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    //! # proc-macro-error
    //!
    //! This crate aims to make error reporting in proc-macros simple and easy to use.
    //! Migrate from `panic!`-based errors for as little effort as possible!
    //!
    //! (Also, you can explicitly [append a dummy token stream](dummy/index.html) to your errors).
    //!
    //! To achieve his, this crate serves as a tiny shim around `proc_macro::Diagnostic` and
    //! `compile_error!`. It detects the best way of emitting available based on compiler's version.
    //! When the underlying diagnostic type is finally stabilized, this crate will simply be
    //! delegating to it requiring no changes in your code!
    //!
    //! So you can just use this crate and have *both* some of `proc_macro::Diagnostic` functionality
    //! available on stable ahead of time *and* your error-reporting code future-proof.
    //!
    //! ## Cargo features
    //!
    //! This crate provides *enabled by default* `syn-error` feature that gates
    //! `impl From<syn::Error> for Diagnostic` conversion. If you don't use `syn` and want
    //! to cut off some of compilation time, you can disable it via
    //!
    //! ```toml
    //! [dependencies]
    //! proc-macro-error = { version = "1", default-features = false }
    //! ```
    //!
    //! ***Please note that disabling this feature makes sense only if you don't depend on `syn`
    //! directly or indirectly, and you very likely do.**
    //!
    //! ## Real world examples
    //!
    //! * [`structopt-derive`](https://github.com/TeXitoi/structopt/tree/master/structopt-derive)
    //!   (abort-like usage)
    //! * [`auto-impl`](https://github.com/auto-impl-rs/auto_impl/) (emit-like usage)
    //!
    //! ## Limitations
    //!
    //! - Warnings are emitted only on nightly, they are ignored on stable.
    //! - "help" suggestions can't have their own span info on stable,
    //!   (essentially inheriting the parent span).
    //! - If a panic occurs somewhere in your macro no errors will be displayed. This is not a
    //!   technical limitation but rather intentional design. `panic` is not for error reporting.
    //!
    //! ### `#[proc_macro_error]` attribute
    //!
    //! **This attribute MUST be present on the top level of your macro** (the function
    //! annotated with any of `#[proc_macro]`, `#[proc_macro_derive]`, `#[proc_macro_attribute]`).
    //!
    //! This attribute performs the setup and cleanup necessary to make things work.
    //!
    //! In most cases you'll need the simple `#[proc_macro_error]` form without any
    //! additional settings. Feel free to [skip the "Syntax" section](#macros).
    //!
    //! #### Syntax
    //!
    //! `#[proc_macro_error]` or `#[proc_macro_error(settings...)]`, where `settings...`
    //! is a comma-separated list of:
    //!
    //! - `proc_macro_hack`:
    //!
    //!     In order to correctly cooperate with `#[proc_macro_hack]`, `#[proc_macro_error]`
    //!     attribute must be placed *before* (above) it, like this:
    //!
    //!     ```no_run
    //!     # use proc_macro2::TokenStream;
    //!     # const IGNORE: &str = "
    //!     #[proc_macro_error]
    //!     #[proc_macro_hack]
    //!     #[proc_macro]
    //!     # ";
    //!     fn my_macro(input: TokenStream) -> TokenStream {
    //!         unimplemented!()
    //!     }
    //!     ```
    //!
    //!     If, for some reason, you can't place it like that you can use
    //!     `#[proc_macro_error(proc_macro_hack)]` instead.
    //!
    //!     # Note
    //!
    //!     If `proc-macro-hack` was detected (by any means) `allow_not_macro`
    //!     and `assert_unwind_safe` will be applied automatically.
    //!
    //! - `allow_not_macro`:
    //!
    //!     By default, the attribute checks that it's applied to a proc-macro.
    //!     If none of `#[proc_macro]`, `#[proc_macro_derive]` nor `#[proc_macro_attribute]` are
    //!     present it will panic. It's the intention - this crate is supposed to be used only with
    //!     proc-macros.
    //!
    //!     This setting is made to bypass the check, useful in certain circumstances.
    //!
    //!     Pay attention: the function this attribute is applied to must return
    //!     `proc_macro::TokenStream`.
    //!
    //!     This setting is implied if `proc-macro-hack` was detected.
    //!
    //! - `assert_unwind_safe`:
    //!
    //!     By default, your code must be [unwind safe]. If your code is not unwind safe,
    //!     but you believe it's correct, you can use this setting to bypass the check.
    //!     You would need this for code that uses `lazy_static` or `thread_local` with
    //!     `Cell/RefCell` inside (and the like).
    //!
    //!     This setting is implied if `#[proc_macro_error]` is applied to a function
    //!     marked as `#[proc_macro]`, `#[proc_macro_derive]` or `#[proc_macro_attribute]`.
    //!
    //!     This setting is also implied if `proc-macro-hack` was detected.
    //!
    //! ## Macros
    //!
    //! Most of the time you want to use the macros. Syntax is described in the next section below.
    //!
    //! You'll need to decide how you want to emit errors:
    //!
    //! * Emit the error and abort. Very much panic-like usage. Served by [`abort!`] and
    //!   [`abort_call_site!`].
