Allow additional trait bounds for #[pallet::constant] (#9050)

* Allow additional trait bounds for constants

* Add ui test for constants with additional trait bounds

* Update trait constant ui test

* Import syn::Error

* Use reference instead of cloning

* Add extra invalid bound ui test

* Out or order valid bounds

* Fix ui test

* Fix ui test

* Apply review suggestion about error message

substrate/frame/support/test/tests/pallet_ui.rs

@@ -23,4 +23,5 @@ fn pallet_ui() {
 
 	let t = trybuild::TestCases::new();
 	t.compile_fail("tests/pallet_ui/*.rs");
+	t.pass("tests/pallet_ui/pass/*.rs");
 }

substrate/frame/support/test/tests/pallet_ui/pass/trait_constant_valid_bounds.rs

@@ -0,0 +1,29 @@
+#[frame_support::pallet]
+mod pallet {
+	use frame_support::pallet_prelude::*;
+	use frame_system::pallet_prelude::BlockNumberFor;
+
+	#[pallet::config]
+	pub trait Config: frame_system::Config {
+		#[pallet::constant]
+		type U: Get<u32>;
+
+		#[pallet::constant]
+		type V: Get<u32> + From<u16>;
+
+		#[pallet::constant]
+		type W: From<u16> + Get<u32>;
+	}
+
+	#[pallet::pallet]
+	pub struct Pallet<T>(core::marker::PhantomData<T>);
+
+	#[pallet::hooks]
+	impl<T: Config> Hooks<BlockNumberFor<T>> for Pallet<T> {}
+
+	#[pallet::call]
+	impl<T: Config> Pallet<T> {}
+}
+
+fn main() {
+}

substrate/frame/support/test/tests/pallet_ui/trait_constant_invalid_bound.stderr

@@ -1,11 +1,5 @@
-error: Invalid usage of `#[pallet::constant]`, syntax must be `type $SomeIdent: Get<$SomeType>;`
+error: Invalid usage of `#[pallet::constant]`: `Get<T>` trait bound not found
  --> $DIR/trait_constant_invalid_bound.rs:9:3
   |
 9 |         type U;
   |         ^^^^
-
-error: expected `:`
- --> $DIR/trait_constant_invalid_bound.rs:9:9
-  |
-9 |         type U;
-  |               ^

substrate/frame/support/test/tests/pallet_ui/trait_constant_invalid_bound_lifetime.rs

@@ -0,0 +1,23 @@
+#[frame_support::pallet]
+mod pallet {
+	use frame_support::pallet_prelude::Hooks;
+	use frame_system::pallet_prelude::BlockNumberFor;
+
+	#[pallet::config]
+	pub trait Config: frame_system::Config {
+		#[pallet::constant]
+		type U: Get<'static>;
+	}
+
+	#[pallet::pallet]
+	pub struct Pallet<T>(core::marker::PhantomData<T>);
+
+	#[pallet::hooks]
+	impl<T: Config> Hooks<BlockNumberFor<T>> for Pallet<T> {}
+
+	#[pallet::call]
+	impl<T: Config> Pallet<T> {}
+}
+
+fn main() {
+}

substrate/frame/support/test/tests/pallet_ui/trait_constant_invalid_bound_lifetime.stderr

@@ -0,0 +1,5 @@
+error: Invalid usage of `#[pallet::constant]`: Expected a type argument
+ --> $DIR/trait_constant_invalid_bound_lifetime.rs:9:15
+  |
+9 |         type U: Get<'static>;
+  |                     ^^^^^^^