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//! # rust-primitive_enum
//! Little utility for dealing with C-style enums
//!
//! This crate exports just the single macro `primitive_enum!`
//! that defines an enum backed by a user specified primitive
//! integer type.
//!
//! The intent is to emulate traditional C-style enums while
//! adding some handy associated functions useful in such
//! contexts (e.g. enumerating over each enum and converting
//! between the underlying types).
//!
//! # Example
//!
//! ```rust
//! #[macro_use] extern crate primitive_enum;
//!
//! primitive_enum! { MyEnum u16 ;
//! A,
//! B,
//! C,
//! D = 500,
//! E, // as you would expect, E maps to 501
//! }
//!
//! fn main() {
//! use MyEnum::*;
//!
//! // Get a slice of all enum elements:
//! assert_eq!(
//! MyEnum::list(),
//! &[A, B, C, D, E],
//! );
//!
//! // Get the enum value given its integer value:
//! assert_eq!(MyEnum::from(0), Some(A));
//! assert_eq!(MyEnum::from(1000), None);
//!
//! // User specified enum values behave as you would expect
//! assert_eq!(D as u16, 500);
//! assert_eq!(MyEnum::from(501), Some(E));
//!
//! // You can also get an enum by its name
//! assert_eq!(MyEnum::from_name("E"), Some(E));
//! }
//! ```
//!
//! # Expansion
//!
//! As of the current version, the macro
//!
//! ```rust
//! #[macro_use] extern crate primitive_enum;
//!
//! primitive_enum! { MyEnum u16 ;
//! A,
//! B,
//! C,
//! D = 500,
//! E,
//! }
//!
//! ```
//!
//! is effectively equivalent to
//!
//! ```rust
//! #[repr(u16)]
//! #[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
//! pub enum MyEnum {
//! A = 0,
//! B = 1,
//! C = 2,
//! D = 500,
//! E = 501,
//! }
//!
//! impl MyEnum {
//! pub fn from(x: u16) -> Option<MyEnum> {
//! // ...
//! None
//! }
//!
//! pub fn from_name(name: &str) -> Option<MyEnum> {
//! // ...
//! None
//! }
//!
//! pub fn list() -> &'static [MyEnum] {
//! &[
//! MyEnum::A,
//! MyEnum::B,
//! MyEnum::C,
//! MyEnum::D,
//! MyEnum::E,
//! ]
//! }
//! }
//! ```
//!
//! # Doc comments
//!
//! Starting from version 1.1.0, doc comments are supported.
//!
//! ```rust
//! #[macro_use] extern crate primitive_enum;
//!
//! primitive_enum! {
//! /// Some comments about 'MyEnum'
//! MyEnum u16 ;
//! A,
//! B,
//!
//! /// Some special comments about variant C
//! C,
//! D = 500,
//! E,
//! }
//! ```
//!
//! Starting from version 1.1.0 this crate is implemented as a procedural macro
//! to improve space efficiency of the generated code.
//! Prior to version 1.1.0, this crate was implemented as a simple declarative macro.
//!
//! # Default trait
//!
//! Originally, enums did not automatically get the `Default` trait. But starting from version `1.2.0`
//! the enum will automatically derive `Default` if you specify `#[default]`.
//!
//! So for example, given
//!
//! ```rust
//! #[macro_use] extern crate primitive_enum;
//!
//! primitive_enum! {
//! EnumWithDefault u16 ;
//! A,
//! B,
//! #[default]
//! C,
//! D,
//! }
//!
//! fn main() {
//! assert_eq!(EnumWithDefault::default(), EnumWithDefault::C);
//! }
//! ```
//!
//! the resulting code is effectively eqivalent to
//!
//! ```rust
//! #[repr(u16)]
//! #[derive(Debug, Clone, Copy, PartialEq, Eq, Hash, Default)]
//! pub enum EnumWithDefault {
//! A = 0,
//! B = 1,
//! #[default]
//! C = 2,
//! D = 3,
//! }
//!
//! impl EnumWithDefault {
//! // ... (same as with the other example above)
//! }
//!
