Add iter macro

compiler/rustc_ast_lowering/src/expr.rs

@@ -1,4 +1,3 @@
-use std::assert_matches::assert_matches;
 use std::ops::ControlFlow;
 use std::sync::Arc;
 
@@ -1190,11 +1189,13 @@ impl<'hir> LoweringContext<'_, 'hir> {
         let closure_def_id = self.local_def_id(closure_id);
         let (binder_clause, generic_params) = self.lower_closure_binder(binder);
 
-        assert_matches!(
-            coroutine_kind,
-            CoroutineKind::Async { .. },
-            "only async closures are supported currently"
-        );
+        let coroutine_desugaring = match coroutine_kind {
+            CoroutineKind::Async { .. } => hir::CoroutineDesugaring::Async,
+            CoroutineKind::Gen { .. } => hir::CoroutineDesugaring::Gen,
+            CoroutineKind::AsyncGen { span, .. } => {
+                span_bug!(span, "only async closures and `iter!` closures are supported currently")
+            }
+        };
 
         let body = self.with_new_scopes(fn_decl_span, |this| {
             let inner_decl =
@@ -1238,7 +1239,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
             // Lower this as a `CoroutineClosure`. That will ensure that HIR typeck
             // knows that a `FnDecl` output type like `-> &str` actually means
             // "coroutine that returns &str", rather than directly returning a `&str`.
-            kind: hir::ClosureKind::CoroutineClosure(hir::CoroutineDesugaring::Async),
+            kind: hir::ClosureKind::CoroutineClosure(coroutine_desugaring),
             constness: hir::Constness::NotConst,
         });
         hir::ExprKind::Closure(c)

compiler/rustc_ast_passes/src/feature_gate.rs

@@ -468,11 +468,12 @@ pub fn check_crate(krate: &ast::Crate, sess: &Session, features: &Features) {
         for span in spans {
             if (!visitor.features.coroutines() && !span.allows_unstable(sym::coroutines))
                 && (!visitor.features.gen_blocks() && !span.allows_unstable(sym::gen_blocks))
+                && (!visitor.features.yield_expr() && !span.allows_unstable(sym::yield_expr))
             {
                 #[allow(rustc::untranslatable_diagnostic)]
-                // Don't know which of the two features to include in the
-                // error message, so I am arbitrarily picking one.
-                feature_err(&visitor.sess, sym::coroutines, *span, "yield syntax is experimental")
+                // Emit yield_expr as the error, since that will be sufficient. You can think of it
+                // as coroutines and gen_blocks imply yield_expr.
+                feature_err(&visitor.sess, sym::yield_expr, *span, "yield syntax is experimental")
                     .emit();
             }
         }

compiler/rustc_borrowck/src/type_check/input_output.rs

@@ -52,7 +52,7 @@ impl<'a, 'tcx> TypeChecker<'a, 'tcx> {
             assert_matches!(
                 self.tcx().coroutine_kind(self.tcx().coroutine_for_closure(mir_def_id)),
                 Some(hir::CoroutineKind::Desugared(
-                    hir::CoroutineDesugaring::Async,
+                    hir::CoroutineDesugaring::Async | hir::CoroutineDesugaring::Gen,
                     hir::CoroutineSource::Closure
                 )),
                 "this needs to be modified if we're lowering non-async closures"

