115 lines
3.7 KiB
Rust
115 lines
3.7 KiB
Rust
//! A trait that can provide the `Span` of the complete contents of a syntax
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//! tree node.
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//!
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//! *This module is available only if Syn is built with both the `"parsing"` and
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//! `"printing"` features.*
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//!
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//! <br>
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//!
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//! # Example
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//!
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//! Suppose in a procedural macro we have a [`Type`] that we want to assert
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//! implements the [`Sync`] trait. Maybe this is the type of one of the fields
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//! of a struct for which we are deriving a trait implementation, and we need to
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//! be able to pass a reference to one of those fields across threads.
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//!
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//! [`Type`]: crate::Type
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//! [`Sync`]: std::marker::Sync
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//!
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//! If the field type does *not* implement `Sync` as required, we want the
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//! compiler to report an error pointing out exactly which type it was.
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//!
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//! The following macro code takes a variable `ty` of type `Type` and produces a
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//! static assertion that `Sync` is implemented for that type.
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//!
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//! ```
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//! # extern crate proc_macro;
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//! #
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//! use proc_macro::TokenStream;
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//! use proc_macro2::Span;
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//! use quote::quote_spanned;
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//! use syn::Type;
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//! use syn::spanned::Spanned;
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//!
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//! # const IGNORE_TOKENS: &str = stringify! {
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//! #[proc_macro_derive(MyMacro)]
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//! # };
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//! pub fn my_macro(input: TokenStream) -> TokenStream {
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//! # let ty = get_a_type();
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//! /* ... */
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//!
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//! let assert_sync = quote_spanned! {ty.span()=>
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//! struct _AssertSync where #ty: Sync;
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//! };
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//!
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//! /* ... */
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//! # input
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//! }
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//! #
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//! # fn get_a_type() -> Type {
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//! # unimplemented!()
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//! # }
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//! ```
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//!
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//! By inserting this `assert_sync` fragment into the output code generated by
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//! our macro, the user's code will fail to compile if `ty` does not implement
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//! `Sync`. The errors they would see look like the following.
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//!
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//! ```text
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//! error[E0277]: the trait bound `*const i32: std::marker::Sync` is not satisfied
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//! --> src/main.rs:10:21
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//! |
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//! 10 | bad_field: *const i32,
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//! | ^^^^^^^^^^ `*const i32` cannot be shared between threads safely
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//! ```
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//!
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//! In this technique, using the `Type`'s span for the error message makes the
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//! error appear in the correct place underlining the right type.
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//!
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//! <br>
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//!
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//! # Limitations
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//!
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//! The underlying [`proc_macro::Span::join`] method is nightly-only. When
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//! called from within a procedural macro in a nightly compiler, `Spanned` will
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//! use `join` to produce the intended span. When not using a nightly compiler,
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//! only the span of the *first token* of the syntax tree node is returned.
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//!
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//! In the common case of wanting to use the joined span as the span of a
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//! `syn::Error`, consider instead using [`syn::Error::new_spanned`] which is
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//! able to span the error correctly under the complete syntax tree node without
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//! needing the unstable `join`.
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//!
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//! [`syn::Error::new_spanned`]: crate::Error::new_spanned
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use proc_macro2::Span;
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use quote::spanned::Spanned as ToTokens;
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/// A trait that can provide the `Span` of the complete contents of a syntax
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/// tree node.
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///
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/// This trait is automatically implemented for all types that implement
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/// [`ToTokens`] from the `quote` crate, as well as for `Span` itself.
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///
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/// [`ToTokens`]: quote::ToTokens
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///
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/// See the [module documentation] for an example.
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///
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/// [module documentation]: self
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///
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/// *This trait is available only if Syn is built with both the `"parsing"` and
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/// `"printing"` features.*
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pub trait Spanned {
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/// Returns a `Span` covering the complete contents of this syntax tree
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/// node, or [`Span::call_site()`] if this node is empty.
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///
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/// [`Span::call_site()`]: proc_macro2::Span::call_site
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fn span(&self) -> Span;
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}
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impl<T: ?Sized + ToTokens> Spanned for T {
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fn span(&self) -> Span {
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self.__span()
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}
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}
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