    //! * Emit the error but do not abort right away, looking for other errors to report.
    //!   Served by [`emit_error!`] and [`emit_call_site_error!`].
    //!
    //! You **can** mix these usages.
    //!
    //! `abort` and `emit_error` take a "source span" as the first argument. This source
    //! will be used to highlight the place the error originates from. It must be one of:
    //!
    //! * *Something* that implements [`ToTokens`] (most types in `syn` and `proc-macro2` do).
    //!   This source is the preferable one since it doesn't lose span information on multi-token
    //!   spans, see [this issue](https://gitlab.com/CreepySkeleton/proc-macro-error/-/issues/6)
    //!   for details.
    //! * [`proc_macro::Span`]
    //! * [`proc-macro2::Span`]
    //!
    //! The rest is your message in format-like style.
    //!
    //! See [the next section](#syntax-1) for detailed syntax.
    //!
    //! - [`abort!`]:
    //!
    //!     Very much panic-like usage - abort right away and show the error.
    //!     Expands to [`!`] (never type).
    //!
    //! - [`abort_call_site!`]:
    //!
    //!     Shortcut for `abort!(Span::call_site(), ...)`. Expands to [`!`] (never type).
    //!
    //! - [`emit_error!`]:
    //!
    //!     [`proc_macro::Diagnostic`]-like usage - emit the error but keep going,
    //!     looking for other errors to report.
    //!     The compilation will fail nonetheless. Expands to [`()`] (unit type).
    //!
    //! - [`emit_call_site_error!`]:
    //!
    //!     Shortcut for `emit_error!(Span::call_site(), ...)`. Expands to [`()`] (unit type).
    //!
    //! - [`emit_warning!`]:
    //!
    //!     Like `emit_error!` but emit a warning instead of error. The compilation won't fail
    //!     because of warnings.
    //!     Expands to [`()`] (unit type).
    //!
    //!     **Beware**: warnings are nightly only, they are completely ignored on stable.
    //!
    //! - [`emit_call_site_warning!`]:
    //!
    //!     Shortcut for `emit_warning!(Span::call_site(), ...)`. Expands to [`()`] (unit type).
    //!
    //! - [`diagnostic`]:
    //!
    //!     Build an instance of `Diagnostic` in format-like style.
    //!
    //! #### Syntax
    //!
    //! All the macros have pretty much the same syntax:
    //!
    //! 1.  ```ignore
    //!     abort!(single_expr)
    //!     ```
    //!     Shortcut for `Diagnostic::from(expr).abort()`.
    //!
    //! 2.  ```ignore
    //!     abort!(span, message)
    //!     ```
    //!     The first argument is an expression the span info should be taken from.
    //!
    //!     The second argument is the error message, it must implement [`ToString`].
    //!
    //! 3.  ```ignore
    //!     abort!(span, format_literal, format_args...)
    //!     ```
    //!
    //!     This form is pretty much the same as 2, except `format!(format_literal, format_args...)`
    //!     will be used to for the message instead of [`ToString`].
    //!
    //! That's it. `abort!`, `emit_warning`, `emit_error` share this exact syntax.
    //!
    //! `abort_call_site!`, `emit_call_site_warning`, `emit_call_site_error` lack 1 form
    //! and do not take span in 2'th and 3'th forms. Those are essentially shortcuts for
    //! `macro!(Span::call_site(), args...)`.
    //!
    //! `diagnostic!` requires a [`Level`] instance between `span` and second argument
    //! (1'th form is the same).
    //!
    //! > **Important!**
    //! >
    //! > If you have some type from `proc_macro` or `syn` to point to, do not call `.span()`
    //! > on it but rather use it directly:
    //! > ```no_run
    //! > # use proc_macro_error::abort;
    //! > # let input = proc_macro2::TokenStream::new();
    //! > let ty: syn::Type = syn::parse2(input).unwrap();
    //! > abort!(ty, "BOOM");
    //! > //     ^^ <-- avoid .span()
    //! > ```
    //! >
    //! > `.span()` calls work too, but you may experience regressions in message quality.
    //!
    //! #### Note attachments
    //!
    //! 3.  Every macro can have "note" attachments (only 2 and 3 form).
    //!   ```ignore
    //!   let opt_help = if have_some_info { Some("did you mean `this`?") } else { None };
    //!
    //!   abort!(
    //!       span, message; // <--- attachments start with `;` (semicolon)
    //!
    //!       help = "format {} {}", "arg1", "arg2"; // <--- every attachment ends with `;`,
    //!                                              //      maybe except the last one
    //!
    //!       note = "to_string"; // <--- one arg uses `.to_string()` instead of `format!()`
    //!
    //!       yay = "I see what {} did here", "you"; // <--- "help =" and "hint =" are mapped
    //!                                              // to Diagnostic::help,
    //!                                              // anything else is Diagnostic::note
    //!
    //!       wow = note_span => "custom span"; // <--- attachments can have their own span
    //!                                         //      it takes effect only on nightly though
    //!
    //!       hint =? opt_help; // <-- "optional" attachment, get displayed only if `Some`
    //!                         //     must be single `Option` expression
    //!
    //!       note =? note_span => opt_help // <-- optional attachments can have custom spans too
    //!   );
    //!   ```
    //!

