Rollup merge of rust-lang#126893 - dtolnay:prec, r=compiler-errors

Eliminate the distinction between PREC_POSTFIX and PREC_PAREN precedence level

I have been tangling with precedence as part of porting some pretty-printer improvements from syn back to rustc (related to parenthesization of closures, returns, and breaks by the AST pretty-printer).

As far as I have been able to tell, there is no difference between the 2 different precedence levels that rustc identifies as `PREC_POSTFIX` (field access, square bracket index, question mark, method call) and `PREC_PAREN` (loops, if, paths, literals).

There are a bunch of places that look at either `prec < PREC_POSTFIX` or `prec >= PREC_POSTFIX`. But there is nothing that needs to distinguish PREC_POSTFIX and PREC_PAREN from one another.

https://github.com/rust-lang/rust/blob/d49994b060684af423339b55769439b2f444a7b9/compiler/rustc_ast/src/util/parser.rs#L236-L237

https://github.com/rust-lang/rust/blob/d49994b060684af423339b55769439b2f444a7b9/compiler/rustc_hir_typeck/src/fn_ctxt/suggestions.rs#L2829

https://github.com/rust-lang/rust/blob/d49994b060684af423339b55769439b2f444a7b9/compiler/rustc_hir_typeck/src/fn_ctxt/suggestions.rs#L1290

In the interest of eliminating a distinction without a difference, this PR collapses these 2 levels down to 1.

There is exactly 1 case where an expression with PREC_POSTFIX precedence needs to be parenthesized in a location that an expression with PREC_PAREN would not, and that's when the receiver of ExprKind::MethodCall is ExprKind::Field. `x.f()` means a different thing than `(x.f)()`. But this does not justify having separate precedence levels because this special case in the grammar is not governed by precedence. Field access does not have "lower precedence than" method call syntax &mdash; you can tell because if it did, then `x.f[0].f()` wouldn't be able to have its unparenthesized field access in the receiver of a method call. Because this Field/MethodCall special case is not governed by precedence, it already requires special handling and is not affected by eliminating the PREC_POSTFIX precedence level.

https://github.com/rust-lang/rust/blob/d49994b060684af423339b55769439b2f444a7b9/compiler/rustc_ast_pretty/src/pprust/state/expr.rs#L217-L221

compiler/rustc_ast/src/util/parser.rs

@@ -233,8 +233,7 @@ pub const PREC_JUMP: i8 = -30;
 pub const PREC_RANGE: i8 = -10;
 // The range 2..=14 is reserved for AssocOp binary operator precedences.
 pub const PREC_PREFIX: i8 = 50;
-pub const PREC_POSTFIX: i8 = 60;
-pub const PREC_PAREN: i8 = 99;
+pub const PREC_UNAMBIGUOUS: i8 = 60;
 pub const PREC_FORCE_PAREN: i8 = 100;
 
 #[derive(Debug, Clone, Copy)]
@@ -325,37 +324,35 @@ impl ExprPrecedence {
             | ExprPrecedence::Let
             | ExprPrecedence::Unary => PREC_PREFIX,
 
