Auto merge of #118527 - Nadrieril:never_patterns_parse, r=compiler-errors

never_patterns: Parse match arms with no body

Never patterns are meant to signal unreachable cases, and thus don't take bodies:
```rust
let ptr: *const Option<!> = ...;
match *ptr {
    None => { foo(); }
    Some(!),
}
```
This PR makes rustc accept the above, and enforces that an arm has a body xor is a never pattern. This affects parsing of match arms even with the feature off, so this is delicate. (Plus this is my first non-trivial change to the parser).

~~The last commit is optional; it introduces a bit of churn to allow the new suggestions to be machine-applicable. There may be a better solution? I'm not sure.~~ EDIT: I removed that commit

r? `@compiler-errors`

Author: bors

compiler/rustc_ast/src/ast.rs

@@ -658,6 +658,24 @@ impl Pat {
     pub fn is_rest(&self) -> bool {
         matches!(self.kind, PatKind::Rest)
     }
+
+    /// Whether this could be a never pattern, taking into account that a macro invocation can
+    /// return a never pattern. Used to inform errors during parsing.
+    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/messages.ftl

@@ -91,6 +91,10 @@ ast_lowering_invalid_register =
 ast_lowering_invalid_register_class =
     invalid register class `{$reg_class}`: {$error}
 
+ast_lowering_match_arm_with_no_body =
+    `match` arm with no body
+    .suggestion = add a body after the pattern
+
 ast_lowering_misplaced_assoc_ty_binding =
     associated type bounds are only allowed in where clauses and function signatures, not in {$position}
 
@@ -104,6 +108,15 @@ ast_lowering_misplaced_impl_trait =
 ast_lowering_misplaced_relax_trait_bound =
     `?Trait` bounds are only permitted at the point where a type parameter is declared
 
+ast_lowering_never_pattern_with_body =
+    a never pattern is always unreachable
+    .label = this will never be executed
+    .suggestion = remove this expression
+
+ast_lowering_never_pattern_with_guard =
+    a guard on a never pattern will never be run
+    .suggestion = remove this guard
+
 ast_lowering_not_supported_for_lifetime_binder_async_closure =
     `for<...>` binders on `async` closures are not currently supported
 

compiler/rustc_ast_lowering/src/errors.rs

@@ -340,6 +340,32 @@ pub struct NotSupportedForLifetimeBinderAsyncClosure {
     pub span: Span,
 }
 
+#[derive(Diagnostic)]
+#[diag(ast_lowering_match_arm_with_no_body)]
+pub struct MatchArmWithNoBody {
+    #[primary_span]
+    pub span: Span,
+    #[suggestion(code = " => todo!(),", applicability = "has-placeholders")]
+    pub suggestion: Span,
+}
+
+#[derive(Diagnostic)]
+#[diag(ast_lowering_never_pattern_with_body)]
+pub struct NeverPatternWithBody {
+    #[primary_span]
+    #[label]
+    #[suggestion(code = "", applicability = "maybe-incorrect")]
+    pub span: Span,
+}
+
+#[derive(Diagnostic)]
+#[diag(ast_lowering_never_pattern_with_guard)]
+pub struct NeverPatternWithGuard {
+    #[primary_span]
+    #[suggestion(code = "", applicability = "maybe-incorrect")]
+    pub span: Span,
+}
+
 #[derive(Diagnostic, Clone, Copy)]
 #[diag(ast_lowering_arbitrary_expression_in_pattern)]
 pub struct ArbitraryExpressionInPattern {

compiler/rustc_ast_lowering/src/expr.rs

@@ -1,8 +1,9 @@
 use super::errors::{
     AsyncCoroutinesNotSupported, AsyncNonMoveClosureNotSupported, AwaitOnlyInAsyncFnAndBlocks,
     BaseExpressionDoubleDot, ClosureCannotBeStatic, CoroutineTooManyParameters,
-    FunctionalRecordUpdateDestructuringAssignment, InclusiveRangeWithNoEnd,
-    NotSupportedForLifetimeBinderAsyncClosure, UnderscoreExprLhsAssign,
+    FunctionalRecordUpdateDestructuringAssignment, InclusiveRangeWithNoEnd, MatchArmWithNoBody,
+    NeverPatternWithBody, NeverPatternWithGuard, NotSupportedForLifetimeBinderAsyncClosure,
+    UnderscoreExprLhsAssign,
 };
 use super::ResolverAstLoweringExt;
 use super::{ImplTraitContext, LoweringContext, ParamMode, ParenthesizedGenericArgs};
@@ -549,7 +550,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
 
     fn lower_arm(&mut self, arm: &Arm) -> hir::Arm<'hir> {
         let pat = self.lower_pat(&arm.pat);
-        let guard = arm.guard.as_ref().map(|cond| {
+        let mut guard = arm.guard.as_ref().map(|cond| {
             if let ExprKind::Let(pat, scrutinee, span, is_recovered) = &cond.kind {
                 hir::Guard::IfLet(self.arena.alloc(hir::Let {
                     hir_id: self.next_id(),
@@ -564,14 +565,43 @@ impl<'hir> LoweringContext<'_, 'hir> {
             }
         });
         let hir_id = self.next_id();
+        let span = self.lower_span(arm.span);
         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 is_never_pattern = pat.is_never_pattern();
+        let body = if let Some(body) = &arm.body
+            && !is_never_pattern
+        {
+            self.lower_expr(body)
+        } else {
+            // Either `body.is_none()` or `is_never_pattern` here.
+            if !is_never_pattern {
+                let suggestion = span.shrink_to_hi();
+                self.tcx.sess.emit_err(MatchArmWithNoBody { span, suggestion });
+            } else if let Some(body) = &arm.body {
+                self.tcx.sess.emit_err(NeverPatternWithBody { span: body.span });
+                guard = None;
+            } else if let Some(g) = &arm.guard {
+                self.tcx.sess.emit_err(NeverPatternWithGuard { span: g.span });
+                guard = None;
+            }
+
+            // We add a fake `loop {}` arm body so that it typecks to `!`.
+            // FIXME(never_patterns): Desugar into a call to `unreachable_unchecked`.
+            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 }
     }
 
     /// 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/messages.ftl

@@ -71,6 +71,8 @@ expand_macro_const_stability =
     .label = invalid const stability attribute
     .label2 = const stability attribute affects this macro
 
+expand_macro_expands_to_match_arm = macros cannot expand to match arms
+
 expand_malformed_feature_attribute =
     malformed `feature` attribute input
     .expected = expected just one word

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,