Rollup merge of #138435 - eholk:prefix-yield, r=oli-obk

Add support for postfix yield expressions

We've been having a discussion about whether we want postfix yield, or want to stick with prefix yield, or have both. I figured it's easy enough to support both for now and let us play around with them while the feature is still experimental.

This PR treats `yield x` and `x.yield` as semantically equivalent. There was a suggestion to make `yield x` have a `()` type (so it only works in coroutines with `Resume = ()`. I think that'd be worth trying, either in a later PR, or before this one merges, depending on people's opinions.

#43122

compiler/rustc_ast/src/ast.rs

@@ -1657,7 +1657,7 @@ pub enum ExprKind {
     Try(P<Expr>),
 
     /// A `yield`, with an optional value to be yielded.
-    Yield(Option<P<Expr>>),
+    Yield(YieldKind),
 
     /// A `do yeet` (aka `throw`/`fail`/`bail`/`raise`/whatever),
     /// with an optional value to be returned.
@@ -1903,6 +1903,44 @@ pub enum MatchKind {
     Postfix,
 }
 
+/// The kind of yield expression
+#[derive(Clone, Encodable, Decodable, Debug)]
+pub enum YieldKind {
+    /// yield expr { ... }
+    Prefix(Option<P<Expr>>),
+    /// expr.yield { ... }
+    Postfix(P<Expr>),
+}
+
+impl YieldKind {
+    /// Returns the expression inside the yield expression, if any.
+    ///
+    /// For postfix yields, this is guaranteed to be `Some`.
+    pub const fn expr(&self) -> Option<&P<Expr>> {
+        match self {
+            YieldKind::Prefix(expr) => expr.as_ref(),
+            YieldKind::Postfix(expr) => Some(expr),
+        }
+    }
+
+    /// Returns a mutable reference to the expression being yielded, if any.
+    pub const fn expr_mut(&mut self) -> Option<&mut P<Expr>> {
+        match self {
+            YieldKind::Prefix(expr) => expr.as_mut(),
+            YieldKind::Postfix(expr) => Some(expr),
+        }
+    }
+
+    /// Returns true if both yields are prefix or both are postfix.
+    pub const fn same_kind(&self, other: &Self) -> bool {
+        match (self, other) {
+            (YieldKind::Prefix(_), YieldKind::Prefix(_)) => true,
+            (YieldKind::Postfix(_), YieldKind::Postfix(_)) => true,
+            _ => false,
+        }
+    }
+}
+
 /// A literal in a meta item.
 #[derive(Clone, Encodable, Decodable, Debug, HashStable_Generic)]
 pub struct MetaItemLit {

compiler/rustc_ast/src/mut_visit.rs

@@ -1813,8 +1813,11 @@ pub fn walk_expr<T: MutVisitor>(vis: &mut T, Expr { kind, id, span, attrs, token
         ExprKind::Paren(expr) => {
             vis.visit_expr(expr);
         }
-        ExprKind::Yield(expr) => {
+        ExprKind::Yield(kind) => {
-            visit_opt(expr, |expr| vis.visit_expr(expr));
+            let expr = kind.expr_mut();
+            if let Some(expr) = expr {
+                vis.visit_expr(expr);
+            }
         }
         ExprKind::Try(expr) => vis.visit_expr(expr),
         ExprKind::TryBlock(body) => vis.visit_block(body),

compiler/rustc_ast/src/util/classify.rs

@@ -182,11 +182,14 @@ pub fn expr_trailing_brace(mut expr: &ast::Expr) -> Option<TrailingBrace<'_>> {
             | Range(_, Some(e), _)
             | Ret(Some(e))
             | Unary(_, e)
-            | Yield(Some(e))
             | Yeet(Some(e))
             | Become(e) => {
                 expr = e;
             }
+            Yield(kind) => match kind.expr() {
+                Some(e) => expr = e,
+                None => break None,
+            },
             Closure(closure) => {
                 expr = &closure.body;
             }
@@ -217,7 +220,6 @@ pub fn expr_trailing_brace(mut expr: &ast::Expr) -> Option<TrailingBrace<'_>> {
             Break(_, None)
             | Range(_, None, _)
             | Ret(None)
-            | Yield(None)
             | Array(_)
             | Call(_, _)
             | MethodCall(_)
@@ -237,7 +239,9 @@ pub fn expr_trailing_brace(mut expr: &ast::Expr) -> Option<TrailingBrace<'_>> {
             | Yeet(None)
             | UnsafeBinderCast(..)
             | Err(_)
-            | Dummy => break None,
+            | Dummy => {
+                break None;
+            }
         }
     }
 }