-            // Unary, postfix
-            ExprPrecedence::Await
+            // Never need parens
+            ExprPrecedence::Array
+            | ExprPrecedence::Await
+            | ExprPrecedence::Block
             | ExprPrecedence::Call
-            | ExprPrecedence::MethodCall
+            | ExprPrecedence::ConstBlock
             | ExprPrecedence::Field
+            | ExprPrecedence::ForLoop
+            | ExprPrecedence::FormatArgs
+            | ExprPrecedence::Gen
+            | ExprPrecedence::If
             | ExprPrecedence::Index
-            | ExprPrecedence::Try
             | ExprPrecedence::InlineAsm
+            | ExprPrecedence::Lit
+            | ExprPrecedence::Loop
             | ExprPrecedence::Mac
-            | ExprPrecedence::FormatArgs
+            | ExprPrecedence::Match
+            | ExprPrecedence::MethodCall
             | ExprPrecedence::OffsetOf
-            | ExprPrecedence::PostfixMatch => PREC_POSTFIX,
-
-            // Never need parens
-            ExprPrecedence::Array
+            | ExprPrecedence::Paren
+            | ExprPrecedence::Path
+            | ExprPrecedence::PostfixMatch
             | ExprPrecedence::Repeat
+            | ExprPrecedence::Struct
+            | ExprPrecedence::Try
+            | ExprPrecedence::TryBlock
             | ExprPrecedence::Tup
-            | ExprPrecedence::Lit
-            | ExprPrecedence::Path
-            | ExprPrecedence::Paren
-            | ExprPrecedence::If
             | ExprPrecedence::While
-            | ExprPrecedence::ForLoop
-            | ExprPrecedence::Loop
-            | ExprPrecedence::Match
-            | ExprPrecedence::ConstBlock
-            | ExprPrecedence::Block
-            | ExprPrecedence::TryBlock
-            | ExprPrecedence::Gen
-            | ExprPrecedence::Struct
-            | ExprPrecedence::Err => PREC_PAREN,
+            | ExprPrecedence::Err => PREC_UNAMBIGUOUS,
         }
     }
 }

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

@@ -217,7 +217,7 @@ impl<'a> State<'a> {
     fn print_expr_call(&mut self, func: &ast::Expr, args: &[P<ast::Expr>], fixup: FixupContext) {
         let prec = match func.kind {
             ast::ExprKind::Field(..) => parser::PREC_FORCE_PAREN,
-            _ => parser::PREC_POSTFIX,
+            _ => parser::PREC_UNAMBIGUOUS,
         };
 
         // Independent of parenthesization related to precedence, we must
@@ -257,7 +257,7 @@ impl<'a> State<'a> {
         // boundaries, `$receiver.method()` can be parsed back as a statement
         // containing an expression if and only if `$receiver` can be parsed as
         // a statement containing an expression.
-        self.print_expr_maybe_paren(receiver, parser::PREC_POSTFIX, fixup);
+        self.print_expr_maybe_paren(receiver, parser::PREC_UNAMBIGUOUS, fixup);
 
         self.word(".");
         self.print_ident(segment.ident);
@@ -489,7 +489,7 @@ impl<'a> State<'a> {
                         self.space();
                     }
                     MatchKind::Postfix => {
-                        self.print_expr_maybe_paren(expr, parser::PREC_POSTFIX, fixup);
+                        self.print_expr_maybe_paren(expr, parser::PREC_UNAMBIGUOUS, fixup);
                         self.word_nbsp(".match");
                     }
                 }
@@ -549,7 +549,7 @@ impl<'a> State<'a> {
                 self.print_block_with_attrs(blk, attrs);
             }
             ast::ExprKind::Await(expr, _) => {
-                self.print_expr_maybe_paren(expr, parser::PREC_POSTFIX, fixup);
+                self.print_expr_maybe_paren(expr, parser::PREC_UNAMBIGUOUS, fixup);
                 self.word(".await");
             }
             ast::ExprKind::Assign(lhs, rhs, _) => {
@@ -568,14 +568,14 @@ impl<'a> State<'a> {
                 self.print_expr_maybe_paren(rhs, prec, fixup.subsequent_subexpression());
             }
             ast::ExprKind::Field(expr, ident) => {
-                self.print_expr_maybe_paren(expr, parser::PREC_POSTFIX, fixup);
+                self.print_expr_maybe_paren(expr, parser::PREC_UNAMBIGUOUS, fixup);
                 self.word(".");
                 self.print_ident(*ident);
             }
             ast::ExprKind::Index(expr, index, _) => {
                 self.print_expr_maybe_paren(
                     expr,
-                    parser::PREC_POSTFIX,
+                    parser::PREC_UNAMBIGUOUS,
                     fixup.leftmost_subexpression(),
                 );
                 self.word("[");
@@ -713,7 +713,7 @@ impl<'a> State<'a> {
                 }
             }
             ast::ExprKind::Try(e) => {
-                self.print_expr_maybe_paren(e, parser::PREC_POSTFIX, fixup);
+                self.print_expr_maybe_paren(e, parser::PREC_UNAMBIGUOUS, fixup);
                 self.word("?")
             }
             ast::ExprKind::TryBlock(blk) => {

