Rust: do not print -> () for signatures that have no results. (#509)

This commit fixes the printing of an unnecessary `-> ()` for function
signatures that have no results.

While we ignore clippy lints in the generated code, Rust analyzer will blindly
copy the unnecessary return specification when implementing missing trait
members. This might cause clippy warnings in user code as a result.

Author: Peter Huene

crates/gen-rust-lib/src/lib.rs

@@ -95,8 +95,7 @@ pub trait RustGenerator<'a> {
         sig: &FnSig,
     ) -> Vec<String> {
         let params = self.print_docs_and_params(func, param_mode, &sig);
-        self.push_str(" -> ");
-        self.print_result_params(&func.results, TypeMode::Owned);
+        self.print_results(&func.results, TypeMode::Owned);
         params
     }
 
@@ -152,12 +151,15 @@ pub trait RustGenerator<'a> {
         params
     }
 
-    fn print_result_params(&mut self, results: &Results, mode: TypeMode) {
+    fn print_results(&mut self, results: &Results, mode: TypeMode) {
         match results.len() {
-            0 => self.push_str("()"),
-            1 => self.print_ty(results.iter_types().next().unwrap(), mode),
+            0 => {}
+            1 => {
+                self.push_str(" -> ");
+                self.print_ty(results.iter_types().next().unwrap(), mode);
+            }
             _ => {
-                self.push_str("(");
+                self.push_str(" -> (");
                 for ty in results.iter_types() {
                     self.print_ty(ty, mode);
                     self.push_str(", ")