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    use crate::{abort_now, check_correctness, sealed::Sealed, SpanRange};
    use proc_macro2::Span;
    use proc_macro2::TokenStream;

    use quote::{quote_spanned, ToTokens};

    /// Represents a diagnostic level
    ///
    /// # Warnings
    ///
    /// Warnings are ignored on stable/beta
    #[derive(Debug, PartialEq)]
    pub enum Level {
        Error,
        Warning,
        #[doc(hidden)]
        NonExhaustive,
    }

    /// Represents a single diagnostic message
    #[derive(Debug)]
    pub struct Diagnostic {
        pub(crate) level: Level,
        pub(crate) span_range: SpanRange,
        pub(crate) msg: String,
        pub(crate) suggestions: Vec<(SuggestionKind, String, Option<SpanRange>)>,
        pub(crate) children: Vec<(SpanRange, String)>,
    }

    /// A collection of methods that do not exist in `proc_macro::Diagnostic`
    /// but still useful to have around.
    ///
    /// This trait is sealed and cannot be implemented outside of `proc_macro_error`.
    pub trait DiagnosticExt: Sealed {
        /// Create a new diagnostic message that points to the `span_range`.
        ///
        /// This function is the same as `Diagnostic::spanned` but produces considerably
        /// better error messages for multi-token spans on stable.
        fn spanned_range(span_range: SpanRange, level: Level, message: String) -> Self;

        /// Add another error message to self such that it will be emitted right after
        /// the main message.
        ///
        /// This function is the same as `Diagnostic::span_error` but produces considerably
        /// better error messages for multi-token spans on stable.
        fn span_range_error(self, span_range: SpanRange, msg: String) -> Self;

        /// Attach a "help" note to your main message, the note will have it's own span on nightly.
        ///
        /// This function is the same as `Diagnostic::span_help` but produces considerably
        /// better error messages for multi-token spans on stable.
        ///
        /// # Span
        ///
        /// The span is ignored on stable, the note effectively inherits its parent's (main message) span
        fn span_range_help(self, span_range: SpanRange, msg: String) -> Self;

        /// Attach a note to your main message, the note will have it's own span on nightly.
        ///
        /// This function is the same as `Diagnostic::span_note` but produces considerably
        /// better error messages for multi-token spans on stable.
        ///
        /// # Span
        ///
        /// The span is ignored on stable, the note effectively inherits its parent's (main message) span
        fn span_range_note(self, span_range: SpanRange, msg: String) -> Self;
    }

    impl DiagnosticExt for Diagnostic {
        fn spanned_range(span_range: SpanRange, level: Level, message: String) -> Self {
            Diagnostic {
                level,
                span_range,
                msg: message,
                suggestions: vec![],
                children: vec![],
            }
        }

        fn span_range_error(mut self, span_range: SpanRange, msg: String) -> Self {
            self.children.push((span_range, msg));
            self
        }

        fn span_range_help(mut self, span_range: SpanRange, msg: String) -> Self {
            self.suggestions
                .push((SuggestionKind::Help, msg, Some(span_range)));
            self
        }

        fn span_range_note(mut self, span_range: SpanRange, msg: String) -> Self {
            self.suggestions
                .push((SuggestionKind::Note, msg, Some(span_range)));
            self
        }
    }

    impl Diagnostic {
        /// Create a new diagnostic message that points to `Span::call_site()`
        pub fn new(level: Level, message: String) -> Self {
            Diagnostic::spanned(Span::call_site(), level, message)
        }

        /// Create a new diagnostic message that points to the `span`
        pub fn spanned(span: Span, level: Level, message: String) -> Self {
            Diagnostic::spanned_range(
                SpanRange {
                    first: span,
                    last: span,
                },
                level,
                message,
            )
        }

        /// Add another error message to self such that it will be emitted right after
        /// the main message.
        pub fn span_error(self, span: Span, msg: String) -> Self {
            self.span_range_error(
                SpanRange {
                    first: span,
                    last: span,
                },
                msg,
            )
        }

        /// Attach a "help" note to your main message, the note will have it's own span on nightly.
        ///
        /// # Span
        ///
        /// The span is ignored on stable, the note effectively inherits its parent's (main message) span
        pub fn span_help(self, span: Span, msg: String) -> Self {
            self.span_range_help(
                SpanRange {
                    first: span,
                    last: span,
                },
                msg,
            )
        }

        /// Attach a "help" note to your main message.
        pub fn help(mut self, msg: String) -> Self {
            self.suggestions.push((SuggestionKind::Help, msg, None));
            self
        }

        /// Attach a note to your main message, the note will have it's own span on nightly.
        ///
        /// # Span
        ///
        /// The span is ignored on stable, the note effectively inherits its parent's (main message) span
        pub fn span_note(self, span: Span, msg: String) -> Self {
            self.span_range_note(
                SpanRange {
                    first: span,
                    last: span,
                },
                msg,
            )
        }

        /// Attach a note to your main message
        pub fn note(mut self, msg: String) -> Self {
            self.suggestions.push((SuggestionKind::Note, msg, None));
            self
        }

