Add ignore value suggestion in closure body

compiler/rustc_hir_typeck/src/coercion.rs

@@ -1883,25 +1883,24 @@ impl<'tcx, 'exprs, E: AsCoercionSite> CoerceMany<'tcx, 'exprs, E> {
             fcx.err_ctxt().report_mismatched_types(cause, fcx.param_env, expected, found, ty_err);
 
         let due_to_block = matches!(fcx.tcx.hir_node(block_or_return_id), hir::Node::Block(..));
-
-        let parent_id = fcx.tcx.parent_hir_id(block_or_return_id);
-        let parent = fcx.tcx.hir_node(parent_id);
+        let parent = fcx.tcx.parent_hir_node(block_or_return_id);
         if let Some(expr) = expression
             && let hir::Node::Expr(hir::Expr {
                 kind: hir::ExprKind::Closure(&hir::Closure { body, .. }),
                 ..
             }) = parent
-            && !matches!(fcx.tcx.hir_body(body).value.kind, hir::ExprKind::Block(..))
         {
-            fcx.suggest_missing_semicolon(&mut err, expr, expected, true);
+            let needs_block =
+                !matches!(fcx.tcx.hir_body(body).value.kind, hir::ExprKind::Block(..));
+            fcx.suggest_missing_semicolon(&mut err, expr, expected, needs_block, true);
         }
         // Verify that this is a tail expression of a function, otherwise the
         // label pointing out the cause for the type coercion will be wrong
         // as prior return coercions would not be relevant (#57664).
         if let Some(expr) = expression
             && due_to_block
         {
-            fcx.suggest_missing_semicolon(&mut err, expr, expected, false);
+            fcx.suggest_missing_semicolon(&mut err, expr, expected, false, false);
             let pointing_at_return_type = fcx.suggest_mismatched_types_on_tail(
                 &mut err,
                 expr,

compiler/rustc_hir_typeck/src/expr.rs

@@ -921,7 +921,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                         self,
                         &cause,
                         |mut err| {
-                            self.suggest_missing_semicolon(&mut err, expr, e_ty, false);
+                            self.suggest_missing_semicolon(&mut err, expr, e_ty, false, false);
                             self.suggest_mismatched_types_on_tail(
                                 &mut err, expr, ty, e_ty, target_id,
                             );

compiler/rustc_hir_typeck/src/fn_ctxt/suggestions.rs

@@ -754,6 +754,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         expression: &'tcx hir::Expr<'tcx>,
         expected: Ty<'tcx>,
         needs_block: bool,
+        parent_is_closure: bool,
     ) {
         if expected.is_unit() {
             // `BlockTailExpression` only relevant if the tail expr would be
@@ -789,6 +790,17 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                         );
                     }
                 }
+                ExprKind::Path(..) | ExprKind::Lit(_)
+                    if parent_is_closure
+                        && !expression.span.in_external_macro(self.tcx.sess.source_map()) =>
+                {
+                    err.span_suggestion_verbose(
+                        expression.span.shrink_to_lo(),
+                        "consider ignoring the value",
+                        "_ = ",
+                        Applicability::MachineApplicable,
+                    );
+                }
                 _ => (),
             }
         }

tests/ui/typeck/closure-ty-mismatch-issue-128561.rs

@@ -0,0 +1,10 @@
+fn main() {
+    b"abc".iter().for_each(|x| x); //~ ERROR: mismatched types
+
+    b"abc".iter().for_each(|x| dbg!(x)); //~ ERROR: mismatched types
+
+    b"abc".iter().for_each(|x| {
+        println!("{}", x);
+        x //~ ERROR: mismatched types
+    })
+}

tests/ui/typeck/closure-ty-mismatch-issue-128561.stderr

@@ -0,0 +1,28 @@
+error[E0308]: mismatched types
+  --> $DIR/closure-ty-mismatch-issue-128561.rs:2:32
+   |
+LL |     b"abc".iter().for_each(|x| x);
+   |                                ^ expected `()`, found `&u8`
+   |
+help: consider ignoring the value
+   |
+LL |     b"abc".iter().for_each(|x| _ = x);
+   |                                +++
+
+error[E0308]: mismatched types
+  --> $DIR/closure-ty-mismatch-issue-128561.rs:4:32
+   |
+LL |     b"abc".iter().for_each(|x| dbg!(x));
+   |                                ^^^^^^^ expected `()`, found `&u8`
+   |
+   = note: this error originates in the macro `dbg` (in Nightly builds, run with -Z macro-backtrace for more info)
+
+error[E0308]: mismatched types
+  --> $DIR/closure-ty-mismatch-issue-128561.rs:8:9
+   |
+LL |         x
+   |         ^ expected `()`, found `&u8`
+
+error: aborting due to 3 previous errors
+
+For more information about this error, try `rustc --explain E0308`.