compiler/rustc_ast/src/visit.rs

@@ -1269,8 +1269,8 @@ pub fn walk_expr<'a, V: Visitor<'a>>(visitor: &mut V, expression: &'a Expr) -> V
             try_visit!(visitor.visit_ty(container));
             walk_list!(visitor, visit_ident, fields.iter());
         }
-        ExprKind::Yield(optional_expression) => {
+        ExprKind::Yield(kind) => {
-            visit_opt!(visitor, visit_expr, optional_expression);
+            visit_opt!(visitor, visit_expr, kind.expr());
         }
         ExprKind::Try(subexpression) => try_visit!(visitor.visit_expr(subexpression)),
         ExprKind::TryBlock(body) => try_visit!(visitor.visit_block(body)),

compiler/rustc_ast_lowering/src/expr.rs

@@ -351,7 +351,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
                         rest,
                     )
                 }
-                ExprKind::Yield(opt_expr) => self.lower_expr_yield(e.span, opt_expr.as_deref()),
+                ExprKind::Yield(kind) => self.lower_expr_yield(e.span, kind.expr().map(|x| &**x)),
                 ExprKind::Err(guar) => hir::ExprKind::Err(*guar),
 
                 ExprKind::UnsafeBinderCast(kind, expr, ty) => hir::ExprKind::UnsafeBinderCast(

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

@@ -8,7 +8,7 @@ use rustc_ast::util::literal::escape_byte_str_symbol;
 use rustc_ast::util::parser::{self, ExprPrecedence, Fixity};
 use rustc_ast::{
     self as ast, BlockCheckMode, FormatAlignment, FormatArgPosition, FormatArgsPiece, FormatCount,
-    FormatDebugHex, FormatSign, FormatTrait, token,
+    FormatDebugHex, FormatSign, FormatTrait, YieldKind, token,
 };
 
 use crate::pp::Breaks::Inconsistent;
@@ -761,7 +761,7 @@ impl<'a> State<'a> {
                 self.print_expr(e, FixupContext::default());
                 self.pclose();
             }
-            ast::ExprKind::Yield(e) => {
+            ast::ExprKind::Yield(YieldKind::Prefix(e)) => {
                 self.word("yield");
 
                 if let Some(expr) = e {
@@ -773,6 +773,14 @@ impl<'a> State<'a> {
                     );
                 }
             }
+            ast::ExprKind::Yield(YieldKind::Postfix(e)) => {
+                self.print_expr_cond_paren(
+                    e,
+                    e.precedence() < ExprPrecedence::Unambiguous,
+                    fixup.leftmost_subexpression_with_dot(),
+                );
+                self.word(".yield");
+            }
             ast::ExprKind::Try(e) => {
                 self.print_expr_cond_paren(
                     e,

compiler/rustc_parse/src/parser/expr.rs

@@ -17,6 +17,7 @@ use rustc_ast::{
     self as ast, AnonConst, Arm, AttrStyle, AttrVec, BinOp, BinOpKind, BlockCheckMode, CaptureBy,
     ClosureBinder, DUMMY_NODE_ID, Expr, ExprField, ExprKind, FnDecl, FnRetTy, Label, MacCall,
     MetaItemLit, Movability, Param, RangeLimits, StmtKind, Ty, TyKind, UnOp, UnsafeBinderCastKind,
+    YieldKind,
 };
 use rustc_ast_pretty::pprust;
 use rustc_data_structures::stack::ensure_sufficient_stack;
@@ -1310,6 +1311,15 @@ impl<'a> Parser<'a> {
             return self.parse_match_block(lo, match_span, self_arg, MatchKind::Postfix);
         }
 
+        // Parse a postfix `yield`.
+        if self.eat_keyword(exp!(Yield)) {
+            let yield_span = self.prev_token.span;
+            self.psess.gated_spans.gate(sym::yield_expr, yield_span);
+            return Ok(
+                self.mk_expr(lo.to(yield_span), ExprKind::Yield(YieldKind::Postfix(self_arg)))
+            );
+        }
+
         let fn_span_lo = self.token.span;
         let mut seg = self.parse_path_segment(PathStyle::Expr, None)?;
         self.check_trailing_angle_brackets(&seg, &[exp!(OpenParen)]);
@@ -1884,7 +1894,7 @@ impl<'a> Parser<'a> {
     /// Parse `"yield" expr?`.
     fn parse_expr_yield(&mut self) -> PResult<'a, P<Expr>> {
         let lo = self.prev_token.span;
-        let kind = ExprKind::Yield(self.parse_expr_opt()?);
+        let kind = ExprKind::Yield(YieldKind::Prefix(self.parse_expr_opt()?));
         let span = lo.to(self.prev_token.span);
         self.psess.gated_spans.gate(sym::yield_expr, span);
         let expr = self.mk_expr(span, kind);

