Improve errors for generate_solution_type macro (#9553)

* add more errors for check attributes in npos elections solution type

* revert local env

* return Ok false if there are no attributes

* fmt

* Update primitives/npos-elections/solution-type/src/lib.rs

Co-authored-by: Squirrel <gilescope@gmail.com>

* Update primitives/npos-elections/solution-type/src/lib.rs

Co-authored-by: Guillaume Thiolliere <gui.thiolliere@gmail.com>

* Update primitives/npos-elections/solution-type/src/lib.rs

Co-authored-by: Guillaume Thiolliere <gui.thiolliere@gmail.com>

* Update primitives/npos-elections/solution-type/src/lib.rs

* improve span by giving extra attribute, nightly fmt

* fix test to test new error msg

Co-authored-by: Squirrel <gilescope@gmail.com>
Co-authored-by: Guillaume Thiolliere <gui.thiolliere@gmail.com>

substrate/primitives/npos-elections/solution-type/src/lib.rs

@@ -134,20 +134,23 @@ struct SolutionDef {
 }
 
 fn check_attributes(input: ParseStream) -> syn::Result<bool> {
-	let attrs = input.call(syn::Attribute::parse_outer).unwrap_or_default();
+	let mut attrs = input.call(syn::Attribute::parse_outer).unwrap_or_default();
 	if attrs.len() > 1 {
-		return Err(syn_err("compact solution can accept only #[compact]"))
+		let extra_attr = attrs.pop().expect("attributes vec with len > 1 can be popped");
+		return Err(syn::Error::new_spanned(
+			extra_attr.clone(),
+			"compact solution can accept only #[compact]",
+		))
+	}
+	if attrs.is_empty() {
+		return Ok(false)
+	}
+	let attr = attrs.pop().expect("attributes vec with len 1 can be popped.");
+	if attr.path.is_ident("compact") {
+		Ok(true)
+	} else {
+		Err(syn::Error::new_spanned(attr.clone(), "compact solution can accept only #[compact]"))
 	}
-
-	Ok(attrs.iter().any(|attr| {
-		if attr.path.segments.len() == 1 {
-			let segment = attr.path.segments.first().expect("Vec with len 1 can be popped.");
-			if segment.ident == Ident::new("compact", Span::call_site()) {
-				return true
-			}
-		}
-		false
-	}))
 }
 
 impl Parse for SolutionDef {

substrate/primitives/npos-elections/solution-type/tests/ui/fail/wrong_attribute.stderr

@@ -0,0 +1,5 @@
+error: compact solution can accept only #[compact]
+ --> $DIR/wrong_attribute.rs:4:2
+  |
+4 |     #[pages(1)] pub struct TestSolution::<
+  |     ^^^^^^^^^^^

substrate/primitives/npos-elections/solution-type/tests/ui/fail/wrong_page.stderr

@@ -1,38 +0,0 @@
-error[E0412]: cannot find type `Perbill` in this scope
- --> $DIR/wrong_page.rs:7:14
-  |
-7 |         Accuracy = Perbill,
-  |                    ^^^^^^^ not found in this scope
-  |
-help: consider importing this struct
-  |
-1 | use sp_arithmetic::Perbill;
-  |
-
-error[E0433]: failed to resolve: use of undeclared type `Perbill`
- --> $DIR/wrong_page.rs:7:14
-  |
-7 |         Accuracy = Perbill,
-  |                    ^^^^^^^ not found in this scope
-  |
-help: consider importing this struct
-  |
-1 | use sp_arithmetic::Perbill;
-  |
-
-error[E0119]: conflicting implementations of trait `std::convert::TryFrom<&[_npos::IndexAssignment<u8, u16, [type error]>]>` for type `TestSolution`
- --> $DIR/wrong_page.rs:3:1
-  |
-3 | / generate_solution_type!(
-4 | |     #[pages(1)] pub struct TestSolution::<
-5 | |         VoterIndex = u8,
-6 | |         TargetIndex = u16,
-7 | |         Accuracy = Perbill,
-8 | |     >(8)
-9 | | );
-  | |__^
-  |
-  = note: conflicting implementation in crate `core`:
-          - impl<T, U> TryFrom<U> for T
-            where U: Into<T>;
-  = note: this error originates in a macro (in Nightly builds, run with -Z macro-backtrace for more info)