1
  2
  3
  4
  5
  6
  7
  8
  9
 10
 11
 12
 13
 14
 15
 16
 17
 18
 19
 20
 21
 22
 23
 24
 25
 26
 27
 28
 29
 30
 31
 32
 33
 34
 35
 36
 37
 38
 39
 40
 41
 42
 43
 44
 45
 46
 47
 48
 49
 50
 51
 52
 53
 54
 55
 56
 57
 58
 59
 60
 61
 62
 63
 64
 65
 66
 67
 68
 69
 70
 71
 72
 73
 74
 75
 76
 77
 78
 79
 80
 81
 82
 83
 84
 85
 86
 87
 88
 89
 90
 91
 92
 93
 94
 95
 96
 97
 98
 99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144

// Copyright 2018 Syn Developers
//
// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
// option. This file may not be copied, modified, or distributed
// except according to those terms.

//! 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.*
//!
//! # 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.
//!
//! ```
//! #[macro_use]
//! extern crate quote;
//!
//! extern crate syn;
//! extern crate proc_macro;
//! extern crate proc_macro2;
//!
//! use syn::Type;
//! use syn::spanned::Spanned;
//! use proc_macro::TokenStream;
//! use proc_macro2::Span;
//!
//! # 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!()
//! # }
//! #
//! # fn main() {}
//! ```
//!
//! 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.

use proc_macro2::{Span, TokenStream};
use quote::{ToTokens, Tokens};

/// 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.
///
/// [`ToTokens`]: https://docs.rs/quote/0.4/quote/trait.ToTokens.html
///
/// See the [module documentation] for an example.
///
/// [module documentation]: index.html
///
/// *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()`]: https://docs.rs/proc-macro2/0.2/proc_macro2/struct.Span.html#method.call_site
    fn span(&self) -> Span;
}

impl<T> Spanned for T
where
    T: ToTokens,
{
    #[cfg(procmacro2_semver_exempt)]
    fn span(&self) -> Span {
        let mut tokens = Tokens::new();
        self.to_tokens(&mut tokens);
        let token_stream = TokenStream::from(tokens);
        let mut iter = token_stream.into_iter();
        let mut span = match iter.next() {
            Some(tt) => tt.span(),
            None => {
                return Span::call_site();
            }
        };
        for tt in iter {
            if let Some(joined) = span.join(tt.span()) {
                span = joined;
            }
        }
        span
    }

    #[cfg(not(procmacro2_semver_exempt))]
    fn span(&self) -> Span {
        let mut tokens = Tokens::new();
        self.to_tokens(&mut tokens);
        let token_stream = TokenStream::from(tokens);
        let mut iter = token_stream.into_iter();

        // We can't join spans without procmacro2_semver_exempt so just grab the
        // first one.
        match iter.next() {
            Some(tt) => tt.span(),
            None => Span::call_site(),
        }
    }
}