RFC: Re-implement crate using a toml-driven code generator

[alteous]

Based on my conclusions in #198, this tracks the re-implementation of the gltf_derive, gltf_json, and gltf crates using a code generation program (written in Rust, of course!)

[alteous]

This would close #187.

[alteous]

Example toml input

[meta]
id = "accessor::Accessor"
kind = "Struct"
docs = "A typed view into a buffer view."

[fields.buffer_view]
ty = "Index"
of = "buffer::View"
json = "bufferView"
docs = "The parent buffer view this accessor reads from."

[fields.byte_offset]
ty = "Integer"
json = "byteOffset"
docs = "The number of components within the buffer view."
default = "0"

[fields.data_type]
ty = "Enum"
of = "accessor::DataType"
json = "componentType"
docs = "The data type of components in the attribute."

[fields.dimensions]
ty = "Enum"
of = "accessor::Dimensions"
json = "type"
docs = "Specifies if the attribute is a scalar, vector, or matrix."

[fields.min]
ty = "Option"
of = "serde_json::value::RawValue"
docs = "Minimum value of each component in this attribute."

[fields.max]
ty = "Option"
of = "serde_json::value::RawValue"
docs = "Maximum value of each component in this attribute."

[fields.normalized]
ty = "Bool"
docs = "Specifies whether integer data values should be normalized."
default = "false"

Example output

$ cargo run schema/accessor/Accessor.toml | rustfmt
pub mod json {
    pub mod accessor {
        /// A typed view into a buffer view.
        #[derive(Clone, Debug, Deserialize, Eq, Hash, PartialEq, Serialize)]
        pub struct Accessor {
            /// The parent buffer view this accessor reads from.
            pub buffer_view: Index<buffer::View>,
            /// The number of components within the buffer view.
            #[serde(default = "byte_offset_default")]
            #[serde(skip_serializing_if = "byte_offset_is_default")]
            pub byte_offset: u32,
            /// The data type of components in the attribute.
            pub data_type: Checked<accessor::DataType>,
            /// Specifies if the attribute is a scalar, vector, or matrix.
            pub dimensions: Checked<accessor::Dimensions>,
            /// Maximum value of each component in this attribute.
            #[serde(default, skip_serializing_if = "Option::is_none")]
            pub max: Option<serde_json::value::RawValue>,
            /// Minimum value of each component in this attribute.
            #[serde(default, skip_serializing_if = "Option::is_none")]
            pub min: Option<serde_json::value::RawValue>,
            /// Specifies whether integer data values should be normalized.
            #[serde(default = "normalized_default")]
            #[serde(skip_serializing_if = "normalized_is_default")]
            pub normalized: bool,
        }
        fn byte_offset_default() -> u32 {
            0
        }
        fn byte_offset_is_default(x: u32) -> u32 {
            x == 0
        }
        fn normalized_default() -> bool {
            false
        }
        fn normalized_is_default(x: bool) -> bool {
            x == false
        }

    }
}
pub mod accessor {
    /// A typed view into a buffer view.
    #[derive(Clone, Debug)]
    pub struct Accessor<'a> {
        pub(crate) document: &'a Document,
        pub(crate) json: &'a json::Accessor,
    }

    impl<'a> Accessor<'a> {
        /// The parent buffer view this accessor reads from.
        pub fn buffer_view(&self) -> buffer::View<'a> {
            self.document.get(&self.buffer_view)
        }
        /// The number of components within the buffer view.
        pub fn byte_offset(&self) -> u32 {
            self.byte_offset
        }
        /// The data type of components in the attribute.
        pub fn data_type(&self) -> accessor::DataType {
            self.data_type.unwrap()
        }
        /// Specifies if the attribute is a scalar, vector, or matrix.
        pub fn dimensions(&self) -> accessor::Dimensions {
            self.dimensions.unwrap()
        }
        /// Maximum value of each component in this attribute.
        pub fn max(&self) -> Option<&'a serde_json::value::RawValue> {
            self.max.as_ref()
        }
        /// Minimum value of each component in this attribute.
        pub fn min(&self) -> Option<&'a serde_json::value::RawValue> {
            self.min.as_ref()
        }
        /// Specifies whether integer data values should be normalized.
        pub fn normalized(&self) -> bool {
            self.normalized
        }
    }
}