Why does adding a type name here speed up typechecking so much?

[taylorswift]

i’ve got a truly overgrown piece of MongoDB SQL that looks like:

$0[.pipeline] = .init
{
    $0.stage
    {
        $0[.match] = .init
        {
            $0[.expr] = .expr
            {
                $0[.and] = .init
                {
                    $0.expr
                    {
                        $0[.eq] = ("$\(Record[.scope])", id)
                    }
                    $0.expr
                    {
                        $0[.or] = .init
                        {
                            $0.expr
                            {
                                $0[.and] = .init
                                {
                                    $0.expr
                                    {
                                        ...

this thing takes about 25s to typecheck, a number i got by timing the build with and without the query body.

when i make a tiny addition:

$0[.pipeline] = .init
{
    $0.stage
    {
        $0[.match] = Mongo.PredicateDocument.init
+++                  ^~~~~~~~~~~~~~~~~~~~~~~
        {

it takes about 3s to typecheck.

i have seen this problem before with generic and heavily overloaded subscripts, but in this case, the subscript called in $0[.match] is a plain old non-generic subscript with no overloads.

extension Mongo.PipelineStage
{
    @inlinable public
    subscript(key:Match) -> Mongo.PredicateDocument?
    {
        get
        {
            nil
        }
        set(value)
        {
            self.bson.push(key, value)
        }
    }
}

why doesn’t it just “know” that the .init(with:) should be a Mongo.PredicateDocument.init(with:)?

[tera]

If you rename "init(with:)" to a less common "init(withXYZ:)" is it still slow?

[taylorswift]

the init(with:) actually comes from a protocol MongoDocumentDSL:

extension Mongo
{
    /// Not to be confused with ``FilterDocument``.
    @frozen public
    struct PredicateDocument:MongoDocumentDSL, Sendable
    {
        public
        var bson:BSON.Document

        @inlinable public
        init(_ bson:BSON.Document)
        {
            self.bson = bson
        }
    }
}

/// A `MongoDocumentDSL` is nothing more than a type that supports an
/// ``init(with:)`` builder API.
///
/// The specific encoding API vended and encodability protocol used is up to
/// the conforming type.
public
protocol MongoDocumentDSL:BSONRepresentable<BSON.Document>, 
    BSONDecodable, 
    BSONEncodable
{
}
extension MongoDocumentDSL
{
    @inlinable public
    init()
    {
        self.init(.init())
    }
    @inlinable public
    init(with populate:(inout Self) throws -> ()) rethrows
    {
        self.init()
        try populate(&self)
    }
}

there are a handful of other types that have init(with:)s, but the only ones that exist on Optional are:

//  These APIs must additionally be extensions on ``Optional`` and not just
//  ``BSONEncodable`` because SE-299 does not support protocol extension
//  member lookup with unnamed closure parameters. Only the APIs that take
//  closure arguments need to be duplicated here.
extension BSON.Document?
{
    @inlinable public
    init<CodingKey>(_:CodingKey.Type = CodingKey.self,
        with populate:(inout BSON.DocumentEncoder<CodingKey>) throws -> ()) rethrows
    {
        self = .some(try .init(with: populate))
    }
    @inlinable public
    init(with populate:(inout BSON.DocumentEncoder<BSON.Key>) throws -> ()) rethrows
    {
        self = .some(try .init(with: populate))
    }
}
extension BSON.List?
{
    @inlinable public
    init(with populate:(inout BSON.ListEncoder) throws -> ()) rethrows
    {
        self = .some(try .init(with: populate))
    }
}

crucially, none of them have Wrapped == Mongo.PredicateDocument, which is the only type that could possible match that subscript.