Parse a pattern with no arm

Author: Nadrieril

compiler/rustc_ast/src/ast.rs

@@ -658,6 +658,24 @@ impl Pat {
     pub fn is_rest(&self) -> bool {
         matches!(self.kind, PatKind::Rest)
     }
+
+    /// Could this be a never pattern? I.e. is it a never pattern modulo macro invocations that
+    /// might return never patterns?
+    pub fn could_be_never_pattern(&self) -> bool {
+        let mut could_be_never_pattern = false;
+        self.walk(&mut |pat| match &pat.kind {
+            PatKind::Never | PatKind::MacCall(_) => {
+                could_be_never_pattern = true;
+                false
+            }
+            PatKind::Or(s) => {
+                could_be_never_pattern = s.iter().all(|p| p.could_be_never_pattern());
+                false
+            }
+            _ => true,
+        });
+        could_be_never_pattern
+    }
 }
 
 /// A single field in a struct pattern.
@@ -1080,8 +1098,8 @@ pub struct Arm {
     pub pat: P<Pat>,
     /// Match arm guard, e.g. `n > 10` in `match foo { n if n > 10 => {}, _ => {} }`
     pub guard: Option<P<Expr>>,
-    /// Match arm body.
-    pub body: P<Expr>,
+    /// Match arm body. Omitted if the pattern is a never pattern.
+    pub body: Option<P<Expr>>,
     pub span: Span,
     pub id: NodeId,
     pub is_placeholder: bool,

compiler/rustc_ast/src/mut_visit.rs

@@ -453,7 +453,7 @@ pub fn noop_flat_map_arm<T: MutVisitor>(mut arm: Arm, vis: &mut T) -> SmallVec<[
     vis.visit_id(id);
     vis.visit_pat(pat);
     visit_opt(guard, |guard| vis.visit_expr(guard));
-    vis.visit_expr(body);
+    visit_opt(body, |body| vis.visit_expr(body));
     vis.visit_span(span);
     smallvec![arm]
 }

compiler/rustc_ast/src/visit.rs

@@ -951,7 +951,7 @@ pub fn walk_param<'a, V: Visitor<'a>>(visitor: &mut V, param: &'a Param) {
 pub fn walk_arm<'a, V: Visitor<'a>>(visitor: &mut V, arm: &'a Arm) {
     visitor.visit_pat(&arm.pat);
     walk_list!(visitor, visit_expr, &arm.guard);
-    visitor.visit_expr(&arm.body);
+    walk_list!(visitor, visit_expr, &arm.body);
     walk_list!(visitor, visit_attribute, &arm.attrs);
 }
 

compiler/rustc_ast_lowering/src/expr.rs

@@ -566,13 +566,28 @@ impl<'hir> LoweringContext<'_, 'hir> {
         });
         let hir_id = self.next_id();
         self.lower_attrs(hir_id, &arm.attrs);
-        hir::Arm {
-            hir_id,
-            pat,
-            guard,
-            body: self.lower_expr(&arm.body),
-            span: self.lower_span(arm.span),
-        }
+        let body = if let Some(body) = &arm.body {
+            self.lower_expr(body)
+        } else {
+            // An arm without a body, meant for never patterns.
+            // We add a fake `loop {}` arm body so that it typecks to `!`.
+            // FIXME(never_patterns): Desugar into a call to `unreachable_unchecked`.
+            let span = pat.span;
+            let block = self.arena.alloc(hir::Block {
+                stmts: &[],
+                expr: None,
+                hir_id: self.next_id(),
+                rules: hir::BlockCheckMode::DefaultBlock,
+                span,
+                targeted_by_break: false,
+            });
+            self.arena.alloc(hir::Expr {
+                hir_id: self.next_id(),
+                kind: hir::ExprKind::Loop(block, None, hir::LoopSource::Loop, span),
+                span,
+            })
+        };
+        hir::Arm { hir_id, pat, guard, body, span: self.lower_span(arm.span) }
     }
 
     /// Lower an `async` construct to a coroutine that implements `Future`.