compiler/rustc_builtin_macros/src/iter.rs

@@ -0,0 +1,78 @@
+use rustc_ast::ptr::P;
+use rustc_ast::tokenstream::TokenStream;
+use rustc_ast::{
+    CaptureBy, ClosureBinder, Const, CoroutineKind, DUMMY_NODE_ID, Expr, ExprKind, ast, token,
+};
+use rustc_errors::PResult;
+use rustc_expand::base::{self, DummyResult, ExpandResult, ExtCtxt, MacroExpanderResult};
+use rustc_span::Span;
+
+pub(crate) fn expand<'cx>(
+    cx: &'cx mut ExtCtxt<'_>,
+    sp: Span,
+    tts: TokenStream,
+) -> MacroExpanderResult<'cx> {
+    let closure = match parse_closure(cx, sp, tts) {
+        Ok(parsed) => parsed,
+        Err(err) => {
+            return ExpandResult::Ready(DummyResult::any(sp, err.emit()));
+        }
+    };
+
+    ExpandResult::Ready(base::MacEager::expr(closure))
+}
+
+fn parse_closure<'a>(
+    cx: &mut ExtCtxt<'a>,
+    span: Span,
+    stream: TokenStream,
+) -> PResult<'a, P<Expr>> {
+    let mut parser = cx.new_parser_from_tts(stream);
+    let mut closure_parser = parser.clone();
+
+    let coroutine_kind = Some(CoroutineKind::Gen {
+        span,
+        closure_id: DUMMY_NODE_ID,
+        return_impl_trait_id: DUMMY_NODE_ID,
+    });
+
+    match closure_parser.parse_expr() {
+        Ok(mut closure) => {
+            if let ast::ExprKind::Closure(c) = &mut closure.kind {
+                if let Some(kind) = c.coroutine_kind {
+                    cx.dcx().span_err(kind.span(), "only plain closures allowed in `iter!`");
+                }
+                c.coroutine_kind = coroutine_kind;
+                if closure_parser.token != token::Eof {
+                    closure_parser.unexpected()?;
+                }
+                return Ok(closure);
+            }
+        }
+        Err(diag) => diag.cancel(),
+    }
+
+    let lo = parser.token.span.shrink_to_lo();
+    let block = parser.parse_block_tail(
+        lo,
+        ast::BlockCheckMode::Default,
+        rustc_parse::parser::AttemptLocalParseRecovery::No,
+    )?;
+    let fn_decl = cx.fn_decl(Default::default(), ast::FnRetTy::Default(span));
+    let closure = ast::Closure {
+        binder: ClosureBinder::NotPresent,
+        capture_clause: CaptureBy::Ref,
+        constness: Const::No,
+        coroutine_kind,
+        movability: ast::Movability::Movable,
+        fn_decl,
+        body: cx.expr_block(block),
+        fn_decl_span: span,
+        fn_arg_span: span,
+    };
+    if parser.token != token::Eof {
+        parser.unexpected()?;
+    }
+    let span = lo.to(parser.token.span);
+    Ok(cx.expr(span, ExprKind::Closure(Box::new(closure))))
+}

compiler/rustc_builtin_macros/src/lib.rs

@@ -47,6 +47,7 @@ mod errors;
 mod format;
 mod format_foreign;
 mod global_allocator;
+mod iter;
 mod log_syntax;
 mod pattern_type;
 mod source_util;
@@ -95,6 +96,7 @@ pub fn register_builtin_macros(resolver: &mut dyn ResolverExpand) {
         include: source_util::expand_include,
         include_bytes: source_util::expand_include_bytes,
         include_str: source_util::expand_include_str,
+        iter: iter::expand,
         line: source_util::expand_line,
         log_syntax: log_syntax::expand_log_syntax,
         module_path: source_util::expand_mod,

compiler/rustc_hir/src/hir.rs

@@ -1899,9 +1899,7 @@ impl CoroutineKind {
             CoroutineKind::Coroutine(mov) => mov,
         }
     }
-}
 
-impl CoroutineKind {
     pub fn is_fn_like(self) -> bool {
         matches!(self, CoroutineKind::Desugared(_, CoroutineSource::Fn))
     }

compiler/rustc_hir_typeck/src/closure.rs

@@ -199,14 +199,19 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                 )
             }
             hir::ClosureKind::CoroutineClosure(kind) => {
-                // async closures always return the type ascribed after the `->` (if present),
-                // and yield `()`.
                 let (bound_return_ty, bound_yield_ty) = match kind {
+                    hir::CoroutineDesugaring::Gen => {
+                        // `iter!` closures always return unit and yield the `Iterator::Item` type
+                        // that we have to infer.
+                        (tcx.types.unit, self.infcx.next_ty_var(expr_span))
+                    }
                     hir::CoroutineDesugaring::Async => {
+                        // async closures always return the type ascribed after the `->` (if present),
+                        // and yield `()`.
                         (bound_sig.skip_binder().output(), tcx.types.unit)
                     }
-                    hir::CoroutineDesugaring::Gen | hir::CoroutineDesugaring::AsyncGen => {
-                        todo!("`gen` and `async gen` closures not supported yet")
+                    hir::CoroutineDesugaring::AsyncGen => {
+                        todo!("`async gen` closures not supported yet")
                     }
                 };
                 // Compute all of the variables that will be used to populate the coroutine.

