e/proc-macros/easy-from/core/src/lib.rs

use proc_macro2::TokenStream;
use quote::{quote, TokenStreamExt};
use syn::{spanned::Spanned, Fields, Ident, ItemEnum, ItemStruct, Meta};

fn handle_enum(e: ItemEnum) -> TokenStream {
	let name = &e.ident;

	let mut stream: TokenStream = e
		.variants
		.iter()
		.filter_map(|variant| {
			let variant_name = &variant.ident;

			let has_from_attr = variant.attrs.iter().any(|a| {
				let path = match &a.meta {
					Meta::Path(p) => p,
					_ => return false,
				};

				path.get_ident().is_some_and(|i| *i == "from")
			});

			if !has_from_attr {
				return None;
			}

			match &variant.fields {
				Fields::Named(_n) => todo!(),
				Fields::Unnamed(u) => {
					match u.unnamed.iter().collect::<Vec<_>>().as_slice() {
						&[] => todo!(),
						&[field] => {
							let ty = &field.ty;
							Some(quote! {
								impl ::core::convert::From<#ty> for #name {
									fn from(value: #ty) -> Self {
										Self::#variant_name(value)
									}
								}
							})
						},
						_sl => todo!(),
					}
				},
				Fields::Unit => todo!(),
			}
		})
		.collect();

	let has_convert_infallible = e.attrs.iter().any(|a| match &a.meta {
		Meta::List(list) => {
			if list.path.get_ident().is_some_and(|i| *i == "from") {
				let ident: Ident = match syn::parse2(list.tokens.clone()) {
					Ok(i) => i,
					_ => return false,
				};

				ident == "infallible"
			} else {
				false
			}
		},
		_ => false,
	});

	if has_convert_infallible {
		stream.append_all(quote! {
			impl ::core::convert::From<::core::convert::Infallible> for #name {
				fn from(value: ::core::convert::Infallible) -> Self {
					unreachable!()
				}
			}
		});
	}

	stream
}

fn handle_struct(s: ItemStruct) -> TokenStream {
	match &s.fields {
		Fields::Unnamed(u) => {
			if u.unnamed.len() != 1 {
				proc_macro_error::abort!(
					s.span(),
					"Tuple struct must have single field"
				);
			}

			let field = u.unnamed.first().expect("Should be len 1");
			let ty = &field.ty;
			let name = &s.ident;
			let generics = &s.generics;

			quote! {
				impl #generics ::core::convert::From<#ty> for #name #generics {
					fn from(value: #ty) -> Self {
						Self(value)
					}
				}
			}
		},
		_ => todo!(),
	}
}

pub fn easy_from_core(input: TokenStream) -> TokenStream {
	match syn::parse2(input.clone()) {
		Ok(e) => handle_enum(e),
		Err(_) => match syn::parse2(input) {
			Ok(s) => handle_struct(s),
			Err(e) => e.to_compile_error(),
		},
	}
}

#[cfg(test)]
mod test {
	use quote::quote;

	use crate::easy_from_core;

	#[test]
	fn test_enum_all_fields_covered() {
		let input = quote! {
			#[derive(EasyFrom)]
			enum Foo {
				#[from]
				Bar(u64),

				#[from]
				Baz(String),
			}
		};
		let output = easy_from_core(input).to_string();
		let expected = quote! {
			impl ::core::convert::From<u64> for Foo {
				fn from(value: u64) -> Self {
					Self::Bar(value)
				}
			}

			impl ::core::convert::From<String> for Foo {
				fn from(value: String) -> Self {
					Self::Baz(value)
				}
			}
		}
		.to_string();

		assert_eq!(output, expected)
	}

	#[test]
	fn test_enum_some_fields_covered() {
		let input = quote! {
			#[derive(EasyFrom)]
			enum Foo {
				#[from]
				Bar(u64),
				Baz(String),
			}
		};
		let output = easy_from_core(input).to_string();
		let expected = quote! {
			impl ::core::convert::From<u64> for Foo {
				fn from(value: u64) -> Self {
					Self::Bar(value)
				}
			}
		}
		.to_string();

		assert_eq!(output, expected)
	}

	#[test]
	fn test_enum_from_infallible() {
		let input = quote! {
			#[derive(EasyFrom)]
			#[from(infallible)]
			enum Never {}
		};
		let output = easy_from_core(input).to_string();
		let expected = quote! {
			impl ::core::convert::From<::core::convert::Infallible> for Never {
				fn from(value: ::core::convert::Infallible) -> Self {
					unreachable!()
				}
			}
		}
		.to_string();

		assert_eq!(output, expected)
	}

	#[test]
	fn test_newtype_struct() {
		let input = quote! {
			#[derive(EasyFrom)]
			struct NewType<T>(#[from] T);
		};
		let output = easy_from_core(input).to_string();
		let expected = quote! {
			impl<T> ::core::convert::From<T> for NewType<T> {
				fn from(value: T) -> Self {
					Self(value)
				}
			}
		}
		.to_string();

		assert_eq!(output, expected)
	}
}