Rollup merge of rust-lang#133603 - dtolnay:precedence, r=lcnr

Eliminate magic numbers from expression precedence

Context: see rust-lang#133140.

This PR continues on backporting Syn's expression precedence design into rustc. Rustc's design used mysterious integer quantities represented variously as `i8` or `usize` (e.g. `PREC_CLOSURE = -40i8`), a special significance around `0` that is never named, and an extra `PREC_FORCE_PAREN` precedence level that does not correspond to any expression. Syn's design uses a C-like enum with variants that clearly correspond to specific sets of expression kinds.

This PR is a refactoring that has no intended behavior change on its own, but it unblocks other precedence work that rustc's precedence design was poorly suited to accommodate.

- Asymmetrical precedence, so that a pretty-printer can tell `(return 1) + 1` needs parens but `1 + return 1` does not.

- Squashing the `Closure` and `Jump` cases into a single precedence level.

- Numerous remaining false positives and false negatives in rustc pretty-printer's parenthesization of macro metavariables, for example in `$e < rhs` where $e is `lhs as Thing<T>`.

FYI `@fmease` &mdash; you don't need to review if rustbot picks someone else, but you mentioned being interested in the followup PRs.

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,
         }
     }
 
@@ -229,26 +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;
-pub const PREC_FORCE_PAREN: i8 = 100;
+#[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