src/tools/clippy/clippy_utils/src/ast_utils/mod.rs

@@ -201,7 +201,8 @@ pub fn eq_expr(l: &Expr, r: &Expr) -> bool {
         (Loop(lt, ll, _), Loop(rt, rl, _)) => eq_label(ll.as_ref(), rl.as_ref()) && eq_block(lt, rt),
         (Block(lb, ll), Block(rb, rl)) => eq_label(ll.as_ref(), rl.as_ref()) && eq_block(lb, rb),
         (TryBlock(l), TryBlock(r)) => eq_block(l, r),
-        (Yield(l), Yield(r)) | (Ret(l), Ret(r)) => eq_expr_opt(l.as_ref(), r.as_ref()),
+        (Yield(l), Yield(r)) => eq_expr_opt(l.expr(), r.expr()) && l.same_kind(r),
+        (Ret(l), Ret(r)) => eq_expr_opt(l.as_ref(), r.as_ref()),
         (Break(ll, le), Break(rl, re)) => eq_label(ll.as_ref(), rl.as_ref()) && eq_expr_opt(le.as_ref(), re.as_ref()),
         (Continue(ll), Continue(rl)) => eq_label(ll.as_ref(), rl.as_ref()),
         (Assign(l1, l2, _), Assign(r1, r2, _)) | (Index(l1, l2, _), Index(r1, r2, _)) => {

src/tools/rustfmt/src/chains.rs

@@ -192,6 +192,7 @@ enum ChainItemKind {
     StructField(symbol::Ident),
     TupleField(symbol::Ident, bool),
     Await,
+    Yield,
     Comment(String, CommentPosition),
 }
 
@@ -203,6 +204,7 @@ impl ChainItemKind {
             | ChainItemKind::StructField(..)
             | ChainItemKind::TupleField(..)
             | ChainItemKind::Await
+            | ChainItemKind::Yield
             | ChainItemKind::Comment(..) => false,
         }
     }
@@ -257,6 +259,10 @@ impl ChainItemKind {
                 let span = mk_sp(nested.span.hi(), expr.span.hi());
                 (ChainItemKind::Await, span)
             }
+            ast::ExprKind::Yield(ast::YieldKind::Postfix(ref nested)) => {
+                let span = mk_sp(nested.span.hi(), expr.span.hi());
+                (ChainItemKind::Yield, span)
+            }
             _ => {
                 return (
                     ChainItemKind::Parent {
@@ -306,6 +312,7 @@ impl Rewrite for ChainItem {
                 rewrite_ident(context, ident)
             ),
             ChainItemKind::Await => ".await".to_owned(),
+            ChainItemKind::Yield => ".yield".to_owned(),
             ChainItemKind::Comment(ref comment, _) => {
                 rewrite_comment(comment, false, shape, context.config)?
             }
@@ -508,7 +515,8 @@ impl Chain {
             }),
             ast::ExprKind::Field(ref subexpr, _)
             | ast::ExprKind::Try(ref subexpr)
-            | ast::ExprKind::Await(ref subexpr, _) => Some(SubExpr {
+            | ast::ExprKind::Await(ref subexpr, _)
+            | ast::ExprKind::Yield(ast::YieldKind::Postfix(ref subexpr)) => Some(SubExpr {
                 expr: Self::convert_try(subexpr, context),
                 is_method_call_receiver: false,
             }),

src/tools/rustfmt/src/expr.rs

@@ -221,7 +221,7 @@ pub(crate) fn format_expr(
                 Ok(format!("break{id_str}"))
             }
         }
-        ast::ExprKind::Yield(ref opt_expr) => {
+        ast::ExprKind::Yield(ast::YieldKind::Prefix(ref opt_expr)) => {
             if let Some(ref expr) = *opt_expr {
                 rewrite_unary_prefix(context, "yield ", &**expr, shape)
             } else {
@@ -243,7 +243,8 @@ pub(crate) fn format_expr(
         ast::ExprKind::Try(..)
         | ast::ExprKind::Field(..)
         | ast::ExprKind::MethodCall(..)
-        | ast::ExprKind::Await(_, _) => rewrite_chain(expr, context, shape),
+        | ast::ExprKind::Await(_, _)
+        | ast::ExprKind::Yield(ast::YieldKind::Postfix(_)) => rewrite_chain(expr, context, shape),
         ast::ExprKind::MacCall(ref mac) => {
             rewrite_macro(mac, None, context, shape, MacroPosition::Expression).or_else(|_| {
                 wrap_str(

