Rework ast::BinOpKind::to_string and ast::UnOp::to_string.

- Rename them both `as_str`, which is the typical name for a function
  that returns a `&str`. (`to_string` is appropriate for functions
  returning `String` or maybe `Cow<'a, str>`.)
- Change `UnOp::as_str` from an associated function (weird!) to a
  method.
- Avoid needless `self` dereferences.

compiler/rustc_ast/src/ast.rs

@@ -858,9 +858,9 @@ pub enum BinOpKind {
 }
 
 impl BinOpKind {
-    pub fn to_string(&self) -> &'static str {
+    pub fn as_str(&self) -> &'static str {
         use BinOpKind::*;
-        match *self {
+        match self {
             Add => "+",
             Sub => "-",
             Mul => "*",
@@ -912,8 +912,8 @@ pub enum UnOp {
 }
 
 impl UnOp {
-    pub fn to_string(op: UnOp) -> &'static str {
+    pub fn as_str(&self) -> &'static str {
-        match op {
+        match self {
             UnOp::Deref => "*",
             UnOp::Not => "!",
             UnOp::Neg => "-",

compiler/rustc_ast_pretty/src/pprust/state/expr.rs

@@ -255,12 +255,12 @@ impl<'a> State<'a> {
 
         self.print_expr_maybe_paren(lhs, left_prec);
         self.space();
-        self.word_space(op.node.to_string());
+        self.word_space(op.node.as_str());
         self.print_expr_maybe_paren(rhs, right_prec)
     }
 
     fn print_expr_unary(&mut self, op: ast::UnOp, expr: &ast::Expr) {
-        self.word(ast::UnOp::to_string(op));
+        self.word(op.as_str());
         self.print_expr_maybe_paren(expr, parser::PREC_PREFIX)
     }
 
@@ -470,7 +470,7 @@ impl<'a> State<'a> {
                 let prec = AssocOp::Assign.precedence() as i8;
                 self.print_expr_maybe_paren(lhs, prec + 1);
                 self.space();
-                self.word(op.node.to_string());
+                self.word(op.node.as_str());
                 self.word_space("=");
                 self.print_expr_maybe_paren(rhs, prec);
             }

compiler/rustc_parse/src/parser/stmt.rs

@@ -387,7 +387,7 @@ impl<'a> Parser<'a> {
             if op.node.lazy() {
                 self.sess.emit_err(errors::InvalidExpressionInLetElse {
                     span: init.span,
-                    operator: op.node.to_string(),
+                    operator: op.node.as_str(),
                     sugg: errors::WrapExpressionInParentheses {
                         left: init.span.shrink_to_lo(),
                         right: init.span.shrink_to_hi(),

src/tools/clippy/clippy_lints/src/formatting.rs

@@ -1,7 +1,7 @@
 use clippy_utils::diagnostics::{span_lint_and_help, span_lint_and_note};
 use clippy_utils::is_span_if;
 use clippy_utils::source::snippet_opt;
-use rustc_ast::ast::{BinOpKind, Block, Expr, ExprKind, StmtKind, UnOp};
+use rustc_ast::ast::{BinOpKind, Block, Expr, ExprKind, StmtKind};
 use rustc_lint::{EarlyContext, EarlyLintPass, LintContext};
 use rustc_middle::lint::in_external_macro;
 use rustc_session::{declare_lint_pass, declare_tool_lint};
@@ -144,7 +144,7 @@ fn check_assign(cx: &EarlyContext<'_>, expr: &Expr) {
             let eq_span = lhs.span.between(rhs.span);
             if let ExprKind::Unary(op, ref sub_rhs) = rhs.kind {
                 if let Some(eq_snippet) = snippet_opt(cx, eq_span) {
-                    let op = UnOp::to_string(op);
+                    let op = op.as_str();
                     let eqop_span = lhs.span.between(sub_rhs.span);
                     if eq_snippet.ends_with('=') {
                         span_lint_and_note(
@@ -177,11 +177,11 @@ fn check_unop(cx: &EarlyContext<'_>, expr: &Expr) {
         && let unop_operand_span = rhs.span.until(un_rhs.span)
         && let Some(binop_snippet) = snippet_opt(cx, binop_span)
         && let Some(unop_operand_snippet) = snippet_opt(cx, unop_operand_span)
-        && let binop_str = BinOpKind::to_string(&binop.node)
+        && let binop_str = binop.node.as_str()
         // no space after BinOp operator and space after UnOp operator
         && binop_snippet.ends_with(binop_str) && unop_operand_snippet.ends_with(' ')
     {
-        let unop_str = UnOp::to_string(op);
+        let unop_str = op.as_str();
         let eqop_span = lhs.span.between(un_rhs.span);
         span_lint_and_help(
             cx,

