Fix some suggestions where a Box<T> is expected.

compiler/rustc_hir_typeck/messages.ftl

@@ -75,3 +75,7 @@ hir_typeck_union_pat_dotdot = `..` cannot be used in union patterns
 
 hir_typeck_arg_mismatch_indeterminate = argument type mismatch was detected, but rustc had trouble determining where
     .note = we would appreciate a bug report: https://github.com/rust-lang/rust/issues/new
+
+hir_typeck_suggest_boxing_note = for more on the distinction between the stack and the heap, read https://doc.rust-lang.org/book/ch15-01-box.html, https://doc.rust-lang.org/rust-by-example/std/box.html, and https://doc.rust-lang.org/std/boxed/index.html
+
+hir_typeck_suggest_boxing_when_appropriate = store this in the heap by calling `Box::new`

compiler/rustc_hir_typeck/src/demand.rs

@@ -51,7 +51,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             || self.suggest_non_zero_new_unwrap(err, expr, expected, expr_ty)
             || self.suggest_calling_boxed_future_when_appropriate(err, expr, expected, expr_ty)
             || self.suggest_no_capture_closure(err, expected, expr_ty)
-            || self.suggest_boxing_when_appropriate(err, expr, expected, expr_ty)
+            || self.suggest_boxing_when_appropriate(err, expr.span, expr.hir_id, expected, expr_ty)
             || self.suggest_block_to_brackets_peeling_refs(err, expr, expr_ty, expected)
             || self.suggest_copied_or_cloned(err, expr, expr_ty, expected)
             || self.suggest_clone_for_ref(err, expr, expr_ty, expected)

compiler/rustc_hir_typeck/src/errors.rs

@@ -267,3 +267,31 @@ pub struct ArgMismatchIndeterminate {
     #[primary_span]
     pub span: Span,
 }
+
+#[derive(Subdiagnostic)]
+pub enum SuggestBoxing {
+    #[note(hir_typeck_suggest_boxing_note)]
+    #[multipart_suggestion(
+        hir_typeck_suggest_boxing_when_appropriate,
+        applicability = "machine-applicable"
+    )]
+    Unit {
+        #[suggestion_part(code = "Box::new(())")]
+        start: Span,
+        #[suggestion_part(code = "")]
+        end: Span,
+    },
+    #[note(hir_typeck_suggest_boxing_note)]
+    AsyncBody,
+    #[note(hir_typeck_suggest_boxing_note)]
+    #[multipart_suggestion(
+        hir_typeck_suggest_boxing_when_appropriate,
+        applicability = "machine-applicable"
+    )]
+    Other {
+        #[suggestion_part(code = "Box::new(")]
+        start: Span,
+        #[suggestion_part(code = ")")]
+        end: Span,
+    },
+}

compiler/rustc_hir_typeck/src/fn_ctxt/checks.rs

@@ -1514,7 +1514,6 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         // case we can ignore the tail expression (e.g., `'a: {
         // break 'a 22; }` would not force the type of the block
         // to be `()`).
-        let tail_expr = blk.expr.as_ref();
         let coerce_to_ty = expected.coercion_target_type(self, blk.span);
         let coerce = if blk.targeted_by_break {
             CoerceMany::new(coerce_to_ty)
@@ -1532,13 +1531,13 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
 
             // check the tail expression **without** holding the
             // `enclosing_breakables` lock below.
-            let tail_expr_ty = tail_expr.map(|t| self.check_expr_with_expectation(t, expected));
+            let tail_expr_ty =
+                blk.expr.map(|expr| (expr, self.check_expr_with_expectation(expr, expected)));
 
             let mut enclosing_breakables = self.enclosing_breakables.borrow_mut();
             let ctxt = enclosing_breakables.find_breakable(blk.hir_id);
             let coerce = ctxt.coerce.as_mut().unwrap();
-            if let Some(tail_expr_ty) = tail_expr_ty {
-                let tail_expr = tail_expr.unwrap();
+            if let Some((tail_expr, tail_expr_ty)) = tail_expr_ty {
                 let span = self.get_expr_coercion_span(tail_expr);
                 let cause = self.cause(span, ObligationCauseCode::BlockTailExpression(blk.hir_id));
                 let ty_for_diagnostic = coerce.merged_ty();
@@ -1591,6 +1590,15 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                         &self.misc(sp),
                         &mut |err| {
                             if let Some(expected_ty) = expected.only_has_type(self) {
+                                if blk.stmts.is_empty() && blk.expr.is_none() {
+                                    self.suggest_boxing_when_appropriate(
+                                        err,
+                                        blk.span,
+                                        blk.hir_id,
+                                        expected_ty,
+                                        self.tcx.mk_unit(),
+                                    );
+                                }
                                 if !self.consider_removing_semicolon(blk, expected_ty, err) {
                                     self.err_ctxt().consider_returning_binding(
                                         blk,
@@ -1603,7 +1611,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                                     // silence this redundant error, as we already emit E0070.
 
