Rollup merge of #133140 - dtolnay:precedence, r=fmease

Inline ExprPrecedence::order into Expr::precedence

The representation of expression precedence in rustc_ast has been an obstacle to further improvements in the pretty-printer (continuing from #119105 and #119427).

Previously the operation of *"does this expression have lower precedence than that one"* (relevant for parenthesis insertion in macro-generated syntax trees) consisted of 3 steps:

1. Convert `Expr` to `ExprPrecedence` using `.precedence()`
2. Convert `ExprPrecedence` to `i8` using `.order()`
3. Compare using `<`

As far as I can guess, the reason for the separation between `precedence()` and `order()` was so that both `rustc_ast::Expr` and `rustc_hir::Expr` could convert as straightforwardly as possible to the same `ExprPrecedence` enum, and then the more finicky logic performed by `order` could be present just once.

The mapping between `Expr` and `ExprPrecedence` was intended to be as straightforward as possible:

```rust
match self.kind {
    ExprKind::Closure(..) => ExprPrecedence::Closure,
    ...
}
```

although there were exceptions of both many-to-one, and one-to-many:

```rust
    ExprKind::Underscore => ExprPrecedence::Path,
    ExprKind::Path(..) => ExprPrecedence::Path,
    ...
    ExprKind::Match(_, _, MatchKind::Prefix) => ExprPrecedence::Match,
    ExprKind::Match(_, _, MatchKind::Postfix) => ExprPrecedence::PostfixMatch,
```

Where the nature of `ExprPrecedence` becomes problematic is when a single expression kind might be associated with multiple different precedence levels depending on context (outside the expression) and contents (inside the expression). For example consider what is the precedence of an ExprKind::Closure `$closure`. Well, on the left-hand side of a binary operator it would need parentheses in order to avoid the trailing binary operator being absorbed into the closure body: `($closure) + Rhs`, so the precedence is something lower than that of `+`. But on the right-hand side of a binary operator, a closure is just a straightforward prefix expression like a unary op, which is a relatively high precedence level, higher than binops but lower than method calls: `Lhs + $closure` is fine without parens but `($closure).method()` needs them. But as a third case, if the closure contains an explicit return type, then the precedence is an even higher level than that, never needing parenthesization even in a binop left-hand side or method call: `|| -> bool { false } + Rhs` or `|| -> bool { false }.method()`.

You can see that trying to capture all of this resolution about expressions into `ExprPrecedence` violates the intention of `ExprPrecedence` being a straightforward one-to-one correspondence from each AST and HIR `ExprKind` variant. It would be possible to attempt that by doing stuff like `ExprPrecedence::Closure(Side::Leading, ReturnType::No)`, but I don't foresee the original envisioned benefit of the `precedence()`/`order()` distinction being retained in this approach. Instead I want to move toward a model that Syn has been using successfully. In Syn, there is a Precedence enum but it differs from rustc in the following ways:

- There are [relatively few variants](https://github.com/dtolnay/syn/blob/2.0.87/src/precedence.rs#L11-L47) compared to rustc's `ExprPrecedence`. For example there is no distinction at the precedence level between returns and closures, or between loops and method calls.

- We distinguish between [leading](https://github.com/dtolnay/syn/blob/2.0.87/src/fixup.rs#L293) and [trailing](https://github.com/dtolnay/syn/blob/2.0.87/src/fixup.rs#L309) precedence, taking into account an expression's context such as what token follows it (for various syntactic bail-outs in Rust's grammar, like ambiguities around break-with-value) and how it relates to operators from the surrounding syntax tree.

- There are no hardcoded mysterious integer quantities like rustc's `PREC_CLOSURE = -40`. All precedence comparisons are performed via PartialOrd on a C-like enum.

This PR is just a first step in these changes. As you can tell from Syn, I definitely think there is value in having a dedicated type to represent precedence, instead of what `order()` is doing with `i8`. But that is a whole separate adventure because rustc_ast doesn't even agree consistently on `i8` being the type for precedence order; `AssocOp::precedence` instead uses `usize` and there are casts in both directions. It is likely that a type called `ExprPrecedence` will re-appear, but it will look substantially different from the one that existed before this PR.