    //! ### Diagnostic type
    //!
    //! [`Diagnostic`] type is intentionally designed to be API compatible with [`proc_macro::Diagnostic`].
    //! Not all API is implemented, only the part that can be reasonably implemented on stable.
    //!
    //!
    //! [`abort!`]: macro.abort.html
    //! [`abort_call_site!`]: macro.abort_call_site.html
    //! [`emit_warning!`]: macro.emit_warning.html
    //! [`emit_error!`]: macro.emit_error.html
    //! [`emit_call_site_warning!`]: macro.emit_call_site_error.html
    //! [`emit_call_site_error!`]: macro.emit_call_site_warning.html
    //! [`diagnostic!`]: macro.diagnostic.html
    //! [`Diagnostic`]: struct.Diagnostic.html
    //!
    //! [`proc_macro::Span`]: https://doc.rust-lang.org/proc_macro/struct.Span.html
    //! [`proc_macro::Diagnostic`]: https://doc.rust-lang.org/proc_macro/struct.Diagnostic.html
    //!
    //! [unwind safe]: https://doc.rust-lang.org/std/panic/trait.UnwindSafe.html#what-is-unwind-safety
    //! [`!`]: https://doc.rust-lang.org/std/primitive.never.html
    //! [`()`]: https://doc.rust-lang.org/std/primitive.unit.html
    //! [`ToString`]: https://doc.rust-lang.org/std/string/trait.ToString.html
    //!
    //! [`proc-macro2::Span`]: https://docs.rs/proc-macro2/1.0.10/proc_macro2/struct.Span.html
    //! [`ToTokens`]: https://docs.rs/quote/1.0.3/quote/trait.ToTokens.html
    //!

