Improve error message for non-copy struct fields

Make sure the error message points to the type in question instead of to
the `#[wasm_bindgen]` macro which can be overly confusing!

crates/backend/src/codegen.rs

@@ -291,11 +291,13 @@ impl ToTokens for ast::StructField {
         let ty = &self.ty;
         let getter = &self.getter;
         let setter = &self.setter;
+
+        let assert_copy = quote! { assert_copy::<#ty>() };
+        let assert_copy = respan(assert_copy, ty);
         (quote! {
-            #[no_mangle]
             #[doc(hidden)]
-            #[cfg(all(target_arch = "wasm32", not(target_os = "emscripten")))]
             #[allow(clippy::all)]
+            #[cfg_attr(all(target_arch = "wasm32", not(target_os = "emscripten")), no_mangle)]
             pub unsafe extern "C" fn #getter(js: u32)
                 -> <#ty as wasm_bindgen::convert::IntoWasmAbi>::Abi
             {
@@ -303,7 +305,7 @@ impl ToTokens for ast::StructField {
                 use wasm_bindgen::convert::{GlobalStack, IntoWasmAbi};
 
                 fn assert_copy<T: Copy>(){}
-                assert_copy::<#ty>();
+                #assert_copy;
 
                 let js = js as *mut WasmRefCell<#struct_name>;
                 assert_not_null(js);
@@ -714,7 +716,8 @@ impl ToTokens for ast::ImportType {
 
                 ()
             };
-        }).to_tokens(tokens);
+        })
+        .to_tokens(tokens);
 
         let deref_target = match self.extends.first() {
             Some(target) => quote! { #target },
@@ -1430,3 +1433,31 @@ impl<'a, T: ToTokens> ToTokens for Descriptor<'a, T> {
         .to_tokens(tokens);
     }
 }
+
+fn respan(
+    input: TokenStream,
+    span: &dyn ToTokens,
+) -> TokenStream {
+    let mut first_span = Span::call_site();
+    let mut last_span = Span::call_site();
+    let mut spans = TokenStream::new();
+    span.to_tokens(&mut spans);
+
+    for (i, token) in spans.into_iter().enumerate() {
+        if i == 0 {
+            first_span = token.span();
+        }
+        last_span = token.span();
+    }
+
+    let mut new_tokens = Vec::new();
+    for (i, mut token) in input.into_iter().enumerate() {
+        if i == 0 {
+            token.set_span(first_span);
+        } else {
+            token.set_span(last_span);
+        }
+        new_tokens.push(token);
+    }
+    new_tokens.into_iter().collect()
+}

crates/macro/ui-tests/pub-not-copy.rs

@@ -0,0 +1,10 @@
+#![crate_type = "rlib"]
+
+extern crate wasm_bindgen;
+
+use wasm_bindgen::prelude::*;
+
+#[wasm_bindgen]
+pub struct A {
+    pub field: String,
+}

crates/macro/ui-tests/pub-not-copy.stderr

@@ -0,0 +1,15 @@
+error[E0277]: the trait bound `std::string::String: std::marker::Copy` is not satisfied
+ --> $DIR/pub-not-copy.rs:9:16
+  |
+9 |     pub field: String,
+  |                ^^^^^^ the trait `std::marker::Copy` is not implemented for `std::string::String`
+  |
+note: required by `__wbg_get_a_field::assert_copy`
+ --> $DIR/pub-not-copy.rs:7:1
+  |
+7 | #[wasm_bindgen]
+  | ^^^^^^^^^^^^^^^
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0277`.