6 files changed, 11 insertions, 11 deletions

diff --git a/src/tools/clippy/clippy_lints/src/dereference.rs b/src/tools/clippy/clippy_lints/src/dereference.rs
index fce7f9985ea..1eebfc22af2 100644
--- a/src/tools/clippy/clippy_lints/src/dereference.rs
+++ b/src/tools/clippy/clippy_lints/src/dereference.rs
@@ -963,7 +963,7 @@ fn report<'tcx>(
             // expr_str (the suggestion) is never shown if is_final_ufcs is true, since it's
             // `expr.kind == ExprKind::Call`. Therefore, this is, afaik, always unnecessary.
             /*
-            expr_str = if !expr_is_macro_call && is_final_ufcs && expr.precedence().order() < PREC_PREFIX {
+            expr_str = if !expr_is_macro_call && is_final_ufcs && expr.precedence() < PREC_PREFIX {
                 Cow::Owned(format!("({expr_str})"))
             } else {
                 expr_str
@@ -1003,7 +1003,7 @@ fn report<'tcx>(
                         Node::Expr(e) => match e.kind {
                             ExprKind::Call(callee, _) if callee.hir_id != data.first_expr.hir_id => (0, false),
                             ExprKind::Call(..) => (PREC_UNAMBIGUOUS, matches!(expr.kind, ExprKind::Field(..))),
-                            _ => (e.precedence().order(), false),
+                            _ => (e.precedence(), false),
                         },
                         _ => (0, false),
                     };
@@ -1016,7 +1016,7 @@ fn report<'tcx>(
                     );
 