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

@@ -631,28 +631,33 @@ impl<'a> State<'a> {
             self.print_expr(e);
             self.space();
         }
-        self.word_space("=>");
 
-        match &arm.body.kind {
-            ast::ExprKind::Block(blk, opt_label) => {
-                if let Some(label) = opt_label {
-                    self.print_ident(label.ident);
-                    self.word_space(":");
-                }
+        if let Some(body) = &arm.body {
+            self.word_space("=>");
+
+            match &body.kind {
+                ast::ExprKind::Block(blk, opt_label) => {
+                    if let Some(label) = opt_label {
+                        self.print_ident(label.ident);
+                        self.word_space(":");
+                    }
 
-                // The block will close the pattern's ibox.
-                self.print_block_unclosed_indent(blk);
+                    // The block will close the pattern's ibox.
+                    self.print_block_unclosed_indent(blk);
 
-                // If it is a user-provided unsafe block, print a comma after it.
-                if let BlockCheckMode::Unsafe(ast::UserProvided) = blk.rules {
+                    // If it is a user-provided unsafe block, print a comma after it.
+                    if let BlockCheckMode::Unsafe(ast::UserProvided) = blk.rules {
+                        self.word(",");
+                    }
+                }
+                _ => {
+                    self.end(); // Close the ibox for the pattern.
+                    self.print_expr(body);
                     self.word(",");
                 }
             }
-            _ => {
-                self.end(); // Close the ibox for the pattern.
-                self.print_expr(&arm.body);
-                self.word(",");
-            }
+        } else {
+            self.word(",");
         }
         self.end(); // Close enclosing cbox.
     }

compiler/rustc_builtin_macros/src/deriving/cmp/partial_ord.rs

@@ -136,7 +136,7 @@ fn cs_partial_cmp(
                     && let Some(last) = arms.last_mut()
                     && let PatKind::Wild = last.pat.kind
                 {
-                    last.body = expr2;
+                    last.body = Some(expr2);
                     expr1
                 } else {
                     let eq_arm = cx.arm(

compiler/rustc_expand/src/build.rs

@@ -505,7 +505,7 @@ impl<'a> ExtCtxt<'a> {
             attrs: AttrVec::new(),
             pat,
             guard: None,
-            body: expr,
+            body: Some(expr),
             span,
             id: ast::DUMMY_NODE_ID,
             is_placeholder: false,

compiler/rustc_expand/src/placeholders.rs

@@ -119,7 +119,7 @@ pub fn placeholder(
         }]),
         AstFragmentKind::Arms => AstFragment::Arms(smallvec![ast::Arm {
             attrs: Default::default(),
-            body: expr_placeholder(),
+            body: Some(expr_placeholder()),
             guard: None,
             id,
             pat: pat(),

compiler/rustc_lint/src/builtin.rs

@@ -1000,8 +1000,10 @@ impl EarlyLintPass for UnusedDocComment {
     }
 
     fn check_arm(&mut self, cx: &EarlyContext<'_>, arm: &ast::Arm) {
-        let arm_span = arm.pat.span.with_hi(arm.body.span.hi());
-        warn_if_doc(cx, arm_span, "match arms", &arm.attrs);
+        if let Some(body) = &arm.body {
+            let arm_span = arm.pat.span.with_hi(body.span.hi());
+            warn_if_doc(cx, arm_span, "match arms", &arm.attrs);
+        }
     }
 
     fn check_pat(&mut self, cx: &EarlyContext<'_>, pat: &ast::Pat) {

compiler/rustc_lint/src/unused.rs

@@ -1113,15 +1113,17 @@ impl EarlyLintPass for UnusedParens {
             }
             ExprKind::Match(ref _expr, ref arm) => {
                 for a in arm {
-                    self.check_unused_delims_expr(
-                        cx,
-                        &a.body,
-                        UnusedDelimsCtx::MatchArmExpr,
-                        false,
-                        None,
-                        None,
-                        true,
-                    );
+                    if let Some(body) = &a.body {
+                        self.check_unused_delims_expr(
+                            cx,
+                            body,
+                            UnusedDelimsCtx::MatchArmExpr,
+                            false,
+                            None,
+                            None,
+                            true,
+                        );
+                    }
                 }
             }
             _ => {}