compiler/rustc_parse/src/parser/stmt.rs

@@ -685,7 +685,7 @@ impl<'a> Parser<'a> {
 
     /// Parses the rest of a block expression or function body.
     /// Precondition: already parsed the '{'.
-    pub(crate) fn parse_block_tail(
+    pub fn parse_block_tail(
         &mut self,
         lo: Span,
         s: BlockCheckMode,

compiler/rustc_trait_selection/messages.ftl

@@ -72,8 +72,6 @@ trait_selection_adjust_signature_remove_borrow = consider adjusting the signatur
 
 trait_selection_ascribe_user_type_prove_predicate = ...so that the where clause holds
 
-trait_selection_async_closure_not_fn = async closure does not implement `{$kind}` because it captures state from its environment
-
 trait_selection_await_both_futures = consider `await`ing on both `Future`s
 trait_selection_await_future = consider `await`ing on the `Future`
 trait_selection_await_note = calling an async function returns a future
@@ -123,6 +121,8 @@ trait_selection_closure_kind_requirement = the requirement to implement `{$trait
 
 trait_selection_compare_impl_item_obligation = ...so that the definition in impl matches the definition from the trait
 trait_selection_consider_specifying_length = consider specifying the actual array length
+trait_selection_coro_closure_not_fn = {$coro_kind}closure does not implement `{$kind}` because it captures state from its environment
+
 trait_selection_data_flows = ...but data{$label_var1_exists ->
     [true] {" "}from `{$label_var1}`
     *[false] {""}

compiler/rustc_trait_selection/src/error_reporting/traits/fulfillment_errors.rs

@@ -37,9 +37,7 @@ use super::{
 use crate::error_reporting::TypeErrCtxt;
 use crate::error_reporting::infer::TyCategory;
 use crate::error_reporting::traits::report_dyn_incompatibility;
-use crate::errors::{
-    AsyncClosureNotFn, ClosureFnMutLabel, ClosureFnOnceLabel, ClosureKindMismatch,
-};
+use crate::errors::{ClosureFnMutLabel, ClosureFnOnceLabel, ClosureKindMismatch, CoroClosureNotFn};
 use crate::infer::{self, InferCtxt, InferCtxtExt as _};
 use crate::traits::query::evaluate_obligation::InferCtxtExt as _;
 use crate::traits::{
@@ -904,9 +902,18 @@ impl<'a, 'tcx> TypeErrCtxt<'a, 'tcx> {
             && let ty::FnPtr(sig_tys, _) = by_ref_captures.kind()
             && !sig_tys.skip_binder().output().is_unit()
         {
-            let mut err = self.dcx().create_err(AsyncClosureNotFn {
+            let coro_kind = match self
+                .tcx
+                .coroutine_kind(self.tcx.coroutine_for_closure(closure_def_id))
+                .unwrap()
+            {
+                rustc_hir::CoroutineKind::Desugared(desugaring, _) => format!("{desugaring:#}"),
+                coro => format!("{coro:#}"),
+            };
+            let mut err = self.dcx().create_err(CoroClosureNotFn {
                 span: self.tcx.def_span(closure_def_id),
                 kind: expected_kind.as_str(),
+                coro_kind,
             });
             self.note_obligation_cause(&mut err, &obligation);
             return Some(err.emit());

compiler/rustc_trait_selection/src/errors.rs

@@ -170,11 +170,12 @@ pub struct ClosureFnMutLabel {
 }
 
 #[derive(Diagnostic)]
-#[diag(trait_selection_async_closure_not_fn)]
-pub(crate) struct AsyncClosureNotFn {
+#[diag(trait_selection_coro_closure_not_fn)]
+pub(crate) struct CoroClosureNotFn {
     #[primary_span]
     pub span: Span,
     pub kind: &'static str,
+    pub coro_kind: String,
 }
 
