# spanned.rs - source
Source of the Rust file `/home/justine/.cargo/registry/src/github.com-1ecc6299db9ec823/syn-1.0.107/src/spanned.rs`.
Keywords: rust, rustlang, rust-lang

    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

    //! A trait that can provide the `Span` of the complete contents of a syntax
    //! tree node.
    //!
    //! *This module is available only 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`]: crate::Type
    //! [`Sync`]: std::marker::Sync
    //!
    //! 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 only 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()
        }
    }