Suggest adding a comma when () followed by a closure is parsed as bitor

compiler/rustc_hir_typeck/src/fn_ctxt/checks.rs

@@ -925,6 +925,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         let mut err = fn_call_diag_ctxt.initial_final_diagnostic();
         fn_call_diag_ctxt.suggest_confusable(&mut err);
         fn_call_diag_ctxt.detect_missing_turbofish(&mut err);
+        fn_call_diag_ctxt.suggest_missing_comma_before_closure(&mut err);
 
         // As we encounter issues, keep track of what we want to provide for the suggestion.
 
@@ -3315,6 +3316,38 @@ impl<'a, 'tcx> ArgMatchingCtxt<'a, 'tcx> {
         }
     }
 
+    fn suggest_missing_comma_before_closure(&self, err: &mut Diag<'_>) {
+        if self.formal_and_expected_inputs.len() <= self.provided_args.len() {
+            return;
+        }
+
+        for &arg in &self.provided_args {
+            let mut curr = arg;
+            let mut bitor_count = 0;
+            while let hir::ExprKind::Binary(op, lhs, _) = curr.kind
+                && op.node == hir::BinOpKind::BitOr
+            {
+                bitor_count += 1;
+                curr = lhs;
+            }
+
+            // `t(() |c| c)` is greedily parsed as `((() | c) | c)`. Unit cannot BitOr,
+            // and closure params need two `|` delimiters.
+            if bitor_count >= 2
+                && matches!(curr.kind, hir::ExprKind::Tup([]))
+                && curr.span.can_be_used_for_suggestions()
+            {
+                err.span_suggestion_verbose(
+                    curr.span.shrink_to_hi(),
+                    "add a comma",
+                    ",",
+                    Applicability::MaybeIncorrect,
+                );
+                break;
+            }
+        }
+    }
+
     /// A "softer" version of the `demand_compatible`, which checks types without persisting them,
     /// and treats error types differently
     /// This will allow us to "probe" for other argument orders that would likely have been correct

tests/ui/argument-suggestions/missing-comma-before-closure.rs

@@ -0,0 +1,20 @@
+fn t1(_: (), _: impl Fn(bool) -> bool) {}
+
+fn main() {
+    t1(() |c| c);
+    //~^ ERROR cannot find value `c` in this scope
+    //~| ERROR cannot find value `c` in this scope
+    //~| ERROR this function takes 2 arguments but 1 argument was supplied
+    t1((()) |c| c);
+    //~^ ERROR cannot find value `c` in this scope
+    //~| ERROR cannot find value `c` in this scope
+    //~| ERROR this function takes 2 arguments but 1 argument was supplied
+    let a = ();
+    t1(() |a| c);
+    //~^ ERROR no implementation for `() | ()`
+    //~| ERROR cannot find value `c` in this scope
+    //~| ERROR this function takes 2 arguments but 1 argument was supplied
+    t1(() | a);
+    //~^ ERROR no implementation for `() | ()`
+    //~| ERROR this function takes 2 arguments but 1 argument was supplied
+}

tests/ui/argument-suggestions/missing-comma-before-closure.stderr

@@ -0,0 +1,132 @@
+error[E0425]: cannot find value `c` in this scope
+  --> $DIR/missing-comma-before-closure.rs:4:12
+   |
+LL |     t1(() |c| c);
+   |            ^ not found in this scope
+
+error[E0425]: cannot find value `c` in this scope
+  --> $DIR/missing-comma-before-closure.rs:4:15
+   |
+LL |     t1(() |c| c);
+   |               ^ not found in this scope
+
+error[E0425]: cannot find value `c` in this scope
+  --> $DIR/missing-comma-before-closure.rs:8:14
+   |
+LL |     t1((()) |c| c);
+   |              ^ not found in this scope
+
+error[E0425]: cannot find value `c` in this scope
+  --> $DIR/missing-comma-before-closure.rs:8:17
+   |
+LL |     t1((()) |c| c);
+   |                 ^ not found in this scope
+
+error[E0425]: cannot find value `c` in this scope
+  --> $DIR/missing-comma-before-closure.rs:13:15
+   |
+LL |     t1(() |a| c);
+   |               ^ not found in this scope
+   |
+help: a local variable with a similar name exists
+   |
+LL -     t1(() |a| c);
+LL +     t1(() |a| a);
+   |
+
+error[E0061]: this function takes 2 arguments but 1 argument was supplied
+  --> $DIR/missing-comma-before-closure.rs:4:5
+   |
+LL |     t1(() |c| c);
+   |     ^^---------- argument #2 is missing
+   |
+note: function defined here
+  --> $DIR/missing-comma-before-closure.rs:1:4
+   |
+LL | fn t1(_: (), _: impl Fn(bool) -> bool) {}
+   |    ^^        ------------------------
+help: add a comma
+   |
+LL |     t1((), |c| c);
+   |          +
+help: provide the argument
+   |
+LL |     t1(() |c| c, /* _ */);
+   |                +++++++++
+
+error[E0061]: this function takes 2 arguments but 1 argument was supplied
+  --> $DIR/missing-comma-before-closure.rs:8:5
+   |
+LL |     t1((()) |c| c);
+   |     ^^------------ argument #2 is missing
+   |
+note: function defined here
+  --> $DIR/missing-comma-before-closure.rs:1:4
+   |
+LL | fn t1(_: (), _: impl Fn(bool) -> bool) {}
+   |    ^^        ------------------------
+help: add a comma
+   |
+LL |     t1((()), |c| c);
+   |            +
+help: provide the argument
+   |
+LL |     t1((()) |c| c, /* _ */);
+   |                  +++++++++
+
+error[E0369]: no implementation for `() | ()`
+  --> $DIR/missing-comma-before-closure.rs:13:11
+   |
+LL |     t1(() |a| c);
+   |        -- ^- ()
+   |        |
+   |        ()
+
+error[E0061]: this function takes 2 arguments but 1 argument was supplied
+  --> $DIR/missing-comma-before-closure.rs:13:5
+   |
+LL |     t1(() |a| c);
+   |     ^^---------- argument #2 is missing
+   |
+note: function defined here
+  --> $DIR/missing-comma-before-closure.rs:1:4
+   |
+LL | fn t1(_: (), _: impl Fn(bool) -> bool) {}
+   |    ^^        ------------------------
+help: add a comma
+   |
+LL |     t1((), |a| c);
+   |          +
+help: provide the argument
+   |
+LL |     t1(() |a| c, /* _ */);
+   |                +++++++++
+
+error[E0369]: no implementation for `() | ()`
+  --> $DIR/missing-comma-before-closure.rs:17:11
+   |
+LL |     t1(() | a);
+   |        -- ^ - ()
+   |        |
+   |        ()
+
+error[E0061]: this function takes 2 arguments but 1 argument was supplied
+  --> $DIR/missing-comma-before-closure.rs:17:5
+   |
+LL |     t1(() | a);
+   |     ^^-------- argument #2 is missing
+   |
+note: function defined here
+  --> $DIR/missing-comma-before-closure.rs:1:4
+   |
+LL | fn t1(_: (), _: impl Fn(bool) -> bool) {}
+   |    ^^        ------------------------
+help: provide the argument
+   |
+LL |     t1(() | a, /* _ */);
+   |              +++++++++
+
+error: aborting due to 11 previous errors
+
+Some errors have detailed explanations: E0061, E0369, E0425.
+For more information about an error, try `rustc --explain E0061`.