compiler/rustc_hir_pretty/src/lib.rs

@@ -1120,7 +1120,7 @@ impl<'a> State<'a> {
     fn print_expr_call(&mut self, func: &hir::Expr<'_>, args: &[hir::Expr<'_>]) {
         let prec = match func.kind {
             hir::ExprKind::Field(..) => parser::PREC_FORCE_PAREN,
-            _ => parser::PREC_POSTFIX,
+            _ => parser::PREC_UNAMBIGUOUS,
         };
 
         self.print_expr_maybe_paren(func, prec);
@@ -1134,7 +1134,7 @@ impl<'a> State<'a> {
         args: &[hir::Expr<'_>],
     ) {
         let base_args = args;
-        self.print_expr_maybe_paren(receiver, parser::PREC_POSTFIX);
+        self.print_expr_maybe_paren(receiver, parser::PREC_UNAMBIGUOUS);
         self.word(".");
         self.print_ident(segment.ident);
 
@@ -1478,12 +1478,12 @@ impl<'a> State<'a> {
                 self.print_expr_maybe_paren(rhs, prec);
             }
             hir::ExprKind::Field(expr, ident) => {
-                self.print_expr_maybe_paren(expr, parser::PREC_POSTFIX);
+                self.print_expr_maybe_paren(expr, parser::PREC_UNAMBIGUOUS);
                 self.word(".");
                 self.print_ident(ident);
             }
             hir::ExprKind::Index(expr, index, _) => {
-                self.print_expr_maybe_paren(expr, parser::PREC_POSTFIX);
+                self.print_expr_maybe_paren(expr, parser::PREC_UNAMBIGUOUS);
                 self.word("[");
                 self.print_expr(index);
                 self.word("]");

compiler/rustc_hir_typeck/src/callee.rs

@@ -3,7 +3,7 @@ use super::method::MethodCallee;
 use super::{Expectation, FnCtxt, TupleArgumentsFlag};
 
 use crate::errors;
-use rustc_ast::util::parser::PREC_POSTFIX;
+use rustc_ast::util::parser::PREC_UNAMBIGUOUS;
 use rustc_errors::{Applicability, Diag, ErrorGuaranteed, StashKey};
 use rustc_hir::def::{self, CtorKind, Namespace, Res};
 use rustc_hir::def_id::DefId;
@@ -656,7 +656,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             };
 
             if let Ok(rest_snippet) = rest_snippet {
-                let sugg = if callee_expr.precedence().order() >= PREC_POSTFIX {
+                let sugg = if callee_expr.precedence().order() >= PREC_UNAMBIGUOUS {
                     vec![
                         (up_to_rcvr_span, "".to_string()),
                         (rest_span, format!(".{}({rest_snippet}", segment.ident)),

compiler/rustc_hir_typeck/src/cast.rs

@@ -946,7 +946,7 @@ impl<'a, 'tcx> CastCheck<'tcx> {
 
     fn lossy_provenance_ptr2int_lint(&self, fcx: &FnCtxt<'a, 'tcx>, t_c: ty::cast::IntTy) {
         let expr_prec = self.expr.precedence().order();
-        let needs_parens = expr_prec < rustc_ast::util::parser::PREC_POSTFIX;
+        let needs_parens = expr_prec < rustc_ast::util::parser::PREC_UNAMBIGUOUS;
 
         let needs_cast = !matches!(t_c, ty::cast::IntTy::U(ty::UintTy::Usize));
         let cast_span = self.expr_span.shrink_to_hi().to(self.cast_span);

compiler/rustc_hir_typeck/src/fn_ctxt/suggestions.rs

@@ -9,7 +9,7 @@ use crate::method::probe::{IsSuggestion, Mode, ProbeScope};
 use core::cmp::min;
 use core::iter;
 use hir::def_id::LocalDefId;
-use rustc_ast::util::parser::{ExprPrecedence, PREC_POSTFIX};
+use rustc_ast::util::parser::{ExprPrecedence, PREC_UNAMBIGUOUS};
 use rustc_data_structures::packed::Pu128;
 use rustc_errors::{Applicability, Diag, MultiSpan};
 use rustc_hir as hir;
@@ -1329,7 +1329,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         {
             let span = expr.span.find_oldest_ancestor_in_same_ctxt();
 
