Ref(ty) => check_type_ref(cx, ty),
+            Type::Fn(_) => cx.errors.push(unimplemented_fn_type(ty)),
             _ => {}
         }
     }
 
-    for api in apis {
+    for api in cx.apis {
         match api {
-            Api::Struct(strct) => check_api_struct(errors, types, strct),
-            Api::CxxFunction(efn) | Api::RustFunction(efn) => check_api_fn(errors, types, efn),
+            Api::Struct(strct) => check_api_struct(cx, strct),
+            Api::CxxFunction(efn) | Api::RustFunction(efn) => check_api_fn(cx, efn),
             _ => {}
         }
     }
 
-    for api in apis {
+    for api in cx.apis {
         if let Api::CxxFunction(efn) = api {
-            errors.extend(check_mut_return_restriction(efn).err());
+            check_mut_return_restriction(cx, efn);
         }
         if let Api::CxxFunction(efn) | Api::RustFunction(efn) = api {
-            errors.extend(check_multiple_arg_lifetimes(efn).err());
+            check_multiple_arg_lifetimes(cx, efn);
         }
     }
 
-    ident::check_all(apis, errors);
-
-    let mut iter = errors.drain(..);
-    let mut all_errors = match iter.next() {
-        Some(err) => err,
-        None => return Ok(()),
-    };
-    for err in iter {
-        all_errors.combine(err);
-    }
-    Err(all_errors)
+    ident::check_all(cx.apis, cx.errors);
 }
 
-fn check_type_ident(errors: &mut Vec<Error>, types: &Types, ident: &Ident) {
+fn check_type_ident(cx: &mut Check, ident: &Ident) {
     if Atom::from(ident).is_none()
-        && !types.structs.contains_key(ident)
-        && !types.cxx.contains(ident)
-        && !types.rust.contains(ident)
+        && !cx.types.structs.contains_key(ident)
+        && !cx.types.cxx.contains(ident)
+        && !cx.types.rust.contains(ident)
     {
-        errors.push(unsupported_type(ident));
+        cx.errors.push(unsupported_type(ident));
     }
 }
 
-fn check_type_box(errors: &mut Vec<Error>, types: &Types, ptr: &Ty1) {
+fn check_type_box(cx: &mut Check, ptr: &Ty1) {
     if let Type::Ident(ident) = &ptr.inner {
-        if types.cxx.contains(ident) {
-            errors.push(unsupported_cxx_type_in_box(ptr));
+        if cx.types.cxx.contains(ident) {
+            cx.errors.push(unsupported_cxx_type_in_box(ptr));
         }
         if Atom::from(ident).is_none() {
             return;
         }
     }
-    errors.push(unsupported_box_target(ptr));
+    cx.errors.push(unsupported_box_target(ptr));
 }
 
-fn check_type_unique_ptr(errors: &mut Vec<Error>, types: &Types, ptr: &Ty1) {
+fn check_type_unique_ptr(cx: &mut Check, ptr: &Ty1) {
     if let Type::Ident(ident) = &ptr.inner {
-        if types.rust.contains(ident) {
-            errors.push(unsupported_rust_type_in_unique_ptr(ptr));
+        if cx.types.rust.contains(ident) {
+            cx.errors.push(unsupported_rust_type_in_unique_ptr(ptr));
         }
         match Atom::from(ident) {
             None | Some(CxxString) => return,
             _ => {}
         }
     }
-    errors.push(unsupported_unique_ptr_target(ptr));
+    cx.errors.push(unsupported_unique_ptr_target(ptr));
 }
 
-fn check_type_ref(errors: &mut Vec<Error>, ty: &Ref) {
+fn check_type_ref(cx: &mut Check, ty: &Ref) {
     if let Type::Void(_) = ty.inner {
-        errors.push(unsupported_reference_type(ty));
+        cx.errors.push(unsupported_reference_type(ty));
     }
 }
 
-fn check_api_struct(errors: &mut Vec<Error>, types: &Types, strct: &Struct) {
+fn check_api_struct(cx: &mut Check, strct: &Struct) {
     if strct.fields.is_empty() {
-        errors.push(struct_empty(strct));
+        cx.errors.push(struct_empty(strct));
     }
     for field in &strct.fields {
-        if is_unsized(&field.ty, types) {
-            errors.push(field_by_value(field, types));
+        if is_unsized(cx, &field.ty) {
+            cx.errors.push(field_by_value(field, cx.types));
         }
     }
 }
 
-fn check_api_fn(errors: &mut Vec<Error>, types: &Types, efn: &ExternFn) {
+fn check_api_fn(cx: &mut Check, efn: &ExternFn) {
     for arg in &efn.args {
-        if is_unsized(&arg.ty, types) {
-            errors.push(argument_by_value(arg, types));
+        if is_unsized(cx, &arg.ty) {
+            cx.errors.push(argument_by_value(arg, cx.types));
         }
     }
     if let Some(ty) = &efn.ret {
-        if is_unsized(ty, types) {
-            errors.push(return_by_value(ty, types));
+        if is_unsized(cx, ty) {
+            cx.errors.push(return_by_value(ty, cx.types));
         }
     }
 }
 
-fn check_mut_return_restriction(efn: &ExternFn) -> Result<()> {
+fn check_mut_return_restriction(cx: &mut Check, efn: &ExternFn) {
     match &efn.ret {
         Some(Type::Ref(ty)) if ty.mutability.is_some() => {}
-        _ => return Ok(()),
+        _ => return,
     }
 
     for arg in &efn.args {
         if let Type::Ref(ty) = &arg.ty {
             if ty.mutability.is_some() {
-                return Ok(());
+                return;
             }
         }
     }
 
-    Err(Error::new_spanned(
+    cx.errors.push(Error::new_spanned(
         efn,
         "&mut return type is not allowed unless there is a &mut argument",
-    ))
+    ));
 }
 
-fn check_multiple_arg_lifetimes(efn: &ExternFn) -> Result<()> {
+fn check_multiple_arg_lifetimes(cx: &mut Check, efn: &ExternFn) {
     match &efn.ret {
         Some(Type::Ref(_)) => {}
-        _ => return Ok(()),
+        _ => return,
     }
 
     let mut reference_args = 0;
@@ -146,23 +151,33 @@ fn check_multiple_arg_lifetimes(efn: &ExternFn) -> Result<()> {
         }
     }
 
-    if reference_args == 1 {
-        Ok(())
-    } else {
-        Err(Error::new_spanned(
+    if reference_args != 1 {
+        cx.errors.push(Error::new_spanned(
             efn,
             "functions that return a reference must take exactly one input reference",
-        ))
+        ));
     }
 }
 
-fn is_unsized(ty: &Type, types: &Types) -> bool {
+fn is_unsized(cx: &mut Check, ty: &Type) -> bool {
     let ident = match ty {
         Type::Ident(ident) => ident,
         Type::Void(_) => return true,
         _ => return false,
     };
-    ident == CxxString || types.cxx.contains(ident) || types.rust.contains(ident)
+    ident == CxxString || cx.types.cxx.contains(ident) || cx.types.rust.contains(ident)
+}
+
+fn combine_errors(errors: Vec<Error>) -> Result<()> {
+    let mut iter = errors.into_iter();
+    let mut all_errors = match iter.next() {
+        Some(err) => err,
+        None => return Ok(()),
+    };
+    for err in iter {
+        all_errors.combine(err);
+    }
+    Err(all_errors)
 }
 
 fn describe(ty: &Type, types: &Types) -> String {