sp-api: Support `mut` in `impl_runtime_apis!` (#7924)

This brings support for declaring variables in parameters as `mut`
inside of `impl_runtime_apis!`.

---------

Co-authored-by: cmd[bot] <41898282+github-actions[bot]@users.noreply.github.com>

prdoc/pr_7924.prdoc

+title: 'sp-api: Support `mut` in `impl_runtime_apis!`'
+doc:
+- audience: Runtime Dev
+  description: |-
+    This brings support for declaring variables in parameters as `mut` inside of `impl_runtime_apis!`.
+crates:
+- name: sp-api-proc-macro
+  bump: patch

substrate/primitives/api/proc-macro/src/utils.rs

@@ -74,10 +74,8 @@ pub fn replace_wild_card_parameter_names(input: &mut Signature) {
 	let mut generated_pattern_counter = 0;
 	input.inputs.iter_mut().for_each(|arg| {
 		if let FnArg::Typed(arg) = arg {
-			arg.pat = Box::new(generate_unique_pattern(
-				(*arg.pat).clone(),
-				&mut generated_pattern_counter,
-			));
+			arg.pat =
+				Box::new(sanitize_pattern((*arg.pat).clone(), &mut generated_pattern_counter));
 		}
 	});
 }
@@ -101,8 +99,11 @@ pub fn fold_fn_decl_for_client_side(
 	};
 }
 
-/// Generate an unique pattern based on the given counter, if the given pattern is a `_`.
-pub fn generate_unique_pattern(pat: Pat, counter: &mut u32) -> Pat {
+/// Sanitize the given pattern.
+///
+/// - `_` patterns are changed to a variable based on `counter`.
+/// - `mut something` removes the `mut`.
+pub fn sanitize_pattern(pat: Pat, counter: &mut u32) -> Pat {
 	match pat {
 		Pat::Wild(_) => {
 			let generated_name =
@@ -111,6 +112,10 @@ pub fn generate_unique_pattern(pat: Pat, counter: &mut u32) -> Pat {
 
 			parse_quote!( #generated_name )
 		},
+		Pat::Ident(mut pat) => {
+			pat.mutability = None;
+			pat.into()
+		},
 		_ => pat,
 	}
 }
@@ -138,8 +143,7 @@ pub fn extract_parameter_names_types_and_borrows(
 					t => (t.clone(), None),
 				};
 
-				let name =
-					generate_unique_pattern((*arg.pat).clone(), &mut generated_pattern_counter);
+				let name = sanitize_pattern((*arg.pat).clone(), &mut generated_pattern_counter);
 				result.push((name, ty, borrow));
 			},
 			FnArg::Receiver(_) if matches!(allow_self, AllowSelfRefInParameters::No) =>

substrate/primitives/api/test/tests/decl_and_impl.rs

@@ -81,7 +81,8 @@ impl_runtime_apis! {
 			unimplemented!()
 		}
 
-		fn something_with_block(_: Block) -> Block {
+		// Ensure that we accept `mut`
+		fn something_with_block(mut _block: Block) -> Block {
 			unimplemented!()
 		}