Eliminate magic numbers from expression precedence

Author: dtolnay

compiler/rustc_ast/src/ast.rs

@@ -39,9 +39,7 @@ pub use crate::format::*;
 use crate::ptr::P;
 use crate::token::{self, CommentKind, Delimiter};
 use crate::tokenstream::{DelimSpan, LazyAttrTokenStream, TokenStream};
-use crate::util::parser::{
-    AssocOp, PREC_CLOSURE, PREC_JUMP, PREC_PREFIX, PREC_RANGE, PREC_UNAMBIGUOUS,
-};
+use crate::util::parser::{AssocOp, ExprPrecedence};
 
 /// A "Label" is an identifier of some point in sources,
 /// e.g. in the following code:
@@ -1317,29 +1315,29 @@ impl Expr {
         Some(P(Ty { kind, id: self.id, span: self.span, tokens: None }))
     }
 
-    pub fn precedence(&self) -> i8 {
+    pub fn precedence(&self) -> ExprPrecedence {
         match self.kind {
-            ExprKind::Closure(..) => PREC_CLOSURE,
+            ExprKind::Closure(..) => ExprPrecedence::Closure,
 
             ExprKind::Break(..)
             | ExprKind::Continue(..)
             | ExprKind::Ret(..)
             | ExprKind::Yield(..)
             | ExprKind::Yeet(..)
-            | ExprKind::Become(..) => PREC_JUMP,
+            | ExprKind::Become(..) => ExprPrecedence::Jump,
 
             // `Range` claims to have higher precedence than `Assign`, but `x .. x = x` fails to
             // parse, instead of parsing as `(x .. x) = x`. Giving `Range` a lower precedence
             // ensures that `pprust` will add parentheses in the right places to get the desired
             // parse.
-            ExprKind::Range(..) => PREC_RANGE,
+            ExprKind::Range(..) => ExprPrecedence::Range,
 
             // Binop-like expr kinds, handled by `AssocOp`.
-            ExprKind::Binary(op, ..) => AssocOp::from_ast_binop(op.node).precedence() as i8,
-            ExprKind::Cast(..) => AssocOp::As.precedence() as i8,
+            ExprKind::Binary(op, ..) => AssocOp::from_ast_binop(op.node).precedence(),
+            ExprKind::Cast(..) => ExprPrecedence::Cast,
 
             ExprKind::Assign(..) |
-            ExprKind::AssignOp(..) => AssocOp::Assign.precedence() as i8,
+            ExprKind::AssignOp(..) => ExprPrecedence::Assign,
 
             // Unary, prefix
             ExprKind::AddrOf(..)
@@ -1348,7 +1346,7 @@ impl Expr {
             // need parens sometimes. E.g. we can print `(let _ = a) && b` as `let _ = a && b`
             // but we need to print `(let _ = a) < b` as-is with parens.
             | ExprKind::Let(..)
-            | ExprKind::Unary(..) => PREC_PREFIX,
+            | ExprKind::Unary(..) => ExprPrecedence::Prefix,
 
             // Never need parens
             ExprKind::Array(_)
@@ -1381,7 +1379,7 @@ impl Expr {
             | ExprKind::Underscore
             | ExprKind::While(..)
             | ExprKind::Err(_)
-            | ExprKind::Dummy => PREC_UNAMBIGUOUS,
+            | ExprKind::Dummy => ExprPrecedence::Unambiguous,
         }
     }
 

