When expecting closure argument but finding block provide suggestion

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

@@ -3442,11 +3442,11 @@ impl<'tcx> TypeErrCtxtExt<'tcx> for TypeErrCtxt<'_, 'tcx> {
         if let Some(Node::Expr(expr)) = hir.find(arg_hir_id)
             && let Some(typeck_results) = &self.typeck_results
         {
-            if let hir::Expr { kind: hir::ExprKind::Block(..), .. } = expr {
-                let expr = expr.peel_blocks();
-                let ty =
-                    typeck_results.expr_ty_adjusted_opt(expr).unwrap_or(Ty::new_misc_error(tcx));
-                let span = expr.span;
+            if let hir::Expr { kind: hir::ExprKind::Block(block, _), .. } = expr {
+                let inner_expr = expr.peel_blocks();
+                let ty = typeck_results.expr_ty_adjusted_opt(inner_expr)
+                    .unwrap_or(Ty::new_misc_error(tcx));
+                let span = inner_expr.span;
                 if Some(span) != err.span.primary_span() {
                     err.span_label(
                         span,
@@ -3457,6 +3457,49 @@ impl<'tcx> TypeErrCtxtExt<'tcx> for TypeErrCtxt<'_, 'tcx> {
                             format!("this tail expression is of type `{ty}`")
                         },
                     );
+                    if let ty::PredicateKind::Clause(clause) = failed_pred.kind().skip_binder()
+                        && let ty::ClauseKind::Trait(pred) = clause
+                        && [
+                            tcx.lang_items().fn_once_trait(),
+                            tcx.lang_items().fn_mut_trait(),
+                            tcx.lang_items().fn_trait(),
+                        ].contains(&Some(pred.def_id()))
+                    {
+                        if let [stmt, ..] = block.stmts
+                            && let hir::StmtKind::Semi(value) = stmt.kind
+                            && let hir::ExprKind::Closure(hir::Closure {
+                                body,
+                                fn_decl_span,
+                                ..
+                            }) = value.kind
+                            && let body = hir.body(*body)
+                            && !matches!(body.value.kind, hir::ExprKind::Block(..))
+                        {
+                            // Check if the failed predicate was an expectation of a closure type
+                            // and if there might have been a `{ |args|` typo instead of `|args| {`.
+                            err.multipart_suggestion(
+                                "you might have meant to open the closure body instead of placing \
+                                 a closure within a block",
+                                vec![
+                                    (expr.span.with_hi(value.span.lo()), String::new()),
+                                    (fn_decl_span.shrink_to_hi(), " {".to_string()),
+                                ],
+                                Applicability::MaybeIncorrect,
+                            );
+                        } else {
+                            // Maybe the bare block was meant to be a closure.
+                            err.span_suggestion_verbose(
+                                expr.span.shrink_to_lo(),
+                                "you might have meant to create the closure instead of a block",
+                                format!(
+                                    "|{}| ",
+                                    (0..pred.trait_ref.args.len() - 1).map(|_| "_")
+                                        .collect::<Vec<_>>()
+                                        .join(", ")),
+                                Applicability::MaybeIncorrect,
+                            );
+                        }
+                    }
                 }
             }
 

tests/ui/expr/malformed_closure/block_instead_of_closure_in_arg.rs

@@ -0,0 +1,10 @@
+fn main() {
+    let number = 2;
+    Some(true).filter({ //~ ERROR expected a `FnOnce<(&bool,)>` closure, found `bool`
+        if number % 2 == 0 {
+            number == 0
+        } else {
+            number != 0
+        }
+    });
+}

tests/ui/expr/malformed_closure/block_instead_of_closure_in_arg.stderr

@@ -0,0 +1,27 @@
+error[E0277]: expected a `FnOnce<(&bool,)>` closure, found `bool`
+  --> $DIR/block_instead_of_closure_in_arg.rs:3:23
+   |
+LL |        Some(true).filter({
+   |  _________________------_^
+   | |                 |
+   | |                 required by a bound introduced by this call
+LL | |/         if number % 2 == 0 {
+LL | ||             number == 0
+LL | ||         } else {
+LL | ||             number != 0
+LL | ||         }
+   | ||_________- this tail expression is of type `bool`
+LL | |      });
+   | |______^ expected an `FnOnce<(&bool,)>` closure, found `bool`
+   |
+   = help: the trait `for<'a> FnOnce<(&'a bool,)>` is not implemented for `bool`
+note: required by a bound in `Option::<T>::filter`
+  --> $SRC_DIR/core/src/option.rs:LL:COL
+help: you might have meant to create the closure instead of a block
+   |
+LL |     Some(true).filter(|_| {
+   |                       +++
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0277`.

tests/ui/expr/malformed_closure/ruby_style_closure_successful_parse.rs

@@ -0,0 +1,7 @@
+const x: usize =42;
+fn main() {
+    let p = Some(45).and_then({|x| //~ ERROR expected a `FnOnce<({integer},)>` closure, found `Option<usize>`
+        1 + 1;
+        Some(x * 2)
+    });
+}

tests/ui/expr/malformed_closure/ruby_style_closure_successful_parse.stderr

@@ -0,0 +1,25 @@
+error[E0277]: expected a `FnOnce<({integer},)>` closure, found `Option<usize>`
+  --> $DIR/ruby_style_closure_successful_parse.rs:3:31
+   |
+LL |       let p = Some(45).and_then({|x|
+   |  ______________________--------_^
+   | |                      |
+   | |                      required by a bound introduced by this call
+LL | |         1 + 1;
+LL | |         Some(x * 2)
+   | |         ----------- this tail expression is of type `Option<usize>`
+LL | |     });
+   | |_____^ expected an `FnOnce<({integer},)>` closure, found `Option<usize>`
+   |
+   = help: the trait `FnOnce<({integer},)>` is not implemented for `Option<usize>`
+note: required by a bound in `Option::<T>::and_then`
+  --> $SRC_DIR/core/src/option.rs:LL:COL
+help: you might have meant to open the closure body instead of placing a closure within a block
+   |
+LL -     let p = Some(45).and_then({|x|
+LL +     let p = Some(45).and_then(|x| {
+   |
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0277`.