Plumb awaitness of for loops

Author: eholk

compiler/rustc_ast/src/ast.rs

@@ -1274,7 +1274,7 @@ impl Expr {
             ExprKind::Let(..) => ExprPrecedence::Let,
             ExprKind::If(..) => ExprPrecedence::If,
             ExprKind::While(..) => ExprPrecedence::While,
-            ExprKind::ForLoop(..) => ExprPrecedence::ForLoop,
+            ExprKind::ForLoop { .. } => ExprPrecedence::ForLoop,
             ExprKind::Loop(..) => ExprPrecedence::Loop,
             ExprKind::Match(..) => ExprPrecedence::Match,
             ExprKind::Closure(..) => ExprPrecedence::Closure,
@@ -1436,10 +1436,10 @@ pub enum ExprKind {
     While(P<Expr>, P<Block>, Option<Label>),
     /// A `for` loop, with an optional label.
     ///
-    /// `'label: for pat in expr { block }`
+    /// `'label: for await? pat in iter { block }`
     ///
     /// This is desugared to a combination of `loop` and `match` expressions.
-    ForLoop(P<Pat>, P<Expr>, P<Block>, Option<Label>),
+    ForLoop { pat: P<Pat>, iter: P<Expr>, body: P<Block>, label: Option<Label>, kind: ForLoopKind },
     /// Conditionless loop (can be exited with `break`, `continue`, or `return`).
     ///
     /// `'label: loop { block }`
@@ -1542,6 +1542,13 @@ pub enum ExprKind {
     Err,
 }
 
+/// Used to differentiate between `for` loops and `for await` loops.
+#[derive(Clone, Copy, Encodable, Decodable, Debug, PartialEq, Eq)]
+pub enum ForLoopKind {
+    For,
+    ForAwait,
+}
+
 /// Used to differentiate between `async {}` blocks and `gen {}` blocks.
 #[derive(Clone, Encodable, Decodable, Debug, PartialEq, Eq)]
 pub enum GenBlockKind {

compiler/rustc_ast/src/mut_visit.rs

@@ -1389,7 +1389,7 @@ pub fn noop_visit_expr<T: MutVisitor>(
             vis.visit_block(body);
             visit_opt(label, |label| vis.visit_label(label));
         }
-        ExprKind::ForLoop(pat, iter, body, label) => {
+        ExprKind::ForLoop { pat, iter, body, label, kind: _ } => {
             vis.visit_pat(pat);
             vis.visit_expr(iter);
             vis.visit_block(body);

compiler/rustc_ast/src/util/classify.rs

@@ -19,7 +19,7 @@ pub fn expr_requires_semi_to_be_stmt(e: &ast::Expr) -> bool {
             | ast::ExprKind::Block(..)
             | ast::ExprKind::While(..)
             | ast::ExprKind::Loop(..)
-            | ast::ExprKind::ForLoop(..)
+            | ast::ExprKind::ForLoop { .. }
             | ast::ExprKind::TryBlock(..)
             | ast::ExprKind::ConstBlock(..)
     )
@@ -48,8 +48,16 @@ pub fn expr_trailing_brace(mut expr: &ast::Expr) -> Option<&ast::Expr> {
             Closure(closure) => {
                 expr = &closure.body;
             }
-            Gen(..) | Block(..) | ForLoop(..) | If(..) | Loop(..) | Match(..) | Struct(..)
-            | TryBlock(..) | While(..) | ConstBlock(_) => break Some(expr),
+            Gen(..)
+            | Block(..)
+            | ForLoop { .. }
+            | If(..)
+            | Loop(..)
+            | Match(..)
+            | Struct(..)
+            | TryBlock(..)
+            | While(..)
+            | ConstBlock(_) => break Some(expr),
 
             // FIXME: These can end in `}`, but changing these would break stable code.
             InlineAsm(_) | OffsetOf(_, _) | MacCall(_) | IncludedBytes(_) | FormatArgs(_) => {

compiler/rustc_ast/src/visit.rs

@@ -844,11 +844,11 @@ pub fn walk_expr<'a, V: Visitor<'a>>(visitor: &mut V, expression: &'a Expr) {
             visitor.visit_expr(subexpression);
             visitor.visit_block(block);
         }
-        ExprKind::ForLoop(pattern, subexpression, block, opt_label) => {
-            walk_list!(visitor, visit_label, opt_label);
-            visitor.visit_pat(pattern);
-            visitor.visit_expr(subexpression);
-            visitor.visit_block(block);
+        ExprKind::ForLoop { pat, iter, body, label, kind: _ } => {
+            walk_list!(visitor, visit_label, label);
+            visitor.visit_pat(pat);
+            visitor.visit_expr(iter);
+            visitor.visit_block(body);
         }
         ExprKind::Loop(block, opt_label, _) => {
             walk_list!(visitor, visit_label, opt_label);

compiler/rustc_ast_lowering/src/expr.rs

@@ -56,12 +56,12 @@ impl<'hir> LoweringContext<'_, 'hir> {
                     return ex;
                 }
                 // Desugar `ExprForLoop`
-                // from: `[opt_ident]: for <pat> in <head> <body>`
+                // from: `[opt_ident]: for await? <pat> in <iter> <body>`
                 //
                 // This also needs special handling because the HirId of the returned `hir::Expr` will not
                 // correspond to the `e.id`, so `lower_expr_for` handles attribute lowering itself.
-                ExprKind::ForLoop(pat, head, body, opt_label) => {
-                    return self.lower_expr_for(e, pat, head, body, *opt_label);
+                ExprKind::ForLoop { pat, iter, body, label, kind } => {
+                    return self.lower_expr_for(e, pat, iter, body, *label, *kind);
                 }
                 _ => (),
             }
@@ -337,7 +337,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
                 ),
                 ExprKind::Try(sub_expr) => self.lower_expr_try(e.span, sub_expr),
 