compiler/rustc_ast/src/util/parser.rs

@@ -128,21 +128,21 @@ impl AssocOp {
     }
 
     /// Gets the precedence of this operator
-    pub fn precedence(&self) -> usize {
+    pub fn precedence(&self) -> ExprPrecedence {
         use AssocOp::*;
         match *self {
-            As => 14,
-            Multiply | Divide | Modulus => 13,
-            Add | Subtract => 12,
-            ShiftLeft | ShiftRight => 11,
-            BitAnd => 10,
-            BitXor => 9,
-            BitOr => 8,
-            Less | Greater | LessEqual | GreaterEqual | Equal | NotEqual => 7,
-            LAnd => 6,
-            LOr => 5,
-            DotDot | DotDotEq => 4,
-            Assign | AssignOp(_) => 2,
+            As => ExprPrecedence::Cast,
+            Multiply | Divide | Modulus => ExprPrecedence::Product,
+            Add | Subtract => ExprPrecedence::Sum,
+            ShiftLeft | ShiftRight => ExprPrecedence::Shift,
+            BitAnd => ExprPrecedence::BitAnd,
+            BitXor => ExprPrecedence::BitXor,
+            BitOr => ExprPrecedence::BitOr,
+            Less | Greater | LessEqual | GreaterEqual | Equal | NotEqual => ExprPrecedence::Compare,
+            LAnd => ExprPrecedence::LAnd,
+            LOr => ExprPrecedence::LOr,
+            DotDot | DotDotEq => ExprPrecedence::Range,
+            Assign | AssignOp(_) => ExprPrecedence::Assign,
         }
     }
 
@@ -229,25 +229,53 @@ impl AssocOp {
     }
 }
 
-pub const PREC_CLOSURE: i8 = -40;
-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_UNAMBIGUOUS: i8 = 60;
+#[derive(Clone, Copy, PartialEq, PartialOrd)]
+pub enum ExprPrecedence {
+    Closure,
+    // return, break, yield
+    Jump,
+    // = += -= *= /= %= &= |= ^= <<= >>=
+    Assign,
+    // .. ..=
+    Range,
+    // ||
+    LOr,
+    // &&
+    LAnd,
+    // == != < > <= >=
+    Compare,
+    // |
+    BitOr,
+    // ^
+    BitXor,
+    // &
+    BitAnd,
+    // << >>
+    Shift,
+    // + -
+    Sum,
+    // * / %
+    Product,
+    // as
+    Cast,
+    // unary - * ! & &mut
+    Prefix,
+    // paths, loops, function calls, array indexing, field expressions, method calls
+    Unambiguous,
+}
 
 /// In `let p = e`, operators with precedence `<=` this one requires parentheses in `e`.
-pub fn prec_let_scrutinee_needs_par() -> usize {
-    AssocOp::LAnd.precedence()
+pub fn prec_let_scrutinee_needs_par() -> ExprPrecedence {
+    ExprPrecedence::LAnd
 }
 
 /// Suppose we have `let _ = e` and the `order` of `e`.
 /// Is the `order` such that `e` in `let _ = e` needs parentheses when it is on the RHS?
 ///
 /// Conversely, suppose that we have `(let _ = a) OP b` and `order` is that of `OP`.
 /// Can we print this as `let _ = a OP b`?
-pub fn needs_par_as_let_scrutinee(order: i8) -> bool {
-    order <= prec_let_scrutinee_needs_par() as i8
+pub fn needs_par_as_let_scrutinee(order: ExprPrecedence) -> bool {
+    order <= prec_let_scrutinee_needs_par()
 }
 
 /// Expressions that syntactically contain an "exterior" struct literal i.e., not surrounded by any

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

@@ -5,7 +5,7 @@ use itertools::{Itertools, Position};
 use rustc_ast::ptr::P;
 use rustc_ast::util::classify;
 use rustc_ast::util::literal::escape_byte_str_symbol;
-use rustc_ast::util::parser::{self, AssocOp, Fixity};
+use rustc_ast::util::parser::{self, AssocOp, ExprPrecedence, Fixity};
 use rustc_ast::{
     self as ast, BlockCheckMode, FormatAlignment, FormatArgPosition, FormatArgsPiece, FormatCount,
     FormatDebugHex, FormatSign, FormatTrait, token,
@@ -214,7 +214,7 @@ impl<'a> State<'a> {
     fn print_expr_call(&mut self, func: &ast::Expr, args: &[P<ast::Expr>], fixup: FixupContext) {
         let needs_paren = match func.kind {
             ast::ExprKind::Field(..) => true,
-            _ => func.precedence() < parser::PREC_UNAMBIGUOUS,
+            _ => func.precedence() < ExprPrecedence::Unambiguous,
         };
 
