We updated from version 1.17.0 to 1.20.3 and saw some large code size additons.
We tracked them down to #1042 and #1183. After some thinking, I came up with an idea to reduce the generated code:

Currently, depending on the type of a field, code like the following is generated:

try { if let v = _storage._singularInt32 {
   try visitor.visitSingularInt32Field(value: v, fieldNumber: 1)
} }()

Through the introduction of an helper function on Vistor that performs the optional unwrapping, we could generate the following code instead:

try {
   try visitor.visitSingularInt32Field(optionalValue: _storage._singularInt32, fieldNumber: 1)
}()

The extension for Vistor looks like this:

extension Visitor {

    mutating func visitSingularInt32Field(optionalValue value: Int32?, fieldNumber: Int) throws {
        if let value {
            try visitSingularInt32Field(value: value, fieldNumber: fieldNumber)
        }
    }
} 

This would move the optional checks out of the traverse method and its closures, but it requires one additional call to the helper function.
I did some naive tests of this apporach on compiler explorer and it seems to reduce the generated machine code for my simplified example.

I am happy to contribute an implementation, but wanted to propose and discuss the change first.
I will also verify that there is a code size reduction for optimized builds as well.

(The approach should work on the 1.x branch as well as main.)