    #![cfg_attr(not(use_fallback), feature(proc_macro_diagnostic))]
    #![forbid(unsafe_code)]
    #![allow(clippy::needless_doctest_main)]

    extern crate proc_macro;

    pub use crate::{
        diagnostic::{Diagnostic, DiagnosticExt, Level},
        dummy::{append_dummy, set_dummy},
    };
    pub use proc_macro_error_attr::proc_macro_error;

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

    use std::cell::Cell;
    use std::panic::{catch_unwind, resume_unwind, UnwindSafe};

    pub mod dummy;

    mod diagnostic;
    mod macros;
    mod sealed;

    #[cfg(use_fallback)]
    #[path = "imp/fallback.rs"]
    mod imp;

    #[cfg(not(use_fallback))]
    #[path = "imp/delegate.rs"]
    mod imp;

    #[derive(Debug, Clone, Copy)]
    pub struct SpanRange {
        pub first: Span,
        pub last: Span,
    }

    impl SpanRange {
        /// Create a range with the `first` and `last` spans being the same.
        pub fn single_span(span: Span) -> Self {
            SpanRange {
                first: span,
                last: span,
            }
        }

        /// Create a `SpanRange` resolving at call site.
        pub fn call_site() -> Self {
            SpanRange::single_span(Span::call_site())
        }

        /// Construct span range from a `TokenStream`. This method always preserves all the
        /// range.
        ///
        /// ### Note
        ///
        /// If the stream is empty, the result is `SpanRange::call_site()`. If the stream
        /// consists of only one `TokenTree`, the result is `SpanRange::single_span(tt.span())`
        /// that doesn't lose anything.
        pub fn from_tokens(ts: &dyn ToTokens) -> Self {
            let mut spans = ts.to_token_stream().into_iter().map(|tt| tt.span());
            let first = spans.next().unwrap_or_else(|| Span::call_site());
            let last = spans.last().unwrap_or(first);

            SpanRange { first, last }
        }

        /// Join two span ranges. The resulting range will start at `self.first` and end at
        /// `other.last`.
        pub fn join_range(self, other: SpanRange) -> Self {
            SpanRange {
                first: self.first,
                last: other.last,
            }
        }

        /// Collapse the range into single span, preserving as much information as possible.
        pub fn collapse(self) -> Span {
            self.first.join(self.last).unwrap_or(self.first)
        }
    }

    /// This traits expands `Result<T, Into<Diagnostic>>` with some handy shortcuts.
    pub trait ResultExt {
        type Ok;

        /// Behaves like `Result::unwrap`: if self is `Ok` yield the contained value,
        /// otherwise abort macro execution via `abort!`.
        fn unwrap_or_abort(self) -> Self::Ok;

        /// Behaves like `Result::expect`: if self is `Ok` yield the contained value,
        /// otherwise abort macro execution via `abort!`.
        /// If it aborts then resulting error message will be preceded with `message`.
        fn expect_or_abort(self, msg: &str) -> Self::Ok;
    }

    /// This traits expands `Option` with some handy shortcuts.
    pub trait OptionExt {
        type Some;

        /// Behaves like `Option::expect`: if self is `Some` yield the contained value,
        /// otherwise abort macro execution via `abort_call_site!`.
        /// If it aborts the `message` will be used for [`compile_error!`][compl_err] invocation.
        ///
        /// [compl_err]: https://doc.rust-lang.org/std/macro.compile_error.html
        fn expect_or_abort(self, msg: &str) -> Self::Some;
    }

    /// Abort macro execution and display all the emitted errors, if any.
    ///
    /// Does nothing if no errors were emitted (warnings do not count).
    pub fn abort_if_dirty() {
        imp::abort_if_dirty();
    }

    impl<T, E: Into<Diagnostic>> ResultExt for Result<T, E> {
        type Ok = T;

        fn unwrap_or_abort(self) -> T {
            match self {
                Ok(res) => res,
                Err(e) => e.into().abort(),
            }
        }

        fn expect_or_abort(self, message: &str) -> T {
            match self {
                Ok(res) => res,
                Err(e) => {
                    let mut e = e.into();
                    e.msg = format!("{}: {}", message, e.msg);
                    e.abort()
                }
            }
        }
    }

    impl<T> OptionExt for Option<T> {
        type Some = T;

        fn expect_or_abort(self, message: &str) -> T {
            match self {
                Some(res) => res,
                None => abort_call_site!(message),
            }
        }
    }

    /// This is the entry point for a proc-macro.
    ///
    /// **NOT PUBLIC API, SUBJECT TO CHANGE WITHOUT ANY NOTICE**
    #[doc(hidden)]
    pub fn entry_point<F>(f: F, proc_macro_hack: bool) -> proc_macro::TokenStream
    where
        F: FnOnce() -> proc_macro::TokenStream + UnwindSafe,
    {
        ENTERED_ENTRY_POINT.with(|flag| flag.set(flag.get() + 1));
        let caught = catch_unwind(f);
        let dummy = dummy::cleanup();
        let err_storage = imp::cleanup();
        ENTERED_ENTRY_POINT.with(|flag| flag.set(flag.get() - 1));

        let gen_error = || {
            if proc_macro_hack {
                quote! {{
                    macro_rules! proc_macro_call {
                        () => ( unimplemented!() )
                    }

