simplify the code generated for unit, externally tagged and internally tagged enums

[icewind1991]

Partial solution for #246

• replace the manual construction of Schema::Object with a helper method
• replace metadata setting with a function call per-metadata

Taking the enum from https://github.com/adamchalmers/rust-problems/blob/achalmers/schemars-llvm-lines/src/lib.rs and running cargo rustc -- -Z unpretty=mir | wc -l it goes from 165725 lines of MIR to 4871 (removing the serde derives from the linked code)

The per-variant generate code goes from:

{
    let schema = schemars::schema::Schema::Object(schemars::schema::SchemaObject {
        instance_type: Some(
            schemars::schema::InstanceType::String.into(),
        ),
        enum_values: Some(
            <[_]>::into_vec(
                #[rustc_box]
                ::alloc::boxed::Box::new(["Af".into()]),
            ),
        ),
        ..Default::default()
    });
    schemars::_private::apply_metadata(
        schema,
        schemars::schema::Metadata {
            description: Some("Afghanistan".to_owned()),
            ..Default::default()
        },
    )
},

to:

{
    let schema = schemars::schema::Schema::Object(
        schemars::schema::SchemaObject::new_unit_enum("Af"),
    );
    schema.with_description("Afghanistan")
},

I still have a crate that takes forever to generate the schema for so there are still plenty of room for improvement.

[icewind1991]

• Added optimizations for externally and internally tagged enums

From:

schemars::schema::Schema::Object(schemars::schema::SchemaObject {
    instance_type: Some(
        schemars::schema::InstanceType::Object.into(),
    ),
    object: Some(
        Box::new(schemars::schema::ObjectValidation {
            properties: {
                let mut props = schemars::Map::new();
                props
                    .insert(
                        "type".to_owned(),
                        schemars::schema::Schema::Object(schemars::schema::SchemaObject {
                            instance_type: Some(
                                schemars::schema::InstanceType::String.into(),
                            ),
                            enum_values: Some(
                                <[_]>::into_vec(
                                    #[rustc_box]
                                    ::alloc::boxed::Box::new(["ServerSpawn".into()]),
                                ),
                            ),
                            ..Default::default()
                        }),
                    );
                props
            },
            required: {
                let mut required = schemars::Set::new();
                required.insert("type".to_owned());
                required
            },
            ..Default::default()
        }),
    ),
    ..Default::default()
})
.flatten(
    <Box<
        ServerSpawnEvent,
    > as schemars::JsonSchema>::json_schema(gen),
),

for a single field in an internally tagged enum

to:

schemars::schema::Schema::new_internally_tagged_enum(
    "type",
    "ServerSpawn",
    false,
)
.flatten(
    <Box<
        ServerSpawnEvent,
    > as schemars::JsonSchema>::json_schema(gen),
),

• Added optimization for property validations for structs

from:

{
    object_validation
        .properties
        .insert("hostname".to_owned(), gen.subschema_for::<String>());
    if !<String as schemars::JsonSchema>::_schemars_private_is_option() {
        object_validation.required.insert("hostname".to_owned());
    }
}

per (non-required, non-default) field

to:

 {
    object_validation
        .insert_property::<
            String,
        >("address", false, false, gen.subschema_for::<String>());
}

Given https://gist.github.com/icewind1991/4908eaf88173559c3623b74bfef45640 this goes from 829858 lines of MIT to 70405.

For my full crate the compile time with schema goes from "giving up and killing the build after 3 hours" to 5 seconds.

[GREsau]

Thanks for the PR! I'd rather not expand the public API with the new functions, so I moved them into the _private module and merged it in #286