 #[derive(Diagnostic)]

library/core/src/iter/mod.rs

@@ -420,6 +420,8 @@ pub use self::adapters::{Intersperse, IntersperseWith};
     issue = "42168"
 )]
 pub use self::range::Step;
+#[unstable(feature = "iter_macro", issue = "none", reason = "generators are unstable")]
+pub use self::sources::iter;
 #[stable(feature = "iter_empty", since = "1.2.0")]
 pub use self::sources::{Empty, empty};
 #[unstable(

library/core/src/iter/sources.rs

@@ -1,6 +1,7 @@
 mod empty;
 mod from_coroutine;
 mod from_fn;
+mod generator;
 mod once;
 mod once_with;
 mod repeat;
@@ -18,6 +19,8 @@ pub use self::empty::{Empty, empty};
 pub use self::from_coroutine::{FromCoroutine, from_coroutine};
 #[stable(feature = "iter_from_fn", since = "1.34.0")]
 pub use self::from_fn::{FromFn, from_fn};
+#[unstable(feature = "iter_macro", issue = "none", reason = "generators are unstable")]
+pub use self::generator::iter;
 #[stable(feature = "iter_once", since = "1.2.0")]
 pub use self::once::{Once, once};
 #[stable(feature = "iter_once_with", since = "1.43.0")]

library/core/src/iter/sources/generator.rs

@@ -0,0 +1,26 @@
+/// Creates a new closure that returns an iterator where each iteration steps the given
+/// generator to the next `yield` statement.
+///
+/// Similar to [`iter::from_fn`], but allows arbitrary control flow.
+///
+/// [`iter::from_fn`]: crate::iter::from_fn
+///
+/// # Examples
+///
+/// ```
+/// #![feature(iter_macro, coroutines)]
+///
+/// let it = std::iter::iter!{
+///     yield 1;
+///     yield 2;
+///     yield 3;
+/// };
+/// let v: Vec<_> = it.collect();
+/// assert_eq!(v, [1, 2, 3]);
+/// ```
+#[unstable(feature = "iter_macro", issue = "none", reason = "generators are unstable")]
+#[allow_internal_unstable(coroutines, iter_from_coroutine)]
+#[cfg_attr(not(bootstrap), rustc_builtin_macro)]
+pub macro iter($($t:tt)*) {
+    /* compiler-builtin */
+}

tests/ui/iterators/generator.rs

@@ -0,0 +1,24 @@
+//@ run-pass
+
+#![feature(iter_macro, yield_expr)]
+
+use std::iter::iter;
+
+fn main() {
+    let i = iter! {
+        yield 0;
+        for x in 5..10 {
+            yield x * 2;
+        }
+    };
+    let mut i = i();
+    assert_eq!(i.next(), Some(0));
+    assert_eq!(i.next(), Some(10));
+    assert_eq!(i.next(), Some(12));
+    assert_eq!(i.next(), Some(14));
+    assert_eq!(i.next(), Some(16));
+    assert_eq!(i.next(), Some(18));
+    assert_eq!(i.next(), None);
+    assert_eq!(i.next(), None);
+    assert_eq!(i.next(), None);
+}

tests/ui/iterators/generator_args.rs

@@ -0,0 +1,24 @@
+//@ run-pass
+
+#![feature(iter_macro, yield_expr)]
+
+use std::iter::iter;
+
+fn main() {
+    let i = iter! {|foo| {
+        yield foo;
+        for x in 5..10 {
+            yield x * 2;
+        }
+    }};
+    let mut i = i(3);
+    assert_eq!(i.next(), Some(3));
+    assert_eq!(i.next(), Some(10));
+    assert_eq!(i.next(), Some(12));
+    assert_eq!(i.next(), Some(14));
+    assert_eq!(i.next(), Some(16));
+    assert_eq!(i.next(), Some(18));
+    assert_eq!(i.next(), None);
+    assert_eq!(i.next(), None);
+    assert_eq!(i.next(), None);
+}

tests/ui/iterators/generator_capture.rs

@@ -0,0 +1,27 @@
+//@ run-pass
+
+#![feature(iter_macro, yield_expr)]
+
+use std::iter::iter;
+
+fn main() {
+    let i = {
+        let s = String::new();
+        iter! { move || {
+            yield s.len();
+            for x in 5..10 {
+                yield x * 2;
+            }
+        }}
+    };
+    let mut i = i();
+    assert_eq!(i.next(), Some(0));
+    assert_eq!(i.next(), Some(10));
+    assert_eq!(i.next(), Some(12));
+    assert_eq!(i.next(), Some(14));
+    assert_eq!(i.next(), Some(16));
+    assert_eq!(i.next(), Some(18));
+    assert_eq!(i.next(), None);
+    assert_eq!(i.next(), None);
+    assert_eq!(i.next(), None);
+}