Moar linting: needless_borrow, let_unit_value, ...

* There are a few needless borrows that don't seem to be needed. I even did a quick assembly comparison and posted a q to stackoveflow on it. See [here](https://stackoverflow.com/questions/74910196/advantages-of-pass-by-ref-val-with-impl-intoiteratoritem-impl-asrefstr)
* removed several `let _ = ...` when they don't look necessary (even a few ones that were not suggested by clippy (?))
* there were a few `then(|| ctor{})` that clippy suggested to replace with `then_some(ctor{})` -- seems reasonable?
* some unneeded assignment+return - keep the code a bit leaner
* a few `writeln!` instead of `write!`, or even consolidate write!
* a nice optimization to use `ch.is_ascii_digit` instead of `ch.is_digit(10)`

crates/parser/src/grammar.rs

@@ -51,7 +51,7 @@ pub(crate) mod entry {
         use super::*;
 
         pub(crate) fn vis(p: &mut Parser<'_>) {
-            let _ = opt_visibility(p, false);
+            opt_visibility(p, false);
         }
 
         pub(crate) fn block(p: &mut Parser<'_>) {
@@ -70,10 +70,10 @@ pub(crate) mod entry {
             types::type_(p);
         }
         pub(crate) fn expr(p: &mut Parser<'_>) {
-            let _ = expressions::expr(p);
+            expressions::expr(p);
         }
         pub(crate) fn path(p: &mut Parser<'_>) {
-            let _ = paths::type_path(p);
+            paths::type_path(p);
         }
         pub(crate) fn item(p: &mut Parser<'_>) {
             items::item_or_macro(p, true);

crates/parser/src/shortcuts.rs

@@ -80,8 +80,8 @@ impl<'a> LexedStr<'a> {
             State::PendingEnter | State::Normal => unreachable!(),
         }
 
-        let is_eof = builder.pos == builder.lexed.len();
-        is_eof
+        // is_eof?
+        builder.pos == builder.lexed.len()
     }
 }
 

crates/parser/src/tests.rs

@@ -93,14 +93,12 @@ fn parse(entry: TopEntryPoint, text: &str) -> (String, bool) {
         crate::StrStep::Token { kind, text } => {
             assert!(depth > 0);
             len += text.len();
-            write!(buf, "{indent}").unwrap();
-            write!(buf, "{kind:?} {text:?}\n").unwrap();
+            writeln!(buf, "{indent}{kind:?} {text:?}").unwrap();
         }
         crate::StrStep::Enter { kind } => {
             assert!(depth > 0 || len == 0);
             depth += 1;
-            write!(buf, "{indent}").unwrap();
-            write!(buf, "{kind:?}\n").unwrap();
+            writeln!(buf, "{indent}{kind:?}").unwrap();
             indent.push_str("  ");
         }
         crate::StrStep::Exit => {