src/tools/rustfmt/src/utils.rs

@@ -4,7 +4,7 @@ use rustc_ast::ast::{
     self, Attribute, MetaItem, MetaItemInner, MetaItemKind, NodeId, Path, Visibility,
     VisibilityKind,
 };
-use rustc_ast::ptr;
+use rustc_ast::{YieldKind, ptr};
 use rustc_ast_pretty::pprust;
 use rustc_span::{BytePos, LocalExpnId, Span, Symbol, SyntaxContext, sym, symbol};
 use unicode_width::UnicodeWidthStr;
@@ -485,7 +485,9 @@ pub(crate) fn is_block_expr(context: &RewriteContext<'_>, expr: &ast::Expr, repr
         | ast::ExprKind::Index(_, ref expr, _)
         | ast::ExprKind::Unary(_, ref expr)
         | ast::ExprKind::Try(ref expr)
-        | ast::ExprKind::Yield(Some(ref expr)) => is_block_expr(context, expr, repr),
+        | ast::ExprKind::Yield(YieldKind::Prefix(Some(ref expr))) => {
+            is_block_expr(context, expr, repr)
+        }
         ast::ExprKind::Closure(ref closure) => is_block_expr(context, &closure.body, repr),
         // This can only be a string lit
         ast::ExprKind::Lit(_) => {
@@ -515,7 +517,7 @@ pub(crate) fn is_block_expr(context: &RewriteContext<'_>, expr: &ast::Expr, repr
         | ast::ExprKind::Tup(..)
         | ast::ExprKind::Use(..)
         | ast::ExprKind::Type(..)
-        | ast::ExprKind::Yield(None)
+        | ast::ExprKind::Yield(..)
         | ast::ExprKind::Underscore => false,
     }
 }

src/tools/rustfmt/tests/target/postfix-yield.rs

@@ -0,0 +1,17 @@
+// This demonstrates a proposed alternate or additional option of having yield in postfix position.
+//@ edition: 2024
+
+#![feature(gen_blocks, coroutines, coroutine_trait, yield_expr)]
+
+use std::ops::{Coroutine, CoroutineState};
+use std::pin::pin;
+
+fn main() {
+    let mut coro = pin!(
+        #[coroutine]
+        |_: i32| {
+            let x = 1.yield;
+            (x + 2).await;
+        }
+    );
+}

tests/pretty/postfix-yield.rs

@@ -0,0 +1,15 @@
+// This demonstrates a proposed alternate or additional option of having yield in postfix position.
+//@ edition: 2024
+//@ pp-exact
+
+#![feature(gen_blocks, coroutines, coroutine_trait, yield_expr)]
+
+use std::ops::{Coroutine, CoroutineState};
+use std::pin::pin;
+
+fn main() {
+    let mut gn = gen { yield 1; 2.yield; (1 + 2).yield; };
+
+    let mut coro =
+        pin!(#[coroutine] |_: i32| { let x = 1.yield; (x + 2).yield; });
+}

tests/ui/coroutine/postfix-yield.rs

@@ -0,0 +1,34 @@
+// This demonstrates a proposed alternate or additional option of having yield in postfix position.
+
+//@ run-pass
+//@ edition: 2024
+
+#![feature(gen_blocks, coroutines, coroutine_trait, yield_expr)]
+
+use std::ops::{Coroutine, CoroutineState};
+use std::pin::pin;
+
+fn main() {
+    // generators (i.e. yield doesn't return anything useful)
+    let mut gn = gen {
+        yield 1;
+        2.yield;
+    };
+
+    assert_eq!(gn.next(), Some(1));
+    assert_eq!(gn.next(), Some(2));
+    assert_eq!(gn.next(), None);
+
+    //coroutines (i.e. yield returns something useful)
+    let mut coro = pin!(
+        #[coroutine]
+        |_: i32| {
+            let x = 1.yield;
+            (x + 2).yield;
+        }
+    );
+
+    assert_eq!(coro.as_mut().resume(0), CoroutineState::Yielded(1));
+    assert_eq!(coro.as_mut().resume(2), CoroutineState::Yielded(4));
+    assert_eq!(coro.as_mut().resume(3), CoroutineState::Complete(()));
+}