                     let sugg = if !snip_is_macro
-                        && (calls_field || expr.precedence().order() < precedence)
+                        && (calls_field || expr.precedence() < precedence)
                         && !has_enclosing_paren(&snip)
                         && !is_in_tuple
                     {
@@ -1071,7 +1071,7 @@ fn report<'tcx>(
                     let (snip, snip_is_macro) =
                         snippet_with_context(cx, expr.span, data.first_expr.span.ctxt(), "..", &mut app);
                     let sugg =
-                        if !snip_is_macro && expr.precedence().order() < precedence && !has_enclosing_paren(&snip) {
+                        if !snip_is_macro && expr.precedence() < precedence && !has_enclosing_paren(&snip) {
                             format!("{prefix}({snip})")
                         } else {
                             format!("{prefix}{snip}")
@@ -1158,7 +1158,7 @@ impl<'tcx> Dereferencing<'tcx> {
                         },
                         Some(parent) if !parent.span.from_expansion() => {
                             // Double reference might be needed at this point.
-                            if parent.precedence().order() == PREC_UNAMBIGUOUS {
+                            if parent.precedence() == PREC_UNAMBIGUOUS {
                                 // Parentheses would be needed here, don't lint.
                                 *outer_pat = None;
                             } else {
diff --git a/src/tools/clippy/clippy_lints/src/loops/single_element_loop.rs b/src/tools/clippy/clippy_lints/src/loops/single_element_loop.rs
index 70f76ced09a..35dc8e9aa4e 100644
--- a/src/tools/clippy/clippy_lints/src/loops/single_element_loop.rs
+++ b/src/tools/clippy/clippy_lints/src/loops/single_element_loop.rs
@@ -84,7 +84,7 @@ pub(super) fn check<'tcx>(
         if !prefix.is_empty()
             && (
                 // Precedence of internal expression is less than or equal to precedence of `&expr`.
-                arg_expression.precedence().order() <= PREC_PREFIX || is_range_literal(arg_expression)
+                arg_expression.precedence() <= PREC_PREFIX || is_range_literal(arg_expression)
             )
         {
             arg_snip = format!("({arg_snip})").into();
diff --git a/src/tools/clippy/clippy_lints/src/matches/manual_utils.rs b/src/tools/clippy/clippy_lints/src/matches/manual_utils.rs
index d38560998a5..9c6df4d8ac0 100644
--- a/src/tools/clippy/clippy_lints/src/matches/manual_utils.rs
+++ b/src/tools/clippy/clippy_lints/src/matches/manual_utils.rs
@@ -117,7 +117,7 @@ where
     // it's being passed by value.
     let scrutinee = peel_hir_expr_refs(scrutinee).0;
     let (scrutinee_str, _) = snippet_with_context(cx, scrutinee.span, expr_ctxt, "..", &mut app);
-    let scrutinee_str = if scrutinee.span.eq_ctxt(expr.span) && scrutinee.precedence().order() < PREC_UNAMBIGUOUS {
+    let scrutinee_str = if scrutinee.span.eq_ctxt(expr.span) && scrutinee.precedence() < PREC_UNAMBIGUOUS {
         format!("({scrutinee_str})")
     } else {
         scrutinee_str.into()
diff --git a/src/tools/clippy/clippy_lints/src/neg_multiply.rs b/src/tools/clippy/clippy_lints/src/neg_multiply.rs
index f84d9fadb85..a0ba2aaf552 100644
--- a/src/tools/clippy/clippy_lints/src/neg_multiply.rs
+++ b/src/tools/clippy/clippy_lints/src/neg_multiply.rs
@@ -58,7 +58,7 @@ fn check_mul(cx: &LateContext<'_>, span: Span, lit: &Expr<'_>, exp: &Expr<'_>) {
     {
         let mut applicability = Applicability::MachineApplicable;
         let (snip, from_macro) = snippet_with_context(cx, exp.span, span.ctxt(), "..", &mut applicability);
-        let suggestion = if !from_macro && exp.precedence().order() < PREC_PREFIX && !has_enclosing_paren(&snip) {
+        let suggestion = if !from_macro && exp.precedence() < PREC_PREFIX && !has_enclosing_paren(&snip) {
             format!("-({snip})")
         } else {
             format!("-{snip}")
diff --git a/src/tools/clippy/clippy_lints/src/redundant_slicing.rs b/src/tools/clippy/clippy_lints/src/redundant_slicing.rs
index 0ac818c21d9..79baa914b03 100644
--- a/src/tools/clippy/clippy_lints/src/redundant_slicing.rs
+++ b/src/tools/clippy/clippy_lints/src/redundant_slicing.rs
@@ -85,7 +85,7 @@ impl<'tcx> LateLintPass<'tcx> for RedundantSlicing {
             let (expr_ty, expr_ref_count) = peel_middle_ty_refs(cx.typeck_results().expr_ty(expr));
             let (indexed_ty, indexed_ref_count) = peel_middle_ty_refs(cx.typeck_results().expr_ty(indexed));
             let parent_expr = get_parent_expr(cx, expr);
-            let needs_parens_for_prefix = parent_expr.is_some_and(|parent| parent.precedence().order() > PREC_PREFIX);
+            let needs_parens_for_prefix = parent_expr.is_some_and(|parent| parent.precedence() > PREC_PREFIX);
 
             if expr_ty == indexed_ty {
                 if expr_ref_count > indexed_ref_count {
diff --git a/src/tools/clippy/clippy_lints/src/transmute/transmutes_expressible_as_ptr_casts.rs b/src/tools/clippy/clippy_lints/src/transmute/transmutes_expressible_as_ptr_casts.rs
index fca332dba40..cad15b1e982 100644
--- a/src/tools/clippy/clippy_lints/src/transmute/transmutes_expressible_as_ptr_casts.rs
+++ b/src/tools/clippy/clippy_lints/src/transmute/transmutes_expressible_as_ptr_casts.rs
@@ -1,7 +1,7 @@
 use super::TRANSMUTES_EXPRESSIBLE_AS_PTR_CASTS;
 use clippy_utils::diagnostics::span_lint_and_sugg;
 use clippy_utils::sugg::Sugg;
-use rustc_ast::ExprPrecedence;
+use rustc_ast::util::parser::AssocOp;
 use rustc_errors::Applicability;
 use rustc_hir::{Expr, Node};
 use rustc_hir_typeck::cast::check_cast;
@@ -44,7 +44,7 @@ pub(super) fn check<'tcx>(
     };
 
     if let Node::Expr(parent) = cx.tcx.parent_hir_node(e.hir_id)
-        && parent.precedence().order() > ExprPrecedence::Cast.order()
+        && parent.precedence() > AssocOp::As.precedence() as i8
     {
         sugg = format!("({sugg})");
     }