//! fn main() {
//! assert_eq!(EnumWithDefault::default(), EnumWithDefault::C);
//! }
//! ```
//!
//! This crate is a clean macro implementation that
//! expands to code shown above and doesn't rely on any
//! outside dependencies or magic.
extern crate proc_macro;
use proc_macro::{Delimiter, Group, Ident, Literal, Punct, Span, TokenStream, TokenTree};
use std::iter::FromIterator;
macro_rules! error {
($message:expr $(,)?) => {
return format!("compile_error!({:?})", $message).parse().unwrap()
};
}
fn at_punc(peek: &Option<TokenTree>, punc_char: char) -> bool {
match peek {
Some(TokenTree::Punct(p)) => p == &punc_char,
_ => false,
}
}
fn ident_token(name: &str) -> TokenTree {
TokenTree::Ident(Ident::new(name, Span::call_site()))
}
fn punct_token(ch: char) -> TokenTree {
TokenTree::Punct(Punct::new(ch, proc_macro::Spacing::Alone))
}
fn punct_cont_token(ch: char) -> TokenTree {
TokenTree::Punct(Punct::new(ch, proc_macro::Spacing::Joint))
}
fn punc2_tokens(ch1: char, ch2: char) -> Vec<TokenTree> {
vec![
TokenTree::Punct(Punct::new(ch1, proc_macro::Spacing::Joint)),
TokenTree::Punct(Punct::new(ch2, proc_macro::Spacing::Alone)),
]
}
fn int_token(value: i32) -> TokenTree {
TokenTree::Literal(Literal::i32_unsuffixed(value))
}
fn group_token(delimiter: Delimiter, tokens: Vec<TokenTree>) -> TokenTree {
TokenTree::Group(Group::new(delimiter, TokenStream::from_iter(tokens)))
}
fn paren_token(tokens: Vec<TokenTree>) -> TokenTree {
group_token(Delimiter::Parenthesis, tokens)
}
fn bracket_token(tokens: Vec<TokenTree>) -> TokenTree {
group_token(Delimiter::Bracket, tokens)
}
fn brace_token(tokens: Vec<TokenTree>) -> TokenTree {
group_token(Delimiter::Brace, tokens)
}
fn concat<T>(mut v1: Vec<T>, mut v2: Vec<T>) -> Vec<T> {
v1.append(&mut v2);
v1
}
fn check_for_default(triples: &mut Vec<(TokenStream, Ident, TokenTree)>) -> Result<bool, String> {
let mut found_default = false;
for (attributes, _variant_name, _variant_value) in triples.into_iter() {
if attributes.to_string().contains("default") {
if found_default {
// TODO: Currently, rustc panics when user specifies more than one default.
// Ideally, we should just pass what we get from the user and let the compiler handle the
// error. But it looks like there might already be a pr out to address this issue.
// Remove this error handling logic when the fix pr is merged and released.
// See https://github.com/rust-lang/rust/issues/118119
// and https://github.com/rust-lang/rust/pull/118131
return Err(format!("Multiple variants marked as default"));
}
found_default = true;
}
}
Ok(found_default)
}
#[proc_macro]
pub fn primitive_enum(tokens: TokenStream) -> TokenStream {
let mut iter = tokens.into_iter();
let mut peek = iter.next();
////////////////////////////////////////////////////////////////////
// Part 1: Parse Contents
////////////////////////////////////////////////////////////////////
let enum_attributes = {
let mut tokens = Vec::<TokenTree>::new();
while at_punc(&peek, '#') {
tokens.push(peek.unwrap());
peek = iter.next();
if peek.is_none() {
error!("Dangling '#'");
}
tokens.push(peek.unwrap());
peek = iter.next();
}
tokens
};
let enum_identifier = match peek {
Some(TokenTree::Ident(ident)) => {
peek = iter.next();
ident
}
Some(token) => error!(format!("Expected enum name but got {:?}", token)),
None => error!("Expected enum name but got end of macro"),
};
let repr_type = {
let mut tokens = Vec::<TokenTree>::new();
while peek.is_some() && !at_punc(&peek, ';') {
tokens.push(peek.unwrap());
peek = iter.next();
}
tokens
};
match peek {
Some(TokenTree::Punct(p)) if p == ';' => {
peek = iter.next();
}
Some(token) => error!(format!("Expected ';' but got {:?}", token)),
None => error!("Expected ';' but got end of macro"),
}
let (triples, has_default) = {
// Each triple contains information about a variant of the enum.
