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Diffstat (limited to 'syn/src/spanned.rs')
| -rw-r--r-- | syn/src/spanned.rs | 114 |
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diff --git a/syn/src/spanned.rs b/syn/src/spanned.rs new file mode 100644 index 0000000..71ffe26 --- /dev/null +++ b/syn/src/spanned.rs @@ -0,0 +1,114 @@ +//! A trait that can provide the `Span` of the complete contents of a syntax +//! tree node. +//! +//! *This module is available if Syn is built with both the `"parsing"` and +//! `"printing"` features.* +//! +//! <br> +//! +//! # Example +//! +//! Suppose in a procedural macro we have a [`Type`] that we want to assert +//! implements the [`Sync`] trait. Maybe this is the type of one of the fields +//! of a struct for which we are deriving a trait implementation, and we need to +//! be able to pass a reference to one of those fields across threads. +//! +//! [`Type`]: ../enum.Type.html +//! [`Sync`]: https://doc.rust-lang.org/std/marker/trait.Sync.html +//! +//! If the field type does *not* implement `Sync` as required, we want the +//! compiler to report an error pointing out exactly which type it was. +//! +//! The following macro code takes a variable `ty` of type `Type` and produces a +//! static assertion that `Sync` is implemented for that type. +//! +//! ``` +//! # extern crate proc_macro; +//! # +//! use proc_macro::TokenStream; +//! use proc_macro2::Span; +//! use quote::quote_spanned; +//! use syn::Type; +//! use syn::spanned::Spanned; +//! +//! # const IGNORE_TOKENS: &str = stringify! { +//! #[proc_macro_derive(MyMacro)] +//! # }; +//! pub fn my_macro(input: TokenStream) -> TokenStream { +//! # let ty = get_a_type(); +//! /* ... */ +//! +//! let assert_sync = quote_spanned! {ty.span()=> +//! struct _AssertSync where #ty: Sync; +//! }; +//! +//! /* ... */ +//! # input +//! } +//! # +//! # fn get_a_type() -> Type { +//! # unimplemented!() +//! # } +//! ``` +//! +//! By inserting this `assert_sync` fragment into the output code generated by +//! our macro, the user's code will fail to compile if `ty` does not implement +//! `Sync`. The errors they would see look like the following. +//! +//! ```text +//! error[E0277]: the trait bound `*const i32: std::marker::Sync` is not satisfied +//! --> src/main.rs:10:21 +//! | +//! 10 | bad_field: *const i32, +//! | ^^^^^^^^^^ `*const i32` cannot be shared between threads safely +//! ``` +//! +//! In this technique, using the `Type`'s span for the error message makes the +//! error appear in the correct place underlining the right type. +//! +//! <br> +//! +//! # Limitations +//! +//! The underlying [`proc_macro::Span::join`] method is nightly-only. When +//! called from within a procedural macro in a nightly compiler, `Spanned` will +//! use `join` to produce the intended span. When not using a nightly compiler, +//! only the span of the *first token* of the syntax tree node is returned. +//! +//! In the common case of wanting to use the joined span as the span of a +//! `syn::Error`, consider instead using [`syn::Error::new_spanned`] which is +//! able to span the error correctly under the complete syntax tree node without +//! needing the unstable `join`. +//! +//! [`syn::Error::new_spanned`]: crate::Error::new_spanned + +use proc_macro2::Span; +use quote::spanned::Spanned as ToTokens; + +/// A trait that can provide the `Span` of the complete contents of a syntax +/// tree node. +/// +/// This trait is automatically implemented for all types that implement +/// [`ToTokens`] from the `quote` crate, as well as for `Span` itself. +/// +/// [`ToTokens`]: quote::ToTokens +/// +/// See the [module documentation] for an example. +/// +/// [module documentation]: self +/// +/// *This trait is available if Syn is built with both the `"parsing"` and +/// `"printing"` features.* +pub trait Spanned { + /// Returns a `Span` covering the complete contents of this syntax tree + /// node, or [`Span::call_site()`] if this node is empty. + /// + /// [`Span::call_site()`]: proc_macro2::Span::call_site + fn span(&self) -> Span; +} + +impl<T: ?Sized + ToTokens> Spanned for T { + fn span(&self) -> Span { + self.__span() + } +} |