                    #(#err_storage)*
                    #dummy

                    unimplemented!()
                }}
            } else {
                quote!( #(#err_storage)* #dummy )
            }
        };

        match caught {
            Ok(ts) => {
                if err_storage.is_empty() {
                    ts
                } else {
                    gen_error().into()
                }
            }

            Err(boxed) => match boxed.downcast::<AbortNow>() {
                Ok(_) => gen_error().into(),
                Err(boxed) => resume_unwind(boxed),
            },
        }
    }

    fn abort_now() -> ! {
        check_correctness();
        panic!(AbortNow)
    }

    thread_local! {
        static ENTERED_ENTRY_POINT: Cell<usize> = Cell::new(0);
    }

    struct AbortNow;

    fn check_correctness() {
        if ENTERED_ENTRY_POINT.with(|flag| flag.get()) == 0 {
            panic!(
                "proc-macro-error API cannot be used outside of `entry_point` invocation, \
                 perhaps you forgot to annotate your #[proc_macro] function with `#[proc_macro_error]"
            );
        }
    }

    /// **ALL THE STUFF INSIDE IS NOT PUBLIC API!!!**
    #[doc(hidden)]
    pub mod __export {
        // reexports for use in macros
        pub extern crate proc_macro;
        pub extern crate proc_macro2;

        use proc_macro2::Span;
        use quote::ToTokens;

        use crate::SpanRange;

        // inspired by
        // https://github.com/dtolnay/case-studies/blob/master/autoref-specialization/README.md#simple-application

        pub trait SpanAsSpanRange {
            #[allow(non_snake_case)]
            fn FIRST_ARG_MUST_EITHER_BE_Span_OR_IMPLEMENT_ToTokens_OR_BE_SpanRange(&self) -> SpanRange;
        }

        pub trait Span2AsSpanRange {
            #[allow(non_snake_case)]
            fn FIRST_ARG_MUST_EITHER_BE_Span_OR_IMPLEMENT_ToTokens_OR_BE_SpanRange(&self) -> SpanRange;
        }

        pub trait ToTokensAsSpanRange {
            #[allow(non_snake_case)]
            fn FIRST_ARG_MUST_EITHER_BE_Span_OR_IMPLEMENT_ToTokens_OR_BE_SpanRange(&self) -> SpanRange;
        }

        pub trait SpanRangeAsSpanRange {
            #[allow(non_snake_case)]
            fn FIRST_ARG_MUST_EITHER_BE_Span_OR_IMPLEMENT_ToTokens_OR_BE_SpanRange(&self) -> SpanRange;
        }

        impl<T: ToTokens> ToTokensAsSpanRange for &T {
            fn FIRST_ARG_MUST_EITHER_BE_Span_OR_IMPLEMENT_ToTokens_OR_BE_SpanRange(&self) -> SpanRange {
                let mut ts = self.to_token_stream().into_iter();
                let first = ts
                    .next()
                    .map(|tt| tt.span())
                    .unwrap_or_else(Span::call_site);
                let last = ts.last().map(|tt| tt.span()).unwrap_or(first);
                SpanRange { first, last }
            }
        }

        impl Span2AsSpanRange for Span {
            fn FIRST_ARG_MUST_EITHER_BE_Span_OR_IMPLEMENT_ToTokens_OR_BE_SpanRange(&self) -> SpanRange {
                SpanRange {
                    first: *self,
                    last: *self,
                }
            }
        }

        impl SpanAsSpanRange for proc_macro::Span {
            fn FIRST_ARG_MUST_EITHER_BE_Span_OR_IMPLEMENT_ToTokens_OR_BE_SpanRange(&self) -> SpanRange {
                SpanRange {
                    first: self.clone().into(),
                    last: self.clone().into(),
                }
            }
        }

        impl SpanRangeAsSpanRange for SpanRange {
            fn FIRST_ARG_MUST_EITHER_BE_Span_OR_IMPLEMENT_ToTokens_OR_BE_SpanRange(&self) -> SpanRange {
                *self
            }
        }
    }