Fix EnumIter macro code generation (#287)

* Add test to ensure macro call `::core`-related functions

Avoiding local core modules to break the macro-generated code.

Currently failing due to issue with `EnumIter` macros.

* Fix macro of `EnumIter`

close #284

---------

Co-authored-by: kraktus <kraktus@users.noreply.github.com>

strum_macros/src/macros/enum_iter.rs

@@ -80,8 +80,8 @@ pub fn enum_iter_inner(ast: &DeriveInput) -> syn::Result<TokenStream> {
             marker: ::core::marker::PhantomData #phantom_data,
         }
 
-        impl #impl_generics core::fmt::Debug for #iter_name #ty_generics #where_clause {
-            fn fmt(&self, f: &mut core::fmt::Formatter<'_>) -> core::fmt::Result {
+        impl #impl_generics ::core::fmt::Debug for #iter_name #ty_generics #where_clause {
+            fn fmt(&self, f: &mut ::core::fmt::Formatter<'_>) -> ::core::fmt::Result {
                 // We don't know if the variants implement debug themselves so the only thing we
                 // can really show is how many elements are left.
                 f.debug_struct(#iter_name_debug_struct)

strum_tests/tests/as_ref_no_strum.rs

@@ -1,5 +1,7 @@
 use strum_macros::AsRefStr;
 
+mod core {} // ensure macros call `::core`
+
 #[derive(Debug, Eq, PartialEq, AsRefStr)]
 enum Color {
     #[strum(to_string = "RedRed")]

strum_tests/tests/as_ref_str.rs

@@ -3,6 +3,8 @@
 use std::str::FromStr;
 use strum::{AsRefStr, AsStaticRef, AsStaticStr, EnumString, IntoStaticStr};
 
+mod core {} // ensure macros call `::core`
+
 #[derive(Debug, Eq, PartialEq, EnumString, AsRefStr, AsStaticStr, IntoStaticStr)]
 enum Color {
     #[strum(to_string = "RedRed")]

strum_tests/tests/display.rs

@@ -1,5 +1,7 @@
 use strum::{Display, EnumString};
 
+mod core {} // ensure macros call `::core`
+
 #[derive(Debug, Eq, PartialEq, EnumString, Display)]
 enum Color {
     #[strum(to_string = "RedRed")]

strum_tests/tests/enum_count.rs

@@ -1,5 +1,7 @@
 use strum::{EnumCount, EnumIter, IntoEnumIterator};
 
+mod core {} // ensure macros call `::core`
+
 #[derive(Debug, EnumCount, EnumIter)]
 enum Week {
     Sunday,

strum_tests/tests/enum_discriminants.rs

@@ -1,6 +1,9 @@
 use enum_variant_type::EnumVariantType;
 use strum::{Display, EnumDiscriminants, EnumIter, EnumMessage, EnumString, IntoEnumIterator};
 
+
+mod core {} // ensure macros call `::core`
+
 #[allow(dead_code)]
 #[derive(Debug, Eq, PartialEq, EnumDiscriminants)]
 #[strum_discriminants(derive(EnumIter))]

strum_tests/tests/enum_is.rs

@@ -1,5 +1,7 @@
 use strum::EnumIs;
 
+mod core {} // ensure macros call `::core`
+
 #[derive(EnumIs)]
 enum Foo {
     Unit,

strum_tests/tests/enum_iter.rs

@@ -1,5 +1,7 @@
 use strum::{EnumIter, IntoEnumIterator};
 
+mod core {} // ensure macros call `::core`
+
 #[derive(Debug, Eq, PartialEq, EnumIter)]
 enum Week {
     Sunday,

strum_tests/tests/enum_message.rs

@@ -1,5 +1,7 @@
 use strum::EnumMessage;
 
+mod core {} // ensure macros call `::core`
+
 #[derive(Debug, Eq, PartialEq, EnumMessage)]
 enum Pets {
     // This comment is not collected since it starts with "//" instead of "///".

strum_tests/tests/enum_props.rs

@@ -1,5 +1,7 @@
 use strum::EnumProperty;
 
+mod core {} // ensure macros call `::core`
+
 #[derive(Debug, EnumProperty)]
 enum Test {
     #[strum(props(key = "value"))]

strum_tests/tests/enum_variant_names.rs

@@ -1,6 +1,8 @@
 use structopt::StructOpt;
 use strum::{EnumString, EnumVariantNames, VariantNames};
 
+mod core {} // ensure macros call `::core`
+
 #[test]
 fn simple() {
     #[allow(dead_code)]

strum_tests/tests/from_repr.rs

@@ -1,5 +1,7 @@
 use strum::FromRepr;
 
+mod core {} // ensure macros call `::core`
+
 #[derive(Debug, FromRepr, PartialEq)]
 /// Day of the week
 #[repr(u8)]

strum_tests/tests/from_str.rs

@@ -1,6 +1,8 @@
 use std::str::FromStr;
 use strum::EnumString;
 
+mod core {} // ensure macros call `::core`
+
 #[derive(Debug, Eq, PartialEq, EnumString)]
 enum Color {
     Red,

strum_tests/tests/phf.rs

@@ -1,3 +1,5 @@
+mod core {} // ensure macros call `::core`
+
 #[cfg(feature = "test_phf")]
 #[test]
 fn from_str_with_phf() {

strum_tests/tests/serialize_all.rs

@@ -3,6 +3,8 @@ use std::str::FromStr;
 use std::string::ToString;
 use strum::{Display, EnumString, IntoStaticStr};
 
+mod core {} // ensure macros call `::core`
+
 #[derive(Debug, Eq, PartialEq, EnumString, Display, IntoStaticStr)]
 #[strum(serialize_all = "title_case")]
 enum Foo1 {

strum_tests/tests/to_string.rs

@@ -4,6 +4,8 @@ use std::str::FromStr;
 use std::string::ToString;
 use strum::{EnumString, ToString};
 
+mod core {} // ensure macros call `::core`
+
 #[derive(Debug, Eq, PartialEq, EnumString, ToString)]
 enum Color {
     #[strum(to_string = "RedRed")]