-                ExprKind::Paren(_) | ExprKind::ForLoop(..) => {
+                ExprKind::Paren(_) | ExprKind::ForLoop{..} => {
                     unreachable!("already handled")
                 }
 
@@ -1673,6 +1673,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
         head: &Expr,
         body: &Block,
         opt_label: Option<Label>,
+        _loop_kind: ForLoopKind,
     ) -> hir::Expr<'hir> {
         let head = self.lower_expr_mut(head);
         let pat = self.lower_pat(pat);

compiler/rustc_ast_passes/src/feature_gate.rs

@@ -526,6 +526,7 @@ pub fn check_crate(krate: &ast::Crate, sess: &Session, features: &Features) {
         "async closures are unstable",
         "to use an async block, remove the `||`: `async {`"
     );
+    gate_all!(async_for_loop, "`for await` loops are experimental");
     gate_all!(
         closure_lifetime_binder,
         "`for<...>` binders for closures are experimental",

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

@@ -1,5 +1,6 @@
 use crate::pp::Breaks::Inconsistent;
 use crate::pprust::state::{AnnNode, PrintState, State, INDENT_UNIT};
+use ast::ForLoopKind;
 use itertools::{Itertools, Position};
 use rustc_ast::ptr::P;
 use rustc_ast::token;
@@ -418,20 +419,23 @@ impl<'a> State<'a> {
                 self.space();
                 self.print_block_with_attrs(blk, attrs);
             }
-            ast::ExprKind::ForLoop(pat, iter, blk, opt_label) => {
-                if let Some(label) = opt_label {
+            ast::ExprKind::ForLoop { pat, iter, body, label, kind } => {
+                if let Some(label) = label {
                     self.print_ident(label.ident);
                     self.word_space(":");
                 }
                 self.cbox(0);
                 self.ibox(0);
                 self.word_nbsp("for");
+                if kind == &ForLoopKind::ForAwait {
+                    self.word_nbsp("await");
+                }
                 self.print_pat(pat);
                 self.space();
                 self.word_space("in");
                 self.print_expr_as_cond(iter);
                 self.space();
-                self.print_block_with_attrs(blk, attrs);
+                self.print_block_with_attrs(body, attrs);
             }
             ast::ExprKind::Loop(blk, opt_label, _) => {
                 if let Some(label) = opt_label {

compiler/rustc_builtin_macros/src/assert/context.rs

@@ -303,7 +303,7 @@ impl<'cx, 'a> Context<'cx, 'a> {
             | ExprKind::Continue(_)
             | ExprKind::Err
             | ExprKind::Field(_, _)
-            | ExprKind::ForLoop(_, _, _, _)
+            | ExprKind::ForLoop {..}
             | ExprKind::FormatArgs(_)
             | ExprKind::IncludedBytes(..)
             | ExprKind::InlineAsm(_)

compiler/rustc_feature/src/unstable.rs

@@ -357,6 +357,8 @@ declare_features! (
     (unstable, async_closure, "1.37.0", Some(62290)),
     /// Allows `#[track_caller]` on async functions.
     (unstable, async_fn_track_caller, "1.73.0", Some(110011)),
+    /// Allows `for await` loops.
+    (unstable, async_for_loop, "CURRENT_RUSTC_VERSION", None),
     /// Allows builtin # foo() syntax
     (unstable, builtin_syntax, "1.71.0", Some(110680)),
     /// Treat `extern "C"` function as nounwind.

compiler/rustc_lint/src/unused.rs

@@ -852,8 +852,8 @@ trait UnusedDelimLint {
                 (cond, UnusedDelimsCtx::WhileCond, true, Some(left), Some(right), true)
             }
 
-            ForLoop(_, ref cond, ref block, ..) => {
-                (cond, UnusedDelimsCtx::ForIterExpr, true, None, Some(block.span.lo()), true)
+            ForLoop { ref iter, ref body, .. } => {
+                (iter, UnusedDelimsCtx::ForIterExpr, true, None, Some(body.span.lo()), true)
             }
 
             Match(ref head, _) if Self::LINT_EXPR_IN_PATTERN_MATCHING_CTX => {
@@ -1085,7 +1085,7 @@ impl EarlyLintPass for UnusedParens {
         }
 
         match e.kind {
-            ExprKind::Let(ref pat, _, _, _) | ExprKind::ForLoop(ref pat, ..) => {
+            ExprKind::Let(ref pat, _, _, _) | ExprKind::ForLoop { ref pat, .. } => {
                 self.check_unused_parens_pat(cx, pat, false, false, (true, true));
             }
             // We ignore parens in cases like `if (((let Some(0) = Some(1))))` because we already