-            let mut sugg = if expr.precedence().order() >= PREC_POSTFIX {
+            let mut sugg = if expr.precedence().order() >= PREC_UNAMBIGUOUS {
                 vec![(span.shrink_to_hi(), ".into()".to_owned())]
             } else {
                 vec![
@@ -2868,7 +2868,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             "change the type of the numeric literal from `{checked_ty}` to `{expected_ty}`",
         );
 
-        let close_paren = if expr.precedence().order() < PREC_POSTFIX {
+        let close_paren = if expr.precedence().order() < PREC_UNAMBIGUOUS {
             sugg.push((expr.span.shrink_to_lo(), "(".to_string()));
             ")"
         } else {
@@ -2893,7 +2893,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                 let len = src.trim_end_matches(&checked_ty.to_string()).len();
                 expr.span.with_lo(expr.span.lo() + BytePos(len as u32))
             },
-            if expr.precedence().order() < PREC_POSTFIX {
+            if expr.precedence().order() < PREC_UNAMBIGUOUS {
                 // Readd `)`
                 format!("{expected_ty})")
             } else {

src/tools/clippy/clippy_lints/src/dereference.rs

@@ -6,7 +6,7 @@ use clippy_utils::{
     expr_use_ctxt, get_parent_expr, is_block_like, is_lint_allowed, path_to_local, DefinedTy, ExprUseNode,
 };
 use core::mem;
-use rustc_ast::util::parser::{PREC_POSTFIX, PREC_PREFIX};
+use rustc_ast::util::parser::{PREC_UNAMBIGUOUS, PREC_PREFIX};
 use rustc_data_structures::fx::FxIndexMap;
 use rustc_errors::Applicability;
 use rustc_hir::intravisit::{walk_ty, Visitor};
@@ -1013,7 +1013,7 @@ fn report<'tcx>(
                     let (precedence, calls_field) = match cx.tcx.parent_hir_node(data.first_expr.hir_id) {
                         Node::Expr(e) => match e.kind {
                             ExprKind::Call(callee, _) if callee.hir_id != data.first_expr.hir_id => (0, false),
-                            ExprKind::Call(..) => (PREC_POSTFIX, matches!(expr.kind, ExprKind::Field(..))),
+                            ExprKind::Call(..) => (PREC_UNAMBIGUOUS, matches!(expr.kind, ExprKind::Field(..))),
                             _ => (e.precedence().order(), false),
                         },
                         _ => (0, false),
@@ -1160,7 +1160,7 @@ impl<'tcx> Dereferencing<'tcx> {
                         },
                         Some(parent) if !parent.span.from_expansion() => {
                             // Double reference might be needed at this point.
-                            if parent.precedence().order() == PREC_POSTFIX {
+                            if parent.precedence().order() == PREC_UNAMBIGUOUS {
                                 // Parentheses would be needed here, don't lint.
                                 *outer_pat = None;
                             } else {

src/tools/clippy/clippy_lints/src/matches/manual_utils.rs

@@ -7,7 +7,7 @@ use clippy_utils::{
     can_move_expr_to_closure, is_else_clause, is_lint_allowed, is_res_lang_ctor, path_res, path_to_local_id,
     peel_blocks, peel_hir_expr_refs, peel_hir_expr_while, CaptureKind,
 };
-use rustc_ast::util::parser::PREC_POSTFIX;
+use rustc_ast::util::parser::PREC_UNAMBIGUOUS;
 use rustc_errors::Applicability;
 use rustc_hir::def::Res;
 use rustc_hir::LangItem::{OptionNone, OptionSome};
@@ -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_POSTFIX {
+    let scrutinee_str = if scrutinee.span.eq_ctxt(expr.span) && scrutinee.precedence().order() < PREC_UNAMBIGUOUS {
         format!("({scrutinee_str})")
     } else {
         scrutinee_str.into()