// (Attributes, Identifier, Value-Expression)
let mut triples = Vec::<(TokenStream, Ident, TokenTree)>::new();
let mut base_value: Option<Vec<TokenTree>> = None;
let mut offset = 0;
while peek.is_some() {
let variant_attributes = {
let mut tokens = Vec::<TokenTree>::new();
while at_punc(&peek, '#') {
tokens.push(peek.unwrap());
peek = iter.next();
if peek.is_none() {
error!("Dangling '#'");
}
tokens.push(peek.unwrap());
peek = iter.next();
}
TokenStream::from_iter(tokens)
};
let variant_name = match peek {
Some(TokenTree::Ident(ident)) => {
peek = iter.next();
ident
}
Some(token) => error!(format!("Expected variant identifier but got {:?}", token)),
None => error!("Expected variant identifier but got end of macro"),
};
if at_punc(&peek, '=') {
// Explicit assignment
peek = iter.next(); // consume '='
let mut expr_tokens = Vec::<TokenTree>::new();
while peek.is_some() && !at_punc(&peek, ',') {
expr_tokens.push(peek.unwrap());
peek = iter.next();
}
base_value = Some(expr_tokens);
offset = 0;
}
let value = match &base_value {
Some(base_value_tokens) => {
let base_value_rep = if base_value_tokens.len() == 1 {
base_value_tokens[0].clone()
} else {
TokenTree::Group(Group::new(
proc_macro::Delimiter::Parenthesis,
TokenStream::from_iter(base_value_tokens.clone()),
))
};
if offset == 0 {
base_value_rep
} else {
paren_token(vec![base_value_rep, punct_token('+'), int_token(offset)])
}
}
None => int_token(offset),
};
if at_punc(&peek, ',') {
peek = iter.next();
} else if let Some(token) = peek {
error!(format!("Expected ',' but got {:?}", token));
}
offset += 1;
triples.push((variant_attributes, variant_name, value));
}
// make sure there's a default, even if the user didn't specify one
let has_default = match check_for_default(&mut triples) {
Err(message) => error!(message),
Ok(has_default) => has_default,
};
(triples, has_default)
};
////////////////////////////////////////////////////////////////////
// Part 2: Code Generation
////////////////////////////////////////////////////////////////////
// Make sure doc comments get passed to the enum itself
let mut out = enum_attributes;
// Basically:
// #[repr(`repr_type`)]
// This would be a lot more elegant with `quote`, but it seems to still
// be considered an unstable API as of April 2023
// https://github.com/rust-lang/rust/issues/54722
out.push(punct_token('#'));
out.push(bracket_token(vec![
ident_token("repr"),
paren_token(repr_type.clone()),
]));
// #[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
out.push(punct_token('#'));
out.push(bracket_token(vec![
ident_token("derive"),
paren_token({
let mut derive_list: Vec<TokenTree> = vec![
ident_token("Debug"),
punct_token(','),
ident_token("Clone"),
punct_token(','),
ident_token("Copy"),
punct_token(','),
ident_token("PartialEq"),
punct_token(','),
ident_token("Eq"),
punct_token(','),
ident_token("Hash"),
];
if has_default {
derive_list.push(punct_token(','));
derive_list.push(ident_token("Default"));
}
derive_list
}),
]));
out.push(ident_token("pub"));
out.push(ident_token("enum"));
out.push(TokenTree::Ident(enum_identifier.clone()));
out.push(brace_token({
let mut tokens = Vec::<TokenTree>::new();
for triple in &triples {
tokens.extend(triple.0.clone());
tokens.push(TokenTree::Ident(triple.1.clone()));
tokens.push(punct_token('='));
tokens.push(triple.2.clone());
tokens.push(punct_token(','));
}
tokens.extend("\n".parse::<TokenStream>().unwrap());
tokens
}));
out.push(ident_token("impl"));
out.push(TokenTree::Ident(enum_identifier.clone()));
out.push(brace_token({
let mut tokens = Vec::new();
// pub fn from(x: u16) -> Option<MyEnum>
tokens.extend(vec![
ident_token("pub"),
ident_token("fn"),
ident_token("from"),
]);
tokens.push(paren_token(concat(
vec![ident_token("x"), punct_token(':')],
repr_type.clone(),
)));
tokens.extend(punc2_tokens('-', '>'));
tokens.push(ident_token("Option"));
tokens.push(punct_token('<'));
tokens.push(TokenTree::Ident(enum_identifier.clone()));
tokens.push(punct_token('>'));
tokens.push(brace_token({
// NOTE: You might be wondering why we use a chain of if statements instead
// of a match statement.
