proc-macro-error 0.1.1

// reexports for use in macros
pub extern crate proc_macro;
pub extern crate proc_macro2;
extern crate quote;
extern crate syn;

use proc_macro2::{Span, TokenStream};
use quote::quote_spanned;

/// Makes a [`MacroError`] instance from provided arguments (`panic!`-like)
/// and triggers panic in hope it will be caught by [`filter_macro_errors!`].
///
/// # Syntax
///
/// This macro is meant to be a `panic!` drop-in replacement so its syntax is very similar to `panic!`,
/// but it has three forms instead of two:
/// 1. "panic-format-like" form: span, formatting [`str`] literal, comma-separated list of args.
///     First argument is a span, all the rest gets passed to [`format!`] to build the error message.
/// 2. "panic-single-arg-like" form: span, expr, no comma at the end.
///     First argument is a span, the second is our error message, it must implement [`ToString`].
/// 3. "trigger_error-like" form: single expr.
///     Literally `trigger_error(arg)`. It's here just for convenience so [`span_error!`] can be used
///     with instances of [`syn::Error`], [`MacroError`], [`&str`], [`String`] and so on...
///
#[macro_export]
macro_rules! span_error {
    ($span:expr, $fmt:literal, $($args:expr),*) => {{
        let msg = format!($fmt, $($args),*);
        // we use $span.into() so it would work with proc_macro::Span and
        // proc_macro2::Span all the same
        let err = $crate::MacroError::new($span.into(), msg);
        $crate::trigger_error(err)
    }};

    ($span:expr, $msg:expr) => {{
        // we use $span.into() so it would work with proc_macro::Span and
        // proc_macro2::Span all the same
        let err = $crate::MacroError::new($span.into(), $msg.to_string());
        $crate::trigger_error(err)
    }};

    ($err:expr) => { $crate::trigger_error($err) };
}

/// Shortcut for `span_error!(Span::call_site(), msg...)`. This macro
/// is still preferable over plain panic, see [Motivation](#motivation)
#[macro_export]
macro_rules! call_site_error {
    ($fmt:literal, $($args:expr),*) => {{
        use $crate::span_error;

        let span = $crate::proc_macro2::Span::call_site();
        span_error!(span, $fmt, $($args),*)
    }};

    ($fmt:expr) => {{
        use $crate::span_error;

        let span = $crate::proc_macro2::Span::call_site();
        span_error!(span, $fmt)
    }};
}

/// This macro is supposed to be used at the top level of your `proc-macro`,
/// the function marked with a `#[proc_macro*]` attribute. It catches all the
/// errors triggered by [`span_error!`], [`call_site_error!`], and [`trigger_error`].
/// Once caught, it converts it to a [`proc_macro::TokenStream`]
/// containing a [`compile_error!`][compl_err] invocation.
///
/// See the [module-level documentation](self) for usage example
///
/// [compl_err]: https://doc.rust-lang.org/std/macro.compile_error.html
#[macro_export]
macro_rules! filter_macro_errors {
    ($($code:tt)*) => {
        let f = move || -> $crate::proc_macro::TokenStream { $($code)* };
        $crate::filter_macro_error_panics(f)
    };
}

/// An error in a proc-macro. This struct preserves
/// the given span so `rustc` can highlight the exact place in user code
/// responsible for the error.
///
/// You're not supposed to use this type directly, use [`span_error!`] and [`call_site_error!`].
#[derive(Debug)]
pub struct MacroError {
    span: Span,
    msg: String,
}

impl MacroError {
    /// Create an error with the span and message provided.
    pub fn new(span: Span, msg: String) -> Self {
        MacroError { span, msg }
    }

    /// A shortcut for `MacroError::new(Span::call_site(), message)`
    pub fn call_site(msg: String) -> Self {
        MacroError::new(Span::call_site(), msg)
    }

    /// Convert this error into a [`TokenStream`] containing these tokens: `compiler_error!(<message>);`.
    /// All these tokens carry the span this error contains attached.
    ///
    /// [compl_err]: https://doc.rust-lang.org/std/macro.compile_error.html
    pub fn into_compile_error(self) -> TokenStream {
        let MacroError { span, msg } = self;
        quote_spanned! { span=> compile_error!(#msg); }
    }

    /// Abandon the old span and replace it with the given one.
    pub fn set_span(&mut self, span: Span) {
        self.span = span;
    }
}

/// This traits expands [`Result<T, Into<MacroError>>`](std::result::Result) 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 [`span_error!`].
    fn unwrap_or_exit(self) -> Self::Ok;

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

/// This traits expands [`Option<T>`][std::option::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 [`call_site_error!`].
    /// 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_exit(self, msg: &str) -> Self::Some;
}

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

    fn unwrap_or_exit(self) -> T {
        match self {
            Ok(res) => res,
            Err(e) => trigger_error(e),
        }
    }

    fn expect_or_exit(self, message: &str) -> T {
        match self {
            Ok(res) => res,
            Err(e) => {
                let MacroError { msg, span } = e.into();
                let msg = format!("{}: {}", message, msg);
                trigger_error(MacroError::new(span, msg))
            }
        }
    }
}

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

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

impl From<syn::Error> for MacroError {
    fn from(e: syn::Error) -> Self {
        MacroError::new(e.span(), e.to_string())
    }
}

impl From<String> for MacroError {
    fn from(msg: String) -> Self {
        MacroError::call_site(msg)
    }
}

impl From<&str> for MacroError {
    fn from(msg: &str) -> Self {
        MacroError::call_site(msg.into())
    }
}

impl ToString for MacroError {
    fn to_string(&self) -> String {
        self.msg.clone()
    }
}

/// Trigger error, aborting the proc-macro's execution.
///
/// You're not supposed to use this function directly.
/// Use [`span_error!`] or [`call_site_error!`] instead.
pub fn trigger_error<T: Into<MacroError>>(err: T) -> ! {
    panic!(Payload(err.into()))
}

/// Execute the closure and catch all the panics triggered by
/// [`trigger_error`], converting them to [`proc_macro::TokenStream`].
/// All the other panics will be passed through as is.
///
/// You're not supposed to use this function directly, use [`filter_macro_errors!`]
/// instead.
pub fn filter_macro_error_panics<F>(f: F) -> proc_macro::TokenStream
where
    F: FnOnce() -> proc_macro::TokenStream,
{
    use std::panic::{catch_unwind, resume_unwind, AssertUnwindSafe};

    match catch_unwind(AssertUnwindSafe(f)) {
        Ok(stream) => stream,
        Err(boxed) => match boxed.downcast::<Payload>() {
            Ok(err) => err.0.into_compile_error().into(),
            Err(other) => resume_unwind(other),
        },
    }
}

struct Payload(MacroError);

// SAFE: Payload is private, a user can't use it to make any harm.
unsafe impl Send for Payload {}
unsafe impl Sync for Payload {}