         // Independent of parenthesization related to precedence, we must
@@ -256,7 +256,7 @@ impl<'a> State<'a> {
         // a statement containing an expression.
         self.print_expr_cond_paren(
             receiver,
-            receiver.precedence() < parser::PREC_UNAMBIGUOUS,
+            receiver.precedence() < ExprPrecedence::Unambiguous,
             fixup,
         );
 
@@ -276,7 +276,7 @@ impl<'a> State<'a> {
         fixup: FixupContext,
     ) {
         let assoc_op = AssocOp::from_ast_binop(op.node);
-        let binop_prec = assoc_op.precedence() as i8;
+        let binop_prec = assoc_op.precedence();
         let left_prec = lhs.precedence();
         let right_prec = rhs.precedence();
 
@@ -317,7 +317,7 @@ impl<'a> State<'a> {
         self.word(op.as_str());
         self.print_expr_cond_paren(
             expr,
-            expr.precedence() < parser::PREC_PREFIX,
+            expr.precedence() < ExprPrecedence::Prefix,
             fixup.subsequent_subexpression(),
         );
     }
@@ -339,7 +339,7 @@ impl<'a> State<'a> {
         }
         self.print_expr_cond_paren(
             expr,
-            expr.precedence() < parser::PREC_PREFIX,
+            expr.precedence() < ExprPrecedence::Prefix,
             fixup.subsequent_subexpression(),
         );
     }
@@ -423,10 +423,9 @@ impl<'a> State<'a> {
                 self.print_token_literal(lit, expr.span)
             }
             ast::ExprKind::Cast(expr, ty) => {
-                let prec = AssocOp::As.precedence() as i8;
                 self.print_expr_cond_paren(
                     expr,
-                    expr.precedence() < prec,
+                    expr.precedence() < ExprPrecedence::Cast,
                     fixup.leftmost_subexpression(),
                 );
                 self.space();
@@ -503,7 +502,7 @@ impl<'a> State<'a> {
                     MatchKind::Postfix => {
                         self.print_expr_cond_paren(
                             expr,
-                            expr.precedence() < parser::PREC_UNAMBIGUOUS,
+                            expr.precedence() < ExprPrecedence::Unambiguous,
                             fixup,
                         );
                         self.word_nbsp(".match");
@@ -567,46 +566,46 @@ impl<'a> State<'a> {
             ast::ExprKind::Await(expr, _) => {
                 self.print_expr_cond_paren(
                     expr,
-                    expr.precedence() < parser::PREC_UNAMBIGUOUS,
+                    expr.precedence() < ExprPrecedence::Unambiguous,
                     fixup,
                 );
                 self.word(".await");
             }
             ast::ExprKind::Assign(lhs, rhs, _) => {
-                let prec = AssocOp::Assign.precedence() as i8;
                 self.print_expr_cond_paren(
                     lhs,
-                    lhs.precedence() <= prec,
+                    // Ranges are allowed on the right-hand side of assignment,
+                    // but not the left. `(a..b) = c` needs parentheses.
+                    lhs.precedence() <= ExprPrecedence::Range,
                     fixup.leftmost_subexpression(),
                 );
                 self.space();
                 self.word_space("=");
                 self.print_expr_cond_paren(
                     rhs,
-                    rhs.precedence() < prec,
+                    rhs.precedence() < ExprPrecedence::Assign,
                     fixup.subsequent_subexpression(),
                 );
             }
             ast::ExprKind::AssignOp(op, lhs, rhs) => {
-                let prec = AssocOp::Assign.precedence() as i8;
                 self.print_expr_cond_paren(
                     lhs,
-                    lhs.precedence() <= prec,
+                    lhs.precedence() <= ExprPrecedence::Range,
                     fixup.leftmost_subexpression(),
                 );
                 self.space();
                 self.word(op.node.as_str());
                 self.word_space("=");
                 self.print_expr_cond_paren(
                     rhs,
-                    rhs.precedence() < prec,
+                    rhs.precedence() < ExprPrecedence::Assign,
                     fixup.subsequent_subexpression(),
                 );
             }