                                     // Our block must be a `assign desugar local; assignment`
-                                    if let Some(hir::Node::Block(hir::Block {
+                                    if let hir::Block {
                                         stmts:
                                             [
                                                 hir::Stmt {
@@ -1625,7 +1633,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                                                 },
                                             ],
                                         ..
-                                    })) = self.tcx.hir().find(blk.hir_id)
+                                    } = blk
                                     {
                                         self.comes_from_while_condition(blk.hir_id, |_| {
                                             err.downgrade_to_delayed_bug();

compiler/rustc_hir_typeck/src/fn_ctxt/suggestions.rs

@@ -1,6 +1,6 @@
 use super::FnCtxt;
 
-use crate::errors::{AddReturnTypeSuggestion, ExpectedReturnTypeLabel};
+use crate::errors::{AddReturnTypeSuggestion, ExpectedReturnTypeLabel, SuggestBoxing};
 use crate::fluent_generated as fluent;
 use crate::method::probe::{IsSuggestion, Mode, ProbeScope};
 use rustc_ast::util::parser::{ExprPrecedence, PREC_POSTFIX};
@@ -9,7 +9,8 @@ use rustc_hir as hir;
 use rustc_hir::def::{CtorKind, CtorOf, DefKind};
 use rustc_hir::lang_items::LangItem;
 use rustc_hir::{
-    Expr, ExprKind, GenericBound, Node, Path, QPath, Stmt, StmtKind, TyKind, WherePredicate,
+    AsyncGeneratorKind, Expr, ExprKind, GeneratorKind, GenericBound, HirId, Node, Path, QPath,
+    Stmt, StmtKind, TyKind, WherePredicate,
 };
 use rustc_hir_analysis::astconv::AstConv;
 use rustc_infer::traits::{self, StatementAsExpression};
@@ -438,33 +439,32 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
     pub(in super::super) fn suggest_boxing_when_appropriate(
         &self,
         err: &mut Diagnostic,
-        expr: &hir::Expr<'_>,
+        span: Span,
+        hir_id: HirId,
         expected: Ty<'tcx>,
         found: Ty<'tcx>,
     ) -> bool {
-        if self.tcx.hir().is_inside_const_context(expr.hir_id) {
-            // Do not suggest `Box::new` in const context.
-            return false;
-        }
-        if !expected.is_box() || found.is_box() {
+        // Do not suggest `Box::new` in const context.
+        if self.tcx.hir().is_inside_const_context(hir_id) || !expected.is_box() || found.is_box() {
             return false;
         }
-        let boxed_found = self.tcx.mk_box(found);
-        if self.can_coerce(boxed_found, expected) {
-            err.multipart_suggestion(
-                "store this in the heap by calling `Box::new`",
-                vec![
-                    (expr.span.shrink_to_lo(), "Box::new(".to_string()),
-                    (expr.span.shrink_to_hi(), ")".to_string()),
-                ],
-                Applicability::MachineApplicable,
-            );
-            err.note(
-                "for more on the distinction between the stack and the heap, read \
-                 https://doc.rust-lang.org/book/ch15-01-box.html, \
-                 https://doc.rust-lang.org/rust-by-example/std/box.html, and \
-                 https://doc.rust-lang.org/std/boxed/index.html",
-            );
+        if self.can_coerce(self.tcx.mk_box(found), expected) {
+            let suggest_boxing = match found.kind() {
+                ty::Tuple(tuple) if tuple.is_empty() => {
+                    SuggestBoxing::Unit { start: span.shrink_to_lo(), end: span }
+                }
+                ty::Generator(def_id, ..)
+                    if matches!(
+                        self.tcx.generator_kind(def_id),
+                        Some(GeneratorKind::Async(AsyncGeneratorKind::Closure))
+                    ) =>
+                {
+                    SuggestBoxing::AsyncBody
+                }
+                _ => SuggestBoxing::Other { start: span.shrink_to_lo(), end: span.shrink_to_hi() },
+            };
+            err.subdiagnostic(suggest_boxing);
+
             true
         } else {
             false

