Add classify::expr_requires_comma_to_be_match_arm

Author: dtolnay

compiler/rustc_ast/src/util/classify.rs

@@ -1,5 +1,6 @@
 //! Routines the parser and pretty-printer use to classify AST nodes.
 
+use crate::ast::ExprKind::*;
 use crate::{ast, token::Delimiter};
 
 /// Does this expression require a semicolon to be treated as a statement?
@@ -14,9 +15,27 @@ use crate::{ast, token::Delimiter};
 /// ```
 ///
 /// isn't parsed as `(if true {...} else {...} | x) | 5`.
+pub fn expr_requires_semi_to_be_stmt(e: &ast::Expr) -> bool {
+    match &e.kind {
+        If(..)
+        | Match(..)
+        | Block(..)
+        | While(..)
+        | Loop(..)
+        | ForLoop { .. }
+        | TryBlock(..)
+        | ConstBlock(..) => false,
+
+        MacCall(mac_call) => mac_call.args.delim != Delimiter::Brace,
+
+        _ => true,
+    }
+}
+
+/// Does this expression require a comma to separate it from the next match arm?
 ///
-/// Nearly the same early bail-out also occurs in the right-hand side of match
-/// arms:
+/// The right-hand side of match arms use almost the same early bail-out as
+/// statements do (see `expr_requires_semi_to_be_stmt`):
 ///
 /// ```ignore (illustrative)
 /// match i {
@@ -27,8 +46,9 @@ use crate::{ast, token::Delimiter};
 ///
 /// Here the `|` is a leading vert in a second match arm. It is not a binary
 /// operator with the If as its left operand. If the first arm were some other
-/// expression for which `expr_requires_semi_to_be_stmt` returns true, then the
-/// `|` on the next line would be a binary operator (leading to a parse error).
+/// expression for which `expr_requires_comma_to_be_match_arm` returns true,
+/// then the `|` on the next line would instead be a binary operator (leading to
+/// a parse error).
 ///
 /// The statement case and the match-arm case are "nearly" the same early
 /// bail-out because of 1 edge case. Macro calls with brace delimiter terminate
@@ -42,29 +62,15 @@ use crate::{ast, token::Delimiter};
 ///     _ => m! {} - 1,  // binary subtraction operator
 /// }
 /// ```
-pub fn expr_requires_semi_to_be_stmt(e: &ast::Expr) -> bool {
-    use ast::ExprKind::*;
-
+pub fn expr_requires_comma_to_be_match_arm(e: &ast::Expr) -> bool {
     match &e.kind {
-        If(..)
-        | Match(..)
-        | Block(..)
-        | While(..)
-        | Loop(..)
-        | ForLoop { .. }
-        | TryBlock(..)
-        | ConstBlock(..) => false,
-
-        MacCall(mac_call) => mac_call.args.delim != Delimiter::Brace,
-
-        _ => true,
+        MacCall(_) => true,
+        _ => expr_requires_semi_to_be_stmt(e),
     }
 }
 
 /// If an expression ends with `}`, returns the innermost expression ending in the `}`
 pub fn expr_trailing_brace(mut expr: &ast::Expr) -> Option<&ast::Expr> {
-    use ast::ExprKind::*;
-
     loop {
         match &expr.kind {
             AddrOf(_, _, e)

compiler/rustc_parse/src/parser/expr.rs

@@ -511,22 +511,19 @@ impl<'a> Parser<'a> {
     /// If $e is something like `{}` or `if … {}`, then terminate the current
     /// arm and parse a new arm.
     fn expr_is_complete(&self, e: &Expr) -> bool {
+        // Surprising special case: even though braced macro calls like
+        // `m! {}` normally introduce a statement boundary when found at
+        // the head of a statement, in match arms they do not terminate
+        // the arm.
+        //
+        //     let _ = { m! {} () };  // macro call followed by unit
+        //
+        //     match ... {
+        //         _ => m! {} (),  // macro that expands to a function, which is then called
+        //     }
+        //
         self.restrictions.contains(Restrictions::STMT_EXPR)
-            && match e.kind {
-                // Surprising special case: even though braced macro calls like
-                // `m! {}` normally introduce a statement boundary when found at
-                // the head of a statement, in match arms they do not terminate
-                // the arm.
-                //
-                //     let _ = { m! {} () };  // macro call followed by unit
-                //
-                //     match ... {
-                //         _ => m! {} (),  // macro that expands to a function, which is then called
-                //     }
-                //
-                ExprKind::MacCall(_) => false,
-                _ => !classify::expr_requires_semi_to_be_stmt(e),
-            }
+            && !classify::expr_requires_comma_to_be_match_arm(e)
     }
 
     /// Parses `x..y`, `x..=y`, and `x..`/`x..=`.
@@ -3181,21 +3178,19 @@ impl<'a> Parser<'a> {
                         err
                     })?;
 
-                let require_comma = match expr.kind {
-                    // Special case: braced macro calls require comma in a match
-                    // arm, even though they do not require semicolon in a
-                    // statement.
-                    //
-                    //     m! {}  // okay without semicolon
-                    //
-                    //     match ... {
-                    //         _ => m! {},  // requires comma
-                    //         _ => ...
-                    //     }
-                    //
-                    ExprKind::MacCall(_) => true,
-                    _ => classify::expr_requires_semi_to_be_stmt(&expr),
-                } && this.token != token::CloseDelim(Delimiter::Brace);
+                // Special case: braced macro calls require comma in a match
+                // arm, even though they do not require semicolon in a
+                // statement.
+                //
+                //     m! {}  // okay without semicolon
+                //
+                //     match ... {
+                //         _ => m! {},  // requires comma
+                //         _ => ...
+                //     }
+                //
+                let require_comma = classify::expr_requires_comma_to_be_match_arm(&expr)
+                    && this.token != token::CloseDelim(Delimiter::Brace);
 
                 if !require_comma {
                     arm_body = Some(expr);