             ast::ExprKind::Field(expr, ident) => {
                 self.print_expr_cond_paren(
                     expr,
-                    expr.precedence() < parser::PREC_UNAMBIGUOUS,
+                    expr.precedence() < ExprPrecedence::Unambiguous,
                     fixup,
                 );
                 self.word(".");
@@ -615,7 +614,7 @@ impl<'a> State<'a> {
             ast::ExprKind::Index(expr, index, _) => {
                 self.print_expr_cond_paren(
                     expr,
-                    expr.precedence() < parser::PREC_UNAMBIGUOUS,
+                    expr.precedence() < ExprPrecedence::Unambiguous,
                     fixup.leftmost_subexpression(),
                 );
                 self.word("[");
@@ -627,7 +626,7 @@ impl<'a> State<'a> {
                 // than `Assign`, but `x .. x = x` gives a parse error instead of `x .. (x = x)`.
                 // Here we use a fake precedence value so that any child with lower precedence than
                 // a "normal" binop gets parenthesized. (`LOr` is the lowest-precedence binop.)
-                let fake_prec = AssocOp::LOr.precedence() as i8;
+                let fake_prec = ExprPrecedence::LOr;
                 if let Some(e) = start {
                     self.print_expr_cond_paren(
                         e,
@@ -662,7 +661,7 @@ impl<'a> State<'a> {
                         expr,
                         // Parenthesize if required by precedence, or in the
                         // case of `break 'inner: loop { break 'inner 1 } + 1`
-                        expr.precedence() < parser::PREC_JUMP
+                        expr.precedence() < ExprPrecedence::Jump
                             || (opt_label.is_none() && classify::leading_labeled_expr(expr)),
                         fixup.subsequent_subexpression(),
                     );
@@ -681,7 +680,7 @@ impl<'a> State<'a> {
                     self.word(" ");
                     self.print_expr_cond_paren(
                         expr,
-                        expr.precedence() < parser::PREC_JUMP,
+                        expr.precedence() < ExprPrecedence::Jump,
                         fixup.subsequent_subexpression(),
                     );
                 }
@@ -694,7 +693,7 @@ impl<'a> State<'a> {
                     self.word(" ");
                     self.print_expr_cond_paren(
                         expr,
-                        expr.precedence() < parser::PREC_JUMP,
+                        expr.precedence() < ExprPrecedence::Jump,
                         fixup.subsequent_subexpression(),
                     );
                 }
@@ -704,7 +703,7 @@ impl<'a> State<'a> {
                 self.word(" ");
                 self.print_expr_cond_paren(
                     result,
-                    result.precedence() < parser::PREC_JUMP,
+                    result.precedence() < ExprPrecedence::Jump,
                     fixup.subsequent_subexpression(),
                 );
             }
@@ -758,13 +757,13 @@ impl<'a> State<'a> {
                     self.space();
                     self.print_expr_cond_paren(
                         expr,
-                        expr.precedence() < parser::PREC_JUMP,
+                        expr.precedence() < ExprPrecedence::Jump,
                         fixup.subsequent_subexpression(),
                     );
                 }
             }
             ast::ExprKind::Try(e) => {
-                self.print_expr_cond_paren(e, e.precedence() < parser::PREC_UNAMBIGUOUS, fixup);
+                self.print_expr_cond_paren(e, e.precedence() < ExprPrecedence::Unambiguous, fixup);
                 self.word("?")
             }
             ast::ExprKind::TryBlock(blk) => {

compiler/rustc_errors/src/diagnostic_impls.rs

@@ -6,6 +6,7 @@ use std::path::{Path, PathBuf};
 use std::process::ExitStatus;
 
 use rustc_abi::TargetDataLayoutErrors;
+use rustc_ast::util::parser::ExprPrecedence;
 use rustc_ast_pretty::pprust;
 use rustc_macros::Subdiagnostic;
 use rustc_span::Span;
@@ -298,6 +299,12 @@ impl IntoDiagArg for hir::def::Namespace {
     }
 }
 
+impl IntoDiagArg for ExprPrecedence {
+    fn into_diag_arg(self) -> DiagArgValue {
+        DiagArgValue::Number(self as i32)
+    }
+}
+
 #[derive(Clone)]
 pub struct DiagSymbolList<S = Symbol>(Vec<S>);