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#![recursion_limit = "128"]
#![feature(proc_macro_diagnostic)]
extern crate proc_macro;
use self::proc_macro::TokenStream;
use quote::{quote, quote_spanned};
use syn::parse::{Parse, ParseStream, Result};
use syn::spanned::Spanned;
use syn::{parse_macro_input, Expr, Ident, Token, Type, Visibility};
/// Parses the following syntax, which aligns with the input of the real
/// `lazy_static` crate.
///
/// lazy_static! {
/// $VISIBILITY static ref $NAME: $TYPE = $EXPR;
/// }
///
/// For example:
///
/// lazy_static! {
/// static ref USERNAME: Regex = Regex::new("^[a-z0-9_-]{3,16}$").unwrap();
/// }
struct LazyStatic {
visibility: Visibility,
name: Ident,
ty: Type,
init: Expr,
}
impl Parse for LazyStatic {
fn parse(input: ParseStream) -> Result<Self> {
let visibility: Visibility = input.parse()?;
input.parse::<Token![static]>()?;
input.parse::<Token![ref]>()?;
let name: Ident = input.parse()?;
input.parse::<Token![:]>()?;
let ty: Type = input.parse()?;
input.parse::<Token![=]>()?;
let init: Expr = input.parse()?;
input.parse::<Token![;]>()?;
Ok(LazyStatic {
visibility,
name,
ty,
init,
})
}
}
#[proc_macro]
pub fn lazy_static(input: TokenStream) -> TokenStream {
let LazyStatic {
visibility,
name,
ty,
init,
} = parse_macro_input!(input as LazyStatic);
// The warning looks like this.
//
// warning: come on, pick a more creative name
// --> src/main.rs:10:16
// |
// 10 | static ref FOO: String = "lazy_static".to_owned();
// | ^^^
if name == "FOO" {
name.span()
.unwrap()
.warning("come on, pick a more creative name")
.emit();
}
// The error looks like this.
//
// error: I can't think of a legitimate use for lazily initializing the value `()`
// --> src/main.rs:10:27
// |
// 10 | static ref UNIT: () = ();
// | ^^
if let Expr::Tuple(ref init) = init {
if init.elems.is_empty() {
init.span()
.unwrap()
.error("I can't think of a legitimate use for lazily initializing the value `()`")
.emit();
return TokenStream::new();
}
}
// Assert that the static type implements Sync. If not, user sees an error
// message like the following. We span this assertion with the field type's
// line/column so that the error message appears in the correct place.
//
// error[E0277]: the trait bound `*const (): std::marker::Sync` is not satisfied
// --> src/main.rs:10:21
// |
// 10 | static ref PTR: *const () = &();
// | ^^^^^^^^^ `*const ()` cannot be shared between threads safely
let assert_sync = quote_spanned! {ty.span()=>
struct _AssertSync where #ty: std::marker::Sync;
};
// Check for Sized. Not vital to check here, but the error message is less
// confusing this way than if they get a Sized error in one of our
// implementation details where it assumes Sized.
//
// error[E0277]: the trait bound `str: std::marker::Sized` is not satisfied
// --> src/main.rs:10:19
// |
// 10 | static ref A: str = "";
// | ^^^ `str` does not have a constant size known at compile-time
let assert_sized = quote_spanned! {ty.span()=>
struct _AssertSized where #ty: std::marker::Sized;
};
let init_ptr = quote_spanned! {init.span()=>
Box::into_raw(Box::new(#init))
};
let expanded = quote! {
#visibility struct #name;
impl std::ops::Deref for #name {
type Target = #ty;
fn deref(&self) -> &#ty {
#assert_sync
#assert_sized
static ONCE: std::sync::Once = std::sync::Once::new();
static mut VALUE: *mut #ty = 0 as *mut #ty;
unsafe {
ONCE.call_once(|| VALUE = #init_ptr);
&*VALUE
}
}
}
};
TokenStream::from(expanded)
}
|
