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/flycheck/src/lib.rs

@@ -297,11 +297,11 @@ impl FlycheckActor {
                 let mut cmd = Command::new(toolchain::cargo());
                 cmd.arg(command);
                 cmd.current_dir(&self.root);
-                cmd.args(&["--workspace", "--message-format=json", "--manifest-path"])
+                cmd.args(["--workspace", "--message-format=json", "--manifest-path"])
                     .arg(self.root.join("Cargo.toml").as_os_str());
 
                 for target in target_triples {
-                    cmd.args(&["--target", target.as_str()]);
+                    cmd.args(["--target", target.as_str()]);
                 }
                 if *all_targets {
                     cmd.arg("--all-targets");

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 => {

crates/proc-macro-api/src/process.rs

@@ -67,7 +67,7 @@ impl Process {
         args: impl IntoIterator<Item = impl AsRef<OsStr>>,
     ) -> io::Result<Process> {
         let args: Vec<OsString> = args.into_iter().map(|s| s.as_ref().into()).collect();
-        let child = JodChild(mk_child(&path, &args)?);
+        let child = JodChild(mk_child(&path, args)?);
         Ok(Process { child })
     }
 

crates/proc-macro-test/build.rs

@@ -63,7 +63,7 @@ fn main() {
     };
 
     cmd.current_dir(&staging_dir)
-        .args(&["build", "-p", "proc-macro-test-impl", "--message-format", "json"])
+        .args(["build", "-p", "proc-macro-test-impl", "--message-format", "json"])
         // Explicit override the target directory to avoid using the same one which the parent
         // cargo is using, or we'll deadlock.
         // This can happen when `CARGO_TARGET_DIR` is set or global config forces all cargo

crates/syntax/src/ast/prec.rs

@@ -119,7 +119,7 @@ impl Expr {
     fn binding_power(&self) -> (u8, u8) {
         use ast::{ArithOp::*, BinaryOp::*, Expr::*, LogicOp::*};
 
-        let dps = match self {
+        match self {
             // (0, 0)   -- paren-like/nullary
             // (0, N)   -- prefix
             // (N, 0)   -- postfix
@@ -170,9 +170,7 @@ impl Expr {
             ArrayExpr(_) | TupleExpr(_) | Literal(_) | PathExpr(_) | ParenExpr(_) | IfExpr(_)
             | WhileExpr(_) | ForExpr(_) | LoopExpr(_) | MatchExpr(_) | BlockExpr(_)
             | RecordExpr(_) | UnderscoreExpr(_) => (0, 0),
-        };
-
-        dps
+        }
     }
 
     fn is_paren_like(&self) -> bool {

crates/syntax/src/ptr.rs

@@ -82,7 +82,7 @@ impl<N: AstNode> AstPtr<N> {
 
     /// Like `SyntaxNodePtr::cast` but the trait bounds work out.
     pub fn try_from_raw(raw: SyntaxNodePtr) -> Option<AstPtr<N>> {
-        N::can_cast(raw.kind()).then(|| AstPtr { raw, _ty: PhantomData })
+        N::can_cast(raw.kind()).then_some(AstPtr { raw, _ty: PhantomData })
     }
 }
 

crates/syntax/src/validation.rs

@@ -196,7 +196,7 @@ pub(crate) fn validate_block_structure(root: &SyntaxNode) {
 
 fn validate_numeric_name(name_ref: Option<ast::NameRef>, errors: &mut Vec<SyntaxError>) {
     if let Some(int_token) = int_token(name_ref) {
-        if int_token.text().chars().any(|c| !c.is_digit(10)) {
+        if int_token.text().chars().any(|c| !c.is_ascii_digit()) {
             errors.push(SyntaxError::new(
                 "Tuple (struct) field access is only allowed through \
                 decimal integers with no underscores or suffix",

crates/test-utils/src/bench_fixture.rs

@@ -36,10 +36,10 @@ struct S{} {{
 
 pub fn glorious_old_parser() -> String {
     let path = project_root().join("bench_data/glorious_old_parser");
-    fs::read_to_string(&path).unwrap()
+    fs::read_to_string(path).unwrap()
 }
 
 pub fn numerous_macro_rules() -> String {
     let path = project_root().join("bench_data/numerous_macro_rules");
-    fs::read_to_string(&path).unwrap()
+    fs::read_to_string(path).unwrap()
 }

crates/test-utils/src/lib.rs

@@ -396,7 +396,7 @@ pub fn skip_slow_tests() -> bool {
         eprintln!("ignoring slow test");
     } else {
         let path = project_root().join("./target/.slow_tests_cookie");
-        fs::write(&path, ".").unwrap();
+        fs::write(path, ".").unwrap();
     }
     should_skip
 }

crates/toolchain/src/lib.rs

@@ -35,7 +35,7 @@ fn get_path_for_executable(executable_name: &'static str) -> PathBuf {
     //      example: for cargo, this tries ~/.cargo/bin/cargo
     //      It seems that this is a reasonable place to try for cargo, rustc, and rustup
     let env_var = executable_name.to_ascii_uppercase();
-    if let Some(path) = env::var_os(&env_var) {
+    if let Some(path) = env::var_os(env_var) {
         return path.into();
     }
 

xtask/src/install.rs

@@ -62,7 +62,7 @@ fn fix_path_for_mac(sh: &Shell) -> Result<()> {
         let mut paths = env::split_paths(&vars).collect::<Vec<_>>();
         paths.append(&mut vscode_path);
         let new_paths = env::join_paths(paths).context("build env PATH")?;
-        sh.set_var("PATH", &new_paths);
+        sh.set_var("PATH", new_paths);
     }
 
     Ok(())

xtask/src/release.rs

@@ -65,7 +65,7 @@ impl flags::Release {
 
         let contents = changelog::get_changelog(sh, changelog_n, &commit, prev_tag, &today)?;
         let path = changelog_dir.join(format!("{today}-changelog-{changelog_n}.adoc"));
-        sh.write_file(&path, &contents)?;
+        sh.write_file(path, contents)?;
 
         Ok(())
     }

xtask/src/release/changelog.rs

@@ -23,7 +23,7 @@ pub(crate) fn get_changelog(
     let mut others = String::new();
     for line in git_log.lines() {
         let line = line.trim_start();
-        if let Some(pr_num) = parse_pr_number(&line) {
+        if let Some(pr_num) = parse_pr_number(line) {
             let accept = "Accept: application/vnd.github.v3+json";
             let authorization = format!("Authorization: token {token}");
             let pr_url = "https://api.github.com/repos/rust-lang/rust-analyzer/issues";