Detect misparsed binop caused by missing semi

When encountering

```rust
foo()
*bar = baz;
```

We currently emit potentially two errors, one for the return type of
`foo` not being multiplyiable by the type of `bar`, and another for
`foo() * bar` not being assignable.

We now check for this case and suggest adding a semicolon in the right
place.

Fix #80446.

Author: estebank

compiler/rustc_hir_typeck/src/expr.rs

@@ -953,12 +953,33 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                 Applicability::MachineApplicable,
             );
         });
+        self.check_for_missing_semi(lhs, &mut err);
 
         adjust_err(&mut err);
 
         err.emit();
     }
 
+    /// Check if the expression that could not be assigned to was a typoed expression that
+    fn check_for_missing_semi(&self, expr: &'tcx hir::Expr<'tcx>, err: &mut Diagnostic) {
+        if let hir::ExprKind::Binary(binop, lhs, rhs) = expr.kind
+            && let hir::BinOpKind::Mul = binop.node
+            && self.tcx.sess.source_map().is_multiline(lhs.span.between(rhs.span))
+            && rhs.is_syntactic_place_expr()
+        {
+            //      v missing semicolon here
+            // foo()
+            // *bar = baz;
+            // (#80446).
+            err.span_suggestion_verbose(
+                lhs.span.shrink_to_hi(),
+                "you might have meant to write a semicolon here",
+                ";".to_string(),
+                Applicability::MachineApplicable,
+            );
+        }
+    }
+
     // Check if an expression `original_expr_id` comes from the condition of a while loop,
     /// as opposed from the body of a while loop, which we can naively check by iterating
     /// parents until we find a loop...

compiler/rustc_hir_typeck/src/op.rs

@@ -379,6 +379,22 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                         (err, output_def_id)
                     }
                 };
+                if self.tcx.sess.source_map().is_multiline(lhs_expr.span.between(rhs_expr.span))
+                    && let IsAssign::No = is_assign
+                    && let hir::BinOpKind::Mul = op.node
+                    && rhs_expr.is_syntactic_place_expr()
+                {
+                    //      v missing semicolon here
+                    // foo()
+                    // *bar = baz;
+                    // (#80446).
+                    err.span_suggestion_verbose(
+                        lhs_expr.span.shrink_to_hi(),
+                        "you might have meant to write a semicolon here",
+                        ";".to_string(),
+                        Applicability::MaybeIncorrect,
+                    );
+                }
 
                 let suggest_deref_binop =
                     |err: &mut DiagnosticBuilder<'_, _>, lhs_deref_ty: Ty<'tcx>| {

tests/ui/binop/false-binop-caused-by-missing-semi.rs

@@ -0,0 +1,9 @@
+fn foo() {}
+fn main() {
+    let mut y = 42;
+    let x = &mut y;
+    foo()
+    *x = 0;  //~ ERROR invalid left-hand side of assignment
+    //~^ ERROR cannot multiply `()` by `&mut {integer}`
+    println!("{y}");
+}

tests/ui/binop/false-binop-caused-by-missing-semi.stderr

@@ -0,0 +1,31 @@
+error[E0369]: cannot multiply `()` by `&mut {integer}`
+  --> $DIR/false-binop-caused-by-missing-semi.rs:6:5
+   |
+LL |     foo()
+   |     ----- ()
+LL |     *x = 0;
+   |     ^- &mut {integer}
+   |
+help: you might have meant to write a semicolon here
+   |
+LL |     foo();
+   |          +
+
+error[E0070]: invalid left-hand side of assignment
+  --> $DIR/false-binop-caused-by-missing-semi.rs:6:8
+   |
+LL | /     foo()
+LL | |     *x = 0;
+   | |      - ^
+   | |______|
+   |        cannot assign to this expression
+   |
+help: you might have meant to write a semicolon here
+   |
+LL |     foo();
+   |          +
+
+error: aborting due to 2 previous errors
+
+Some errors have detailed explanations: E0070, E0369.
+For more information about an error, try `rustc --explain E0070`.