src/tools/clippy/clippy_lints/src/precedence.rs

@@ -78,7 +78,7 @@ impl EarlyLintPass for Precedence {
                     let sugg = format!(
                         "({}) {} ({})",
                         snippet_with_applicability(cx, left.span, "..", &mut applicability),
-                        op.to_string(),
+                        op.as_str(),
                         snippet_with_applicability(cx, right.span, "..", &mut applicability)
                     );
                     span_sugg(expr, sugg, applicability);
@@ -87,7 +87,7 @@ impl EarlyLintPass for Precedence {
                     let sugg = format!(
                         "({}) {} {}",
                         snippet_with_applicability(cx, left.span, "..", &mut applicability),
-                        op.to_string(),
+                        op.as_str(),
                         snippet_with_applicability(cx, right.span, "..", &mut applicability)
                     );
                     span_sugg(expr, sugg, applicability);
@@ -96,7 +96,7 @@ impl EarlyLintPass for Precedence {
                     let sugg = format!(
                         "{} {} ({})",
                         snippet_with_applicability(cx, left.span, "..", &mut applicability),
-                        op.to_string(),
+                        op.as_str(),
                         snippet_with_applicability(cx, right.span, "..", &mut applicability)
                     );
                     span_sugg(expr, sugg, applicability);

src/tools/clippy/clippy_lints/src/suspicious_operation_groupings.rs

@@ -298,7 +298,7 @@ fn replace_left_sugg(
 ) -> String {
     format!(
         "{left_suggestion} {} {}",
-        binop.op.to_string(),
+        binop.op.as_str(),
         snippet_with_applicability(cx, binop.right.span, "..", applicability),
     )
 }
@@ -312,7 +312,7 @@ fn replace_right_sugg(
     format!(
         "{} {} {right_suggestion}",
         snippet_with_applicability(cx, binop.left.span, "..", applicability),
-        binop.op.to_string(),
+        binop.op.as_str(),
     )
 }
 

src/tools/clippy/clippy_utils/src/sugg.rs

@@ -382,7 +382,7 @@ fn binop_to_string(op: AssocOp, lhs: &str, rhs: &str) -> String {
         | AssocOp::GreaterEqual => {
             format!(
                 "{lhs} {} {rhs}",
-                op.to_ast_binop().expect("Those are AST ops").to_string()
+                op.to_ast_binop().expect("Those are AST ops").as_str()
             )
         },
         AssocOp::Assign => format!("{lhs} = {rhs}"),

src/tools/rustfmt/src/expr.rs

@@ -1933,7 +1933,7 @@ fn rewrite_unary_op(
     shape: Shape,
 ) -> Option<String> {
     // For some reason, an UnOp is not spanned like BinOp!
-    rewrite_unary_prefix(context, ast::UnOp::to_string(op), expr, shape)
+    rewrite_unary_prefix(context, op.as_str(), expr, shape)
 }
 
 pub(crate) enum RhsAssignKind<'ast> {

src/tools/rustfmt/src/pairs.rs

@@ -339,7 +339,7 @@ impl FlattenPair for ast::Expr {
                     if let Some(pop) = stack.pop() {
                         match pop.kind {
                             ast::ExprKind::Binary(op, _, ref rhs) => {
-                                separators.push(op.node.to_string());
+                                separators.push(op.node.as_str());
                                 node = rhs;
                             }
                             _ => unreachable!(),