tests/ui/issues/auxiliary/issue-111011.rs

@@ -0,0 +1,12 @@
+#![feature(async_closure)]
+
+// edition:2021
+
+fn foo<X>(x: impl FnOnce() -> Box<X>) {}
+// just to make sure async closures can still be suggested for boxing.
+fn bar<X>(x: Box<dyn FnOnce() -> X>) {}
+
+fn main() {
+    foo(async move || {}); //~ ERROR mismatched types
+    bar(async move || {}); //~ ERROR mismatched types
+}

tests/ui/issues/auxiliary/issue-111011.stderr

@@ -0,0 +1,34 @@
+error[E0308]: mismatched types
+  --> $DIR/issue-111011.rs:10:23
+   |
+LL |     foo(async move || {});
+   |                       ^^ expected `Box<_>`, found `async` closure body
+   |
+   = note:            expected struct `Box<_>`
+           found `async` closure body `[async closure body@$DIR/issue-111011.rs:10:23: 10:25]`
+   = note: for more on the distinction between the stack and the heap, read https://doc.rust-lang.org/book/ch15-01-box.html, https://doc.rust-lang.org/rust-by-example/std/box.html, and https://doc.rust-lang.org/std/boxed/index.html
+
+error[E0308]: mismatched types
+  --> $DIR/issue-111011.rs:11:9
+   |
+LL |     bar(async move || {});
+   |     --- ^^^^^^^^^^^^^^^^ expected `Box<dyn FnOnce() -> _>`, found closure
+   |     |
+   |     arguments to this function are incorrect
+   |
+   = note: expected struct `Box<(dyn FnOnce() -> _ + 'static)>`
+             found closure `[closure@$DIR/issue-111011.rs:11:9: 11:22]`
+   = note: for more on the distinction between the stack and the heap, read https://doc.rust-lang.org/book/ch15-01-box.html, https://doc.rust-lang.org/rust-by-example/std/box.html, and https://doc.rust-lang.org/std/boxed/index.html
+note: function defined here
+  --> $DIR/issue-111011.rs:7:4
+   |
+LL | fn bar<X>(x: Box<dyn FnOnce() -> X>) {}
+   |    ^^^    -------------------------
+help: store this in the heap by calling `Box::new`
+   |
+LL |     bar(Box::new(async move || {}));
+   |         +++++++++                +
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0308`.

tests/ui/suggestions/suggest-boxed-empty-block.fixed

@@ -0,0 +1,12 @@
+#![feature(async_closure)]
+
+// edition:2021
+// run-rustfix
+
+fn foo<T>(_: Box<T>) {}
+fn bar<T>(_: impl Fn() -> Box<T>) {}
+
+fn main() {
+    foo(Box::new(())); //~ ERROR mismatched types
+    bar(|| Box::new(())); //~ ERROR mismatched types
+}

tests/ui/suggestions/suggest-boxed-empty-block.rs

@@ -0,0 +1,12 @@
+#![feature(async_closure)]
+
+// edition:2021
+// run-rustfix
+
+fn foo<T>(_: Box<T>) {}
+fn bar<T>(_: impl Fn() -> Box<T>) {}
+
+fn main() {
+    foo({}); //~ ERROR mismatched types
+    bar(|| {}); //~ ERROR mismatched types
+}

tests/ui/suggestions/suggest-boxed-empty-block.stderr

@@ -0,0 +1,33 @@
+error[E0308]: mismatched types
+  --> $DIR/suggest-boxed-empty-block.rs:10:9
+   |
+LL |     foo({});
+   |         ^^ expected `Box<_>`, found `()`
+   |
+   = note: expected struct `Box<_>`
+           found unit type `()`
+   = note: for more on the distinction between the stack and the heap, read https://doc.rust-lang.org/book/ch15-01-box.html, https://doc.rust-lang.org/rust-by-example/std/box.html, and https://doc.rust-lang.org/std/boxed/index.html
+help: store this in the heap by calling `Box::new`
+   |
+LL -     foo({});
+LL +     foo(Box::new(()));
+   |
+
+error[E0308]: mismatched types
+  --> $DIR/suggest-boxed-empty-block.rs:11:12
+   |
+LL |     bar(|| {});
+   |            ^^ expected `Box<_>`, found `()`
+   |
+   = note: expected struct `Box<_>`
+           found unit type `()`
+   = note: for more on the distinction between the stack and the heap, read https://doc.rust-lang.org/book/ch15-01-box.html, https://doc.rust-lang.org/rust-by-example/std/box.html, and https://doc.rust-lang.org/std/boxed/index.html
+help: store this in the heap by calling `Box::new`
+   |
+LL -     bar(|| {});
+LL +     bar(|| Box::new(()));
+   |
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0308`.