Debug implementation: use core if selected

Contributes-To: #1100

Author: chrysn

src/codegen/impl_debug.rs

@@ -42,8 +42,10 @@ pub fn gen_debug_impl(
     format_string.push_str(" }}");
     tokens.insert(0, quote! { #format_string });
 
+    let prefix = ctx.trait_prefix();
+
     quote! {
-        fn fmt(&self, f: &mut ::std::fmt::Formatter) -> ::std::fmt::Result {
+        fn fmt(&self, f: &mut ::#prefix::fmt::Formatter) -> ::#prefix ::fmt::Result {
             write!(f, #( #tokens ),*)
         }
     }

src/codegen/mod.rs

@@ -1920,8 +1920,10 @@ impl CodeGenerator for CompInfo {
                 self.kind(),
             );
 
+            let prefix = ctx.trait_prefix();
+
             result.push(quote! {
-                impl #generics ::std::fmt::Debug for #ty_for_impl {
+                impl #generics ::#prefix::fmt::Debug for #ty_for_impl {
                     #impl_
                 }
             });