// The problem is that if a user provides an expression for one of the
// values, it may not always be possible to infer the exact literal value
// during macro expansion (e.g. what if a const variable is used?).
// And when we have to use user provided expressions for some of the values,
// it's tricky to find a match pattern that will allow us to match against it.
// And besides, the Rust compiler is probably smart enough to optimize
// a chain of if statements that tests a variable against a bunch of constants
// as much as a simple match.
let mut tokens = Vec::new();
for (_, variant_name, variant_value) in &triples {
tokens.push(ident_token("if"));
tokens.push(ident_token("x"));
tokens.extend(punc2_tokens('=', '='));
tokens.push(variant_value.clone());
tokens.push(brace_token(vec![
ident_token("return"),
ident_token("Some"),
paren_token(vec![
TokenTree::Ident(enum_identifier.clone()),
punct_cont_token(':'),
punct_token(':'),
TokenTree::Ident(variant_name.clone()),
]),
]));
}
tokens.push(ident_token("None"));
tokens
}));
// pub fn from_name(name: &str) -> Option<MyEnum>
tokens.extend(vec![
ident_token("pub"),
ident_token("fn"),
ident_token("from_name"),
]);
tokens.push(paren_token(vec![
ident_token("name"),
punct_token(':'),
punct_token('&'),
ident_token("str"),
]));
tokens.extend(punc2_tokens('-', '>'));
tokens.push(ident_token("Option"));
tokens.push(punct_token('<'));
tokens.push(TokenTree::Ident(enum_identifier.clone()));
tokens.push(punct_token('>'));
tokens.push(brace_token({
let mut tokens = Vec::new();
for (_, variant_name, _) in &triples {
tokens.push(ident_token("if"));
tokens.push(ident_token("name"));
tokens.extend(punc2_tokens('=', '='));
tokens.push(TokenTree::Literal(Literal::string(
&variant_name.to_string(),
)));
tokens.push(brace_token(vec![
ident_token("return"),
ident_token("Some"),
paren_token(vec![
TokenTree::Ident(enum_identifier.clone()),
punct_cont_token(':'),
punct_token(':'),
TokenTree::Ident(variant_name.clone()),
]),
]));
}
tokens.push(ident_token("None"));
tokens
}));
// pub fn list() -> &'static [MyEnum]
tokens.extend(vec![
ident_token("pub"),
ident_token("fn"),
ident_token("list"),
]);
tokens.push(paren_token(vec![]));
tokens.extend(punc2_tokens('-', '>'));
tokens.push(punct_token('&'));
tokens.push(punct_cont_token('\''));
tokens.push(ident_token("static"));
tokens.push(bracket_token(vec![TokenTree::Ident(
enum_identifier.clone(),
)]));
tokens.push(brace_token(vec![
punct_token('&'),
bracket_token({
let mut tokens = Vec::new();
for (_, variant_name, _) in &triples {
tokens.push(TokenTree::Ident(enum_identifier.clone()));
tokens.push(punct_cont_token(':'));
tokens.push(punct_token(':'));
tokens.push(TokenTree::Ident(variant_name.clone()));
tokens.push(punct_token(','));
}
tokens
}),
]));
tokens
}));
return TokenStream::from_iter(out.into_iter());
}