Fixes path issue in derive-impl (#1823)

Needs sam0x17/macro_magic#13

The associated PR allows the export of tokens from macro_magic at the
specified path. This fixes the path issue in derive-impl. Now, we can
import the default config using the standard rust syntax:

```rust
use frame_system::config_preludes::TestDefaultConfig;

[derive_impl(TestDefaultConfig as frame_system::DefaultConfig)]
impl frame_system::DefaultConfig for Test {
   //....
}
```

Author: gupnik

substrate/frame/examples/default-config/src/lib.rs

@@ -26,7 +26,7 @@
 //! Study the following types:
 //!
 //! - [`pallet::DefaultConfig`], and how it differs from [`pallet::Config`].
-//! - [`pallet::config_preludes::TestDefaultConfig`] and how it implements
+//! - [`struct@pallet::config_preludes::TestDefaultConfig`] and how it implements
 //!   [`pallet::DefaultConfig`].
 //! - Notice how [`pallet::DefaultConfig`] is independent of [`frame_system::Config`].
 
@@ -83,11 +83,12 @@ pub mod pallet {
 		// This will help use not need to disambiguate anything when using `derive_impl`.
 		use super::*;
 		use frame_support::derive_impl;
+		use frame_system::config_preludes::TestDefaultConfig as SystemTestDefaultConfig;
 
 		/// A type providing default configurations for this pallet in testing environment.
 		pub struct TestDefaultConfig;
 
-		#[derive_impl(frame_system::config_preludes::TestDefaultConfig as frame_system::DefaultConfig, no_aggregated_types)]
+		#[derive_impl(SystemTestDefaultConfig as frame_system::DefaultConfig, no_aggregated_types)]
 		impl frame_system::DefaultConfig for TestDefaultConfig {}
 
 		#[frame_support::register_default_impl(TestDefaultConfig)]
@@ -109,7 +110,7 @@ pub mod pallet {
 		/// example, we simple derive `frame_system::config_preludes::TestDefaultConfig` again.
 		pub struct OtherDefaultConfig;
 
-		#[derive_impl(frame_system::config_preludes::TestDefaultConfig as frame_system::DefaultConfig, no_aggregated_types)]
+		#[derive_impl(SystemTestDefaultConfig as frame_system::DefaultConfig, no_aggregated_types)]
 		impl frame_system::DefaultConfig for OtherDefaultConfig {}
 
 		#[frame_support::register_default_impl(OtherDefaultConfig)]

substrate/frame/support/procedural/src/lib.rs

@@ -34,7 +34,7 @@ mod transactional;
 mod tt_macro;
 
 use frame_support_procedural_tools::generate_crate_access_2018;
-use macro_magic::import_tokens_attr;
+use macro_magic::{import_tokens_attr, import_tokens_attr_verbatim};
 use proc_macro::TokenStream;
 use quote::{quote, ToTokens};
 use std::{cell::RefCell, str::FromStr};
@@ -751,7 +751,7 @@ pub fn storage_alias(attributes: TokenStream, input: TokenStream) -> TokenStream
 /// Items that lack a `syn::Ident` for whatever reason are first checked to see if they exist,
 /// verbatim, in the local/destination trait before they are copied over, so you should not need to
 /// worry about collisions between identical unnamed items.
-#[import_tokens_attr {
+#[import_tokens_attr_verbatim {
     format!(
         "{}::macro_magic",
         match generate_crate_access_2018("frame-support") {
@@ -868,7 +868,7 @@ pub fn register_default_impl(attrs: TokenStream, tokens: TokenStream) -> TokenSt
 		attrs,
 		item_impl.to_token_stream(),
 		true,
-		true,
+		false,
 	) {
 		Ok(tokens) => tokens.into(),
 		Err(err) => err.to_compile_error().into(),

substrate/frame/support/test/tests/derive_impl_ui/bad_default_impl_path.stderr

@@ -1,7 +1,5 @@
-error: cannot find macro `__export_tokens_tt_tiger` in this scope
-  --> tests/derive_impl_ui/bad_default_impl_path.rs:59:1
+error: cannot find macro `Tiger` in this scope
+  --> tests/derive_impl_ui/bad_default_impl_path.rs:59:15
    |
 59 | #[derive_impl(Tiger as Animal)]
-   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
-   |
-   = note: this error originates in the macro `frame_support::macro_magic::forward_tokens` (in Nightly builds, run with -Z macro-backtrace for more info)
+   |               ^^^^^

substrate/frame/support/test/tests/derive_impl_ui/inject_runtime_type_fails_when_type_not_in_scope.stderr

@@ -7,4 +7,4 @@ error[E0412]: cannot find type `RuntimeCall` in this scope
 35 | #[derive_impl(Pallet)] // Injects type RuntimeCall = RuntimeCall;
    | ---------------------- in this macro invocation
    |
-   = note: this error originates in the macro `__export_tokens_tt_pallet` which comes from the expansion of the macro `frame_support::macro_magic::forward_tokens` (in Nightly builds, run with -Z macro-backtrace for more info)
+   = note: this error originates in the macro `Pallet` which comes from the expansion of the macro `frame_support::macro_magic::forward_tokens_verbatim` (in Nightly builds, run with -Z macro-backtrace for more info)