        /// The message of main warning/error (no notes attached)
        pub fn message(&self) -> &str {
            &self.msg
        }

        /// Abort the proc-macro's execution and display the diagnostic.
        ///
        /// # Warnings
        ///
        /// Warnings are not emitted on stable and beta, but this function will abort anyway.
        pub fn abort(self) -> ! {
            self.emit();
            abort_now()
        }

        /// Display the diagnostic while not aborting macro execution.
        ///
        /// # Warnings
        ///
        /// Warnings are ignored on stable/beta
        pub fn emit(self) {
            check_correctness();
            crate::imp::emit_diagnostic(self);
        }
    }

    /// **NOT PUBLIC API! NOTHING TO SEE HERE!!!**
    #[doc(hidden)]
    impl Diagnostic {
        pub fn span_suggestion(self, span: Span, suggestion: &str, msg: String) -> Self {
            match suggestion {
                "help" | "hint" => self.span_help(span, msg),
                _ => self.span_note(span, msg),
            }
        }

        pub fn suggestion(self, suggestion: &str, msg: String) -> Self {
            match suggestion {
                "help" | "hint" => self.help(msg),
                _ => self.note(msg),
            }
        }
    }

    impl ToTokens for Diagnostic {
        fn to_tokens(&self, ts: &mut TokenStream) {
            use std::borrow::Cow;

            fn ensure_lf(buf: &mut String, s: &str) {
                if s.ends_with('\n') {
                    buf.push_str(s);
                } else {
                    buf.push_str(s);
                    buf.push('\n');
                }
            }

            fn diag_to_tokens(
                span_range: SpanRange,
                level: &Level,
                msg: &str,
                suggestions: &[(SuggestionKind, String, Option<SpanRange>)],
            ) -> TokenStream {
                if *level == Level::Warning {
                    return TokenStream::new();
                }

                let message = if suggestions.is_empty() {
                    Cow::Borrowed(msg)
                } else {
                    let mut message = String::new();
                    ensure_lf(&mut message, msg);
                    message.push('\n');

                    for (kind, note, _span) in suggestions {
                        message.push_str("  = ");
                        message.push_str(kind.name());
                        message.push_str(": ");
                        ensure_lf(&mut message, note);
                    }
                    message.push('\n');

                    Cow::Owned(message)
                };

                let mut msg = proc_macro2::Literal::string(&message);
                msg.set_span(span_range.last);
                let group = quote_spanned!(span_range.last=> { #msg } );
                quote_spanned!(span_range.first=> compile_error!#group)
            }

            ts.extend(diag_to_tokens(
                self.span_range,
                &self.level,
                &self.msg,
                &self.suggestions,
            ));
            ts.extend(
                self.children
                    .iter()
                    .map(|(span_range, msg)| diag_to_tokens(*span_range, &Level::Error, &msg, &[])),
            );
        }
    }

    #[derive(Debug)]
    pub(crate) enum SuggestionKind {
        Help,
        Note,
    }

    impl SuggestionKind {
        fn name(&self) -> &'static str {
            match self {
                SuggestionKind::Note => "note",
                SuggestionKind::Help => "help",
            }
        }
    }

    #[cfg(feature = "syn-error")]
    impl From<syn::Error> for Diagnostic {
        fn from(err: syn::Error) -> Self {
            use proc_macro2::{Delimiter, TokenTree};

            fn gut_error(ts: &mut impl Iterator<Item = TokenTree>) -> Option<(SpanRange, String)> {
                let first = match ts.next() {
                    // compile_error
                    None => return None,
                    Some(tt) => tt.span(),
                };
                ts.next().unwrap(); // !

                let lit = match ts.next().unwrap() {
                    TokenTree::Group(group) => {
                        // Currently `syn` builds `compile_error!` invocations
                        // exclusively in `ident{"..."}` (braced) form which is not
                        // followed by `;` (semicolon).
                        //
                        // But if it changes to `ident("...");` (parenthesized)
                        // or `ident["..."];` (bracketed) form,
                        // we will need to skip the `;` as well.
                        // Highly unlikely, but better safe than sorry.

                        if group.delimiter() == Delimiter::Parenthesis
                            || group.delimiter() == Delimiter::Bracket
                        {
                            ts.next().unwrap(); // ;
                        }

                        match group.stream().into_iter().next().unwrap() {
                            TokenTree::Literal(lit) => lit,
                            _ => unreachable!(),
                        }
                    }
                    _ => unreachable!(),
                };

                let last = lit.span();
                let mut msg = lit.to_string();

                // "abc" => abc
                msg.pop();
                msg.remove(0);

                Some((SpanRange { first, last }, msg))
            }

            let mut ts = err.to_compile_error().into_iter();

            let (span_range, msg) = gut_error(&mut ts).unwrap();
            let mut res = Diagnostic::spanned_range(span_range, Level::Error, msg);

            while let Some((span_range, msg)) = gut_error(&mut ts) {
                res = res.span_range_error(span_range, msg);
            }

            res
        }
    }