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//! A procedural macro attribute for instrumenting functions with [`tracing`].
//!
//! [`tracing`] is a framework for instrumenting Rust programs to collect
//! structured, event-based diagnostic information. This crate provides the
//! [`#[instrument]`][instrument] procedural macro attribute.
//!
//! Note that this macro is also re-exported by the main `tracing` crate.
//!
//! *Compiler support: [requires `rustc` 1.42+][msrv]*
//!
//! [msrv]: #supported-rust-versions
//!
//! ## Usage
//!
//! First, add this to your `Cargo.toml`:
//!
//! ```toml
//! [dependencies]
//! tracing-attributes = "0.1.11"
//! ```
//!
//! The [`#[instrument]`][instrument] attribute can now be added to a function
//! to automatically create and enter `tracing` [span] when that function is
//! called. For example:
//!
//! ```
//! use tracing_attributes::instrument;
//!
//! #[instrument]
//! pub fn my_function(my_arg: usize) {
//!     // ...
//! }
//!
//! # fn main() {}
//! ```
//!
//! [`tracing`]: https://crates.io/crates/tracing
//! [instrument]: macro@instrument
//! [span]: https://docs.rs/tracing/latest/tracing/span/index.html
//!
//! ## Supported Rust Versions
//!
//! Tracing is built against the latest stable release. The minimum supported
//! version is 1.42. The current Tracing version is not guaranteed to build on
//! Rust versions earlier than the minimum supported version.
//!
//! Tracing follows the same compiler support policies as the rest of the Tokio
//! project. The current stable Rust compiler and the three most recent minor
//! versions before it will always be supported. For example, if the current
//! stable compiler version is 1.45, the minimum supported version will not be
//! increased past 1.42, three minor versions prior. Increasing the minimum
//! supported compiler version is not considered a semver breaking change as
//! long as doing so complies with this policy.
//!
#![doc(html_root_url = "https://docs.rs/tracing-attributes/0.1.11")]
#![doc(
    html_logo_url = "https://raw.githubusercontent.com/tokio-rs/tracing/master/assets/logo-type.png",
    html_favicon_url = "https://raw.githubusercontent.com/tokio-rs/tracing/master/assets/favicon.ico",
    issue_tracker_base_url = "https://github.com/tokio-rs/tracing/issues/"
)]
#![warn(
    missing_debug_implementations,
    missing_docs,
    rust_2018_idioms,
    unreachable_pub,
    bad_style,
    const_err,
    dead_code,
    improper_ctypes,
    non_shorthand_field_patterns,
    no_mangle_generic_items,
    overflowing_literals,
    path_statements,
    patterns_in_fns_without_body,
    private_in_public,
    unconditional_recursion,
    unused_allocation,
    unused_comparisons,
    unused_parens,
    while_true
)]

use proc_macro2::TokenStream;
use quote::ToTokens;
use syn::parse::{Parse, ParseStream};
use syn::{Attribute, Block, ItemFn, Signature, Visibility};

mod attr;
mod expand;
/// Instruments a function to create and enter a `tracing` [span] every time
/// the function is called.
///
/// Unless overriden, a span with `info` level will be generated.
/// The generated span's name will be the name of the function.
/// By default, all arguments to the function are included as fields on the
/// span. Arguments that are `tracing` [primitive types] implementing the
/// [`Value` trait] will be recorded as fields of that type. Types which do
/// not implement `Value` will be recorded using [`std::fmt::Debug`].
///
/// [primitive types]: https://docs.rs/tracing/latest/tracing/field/trait.Value.html#foreign-impls
/// [`Value` trait]: https://docs.rs/tracing/latest/tracing/field/trait.Value.html
///
/// To skip recording a function's or method's argument, pass the argument's name
/// to the `skip` argument on the `#[instrument]` macro. For example,
/// `skip` can be used when an argument to an instrumented function does
/// not implement [`fmt::Debug`], or to exclude an argument with a verbose
/// or costly Debug implementation. Note that:
/// - multiple argument names can be passed to `skip`.
/// - arguments passed to `skip` do _not_ need to implement `fmt::Debug`.
///
/// You can also pass additional fields (key-value pairs with arbitrary data)
/// to the generated span. This is achieved using the `fields` argument on the
/// `#[instrument]` macro. You can use a string, integer or boolean literal as
/// a value for each field. The name of the field must be a single valid Rust
/// identifier, nested (dotted) field names are not supported.
///
/// Note that overlap between the names of fields and (non-skipped) arguments
/// will result in a compile error.
///
/// # Examples
/// Instrumenting a function:
/// ```
/// # use tracing_attributes::instrument;
/// #[instrument]
/// pub fn my_function(my_arg: usize) {
///     // This event will be recorded inside a span named `my_function` with the
///     // field `my_arg`.
///     tracing::info!("inside my_function!");
///     // ...
/// }
/// ```
/// Setting the level for the generated span:
/// ```
/// # use tracing_attributes::instrument;
/// #[instrument(level = "debug")]
/// pub fn my_function() {
///     // ...
/// }
/// ```
/// Overriding the generated span's name:
/// ```
/// # use tracing_attributes::instrument;
/// #[instrument(name = "my_name")]
/// pub fn my_function() {
///     // ...
/// }
/// ```
/// Overriding the generated span's target:
/// ```
/// # use tracing_attributes::instrument;
/// #[instrument(target = "my_target")]
/// pub fn my_function() {
///     // ...
/// }
/// ```
///
/// To skip recording an argument, pass the argument's name to the `skip`:
///
/// ```
/// # use tracing_attributes::instrument;
/// struct NonDebug;
///
/// #[instrument(skip(non_debug))]
/// fn my_function(arg: usize, non_debug: NonDebug) {
///     // ...
/// }
/// ```
///
/// To add an additional context to the span, you can pass key-value pairs to `fields`:
///
/// ```
/// # use tracing_attributes::instrument;
/// #[instrument(fields(foo="bar", id=1, show=true))]
/// fn my_function(arg: usize) {
///     // ...
/// }
/// ```
///
/// If the function returns a `Result<T, E>` and `E` implements `std::fmt::Display`, you can add
/// `err` or `err(Display)` to emit error events when the function returns `Err`:
///
/// ```
/// # use tracing_attributes::instrument;
/// #[instrument(err)]
/// fn my_function(arg: usize) -> Result<(), std::io::Error> {
///     Ok(())
/// }
/// ```
///
/// By default, error values will be recorded using their `std::fmt::Display` implementations.
/// If an error implements `std::fmt::Debug`, it can be recorded using its `Debug` implementation
/// instead, by writing `err(Debug)`:
///
/// ```
/// # use tracing_attributes::instrument;
/// #[instrument(err(Debug))]
/// fn my_function(arg: usize) -> Result<(), std::io::Error> {
///     Ok(())
/// }
/// ```
///
/// `async fn`s may also be instrumented:
///
/// ```
/// # use tracing_attributes::instrument;
/// #[instrument]
/// pub async fn my_function() -> Result<(), ()> {
///     // ...
///     # Ok(())
/// }
/// ```
///
/// It also works with [async-trait](https://crates.io/crates/async-trait)
/// (a crate that allows defining async functions in traits,
/// something not currently possible in Rust),
/// and hopefully most libraries that exhibit similar behaviors:
///
/// ```
/// # use tracing::instrument;
/// use async_trait::async_trait;
///
/// #[async_trait]
/// pub trait Foo {
///     async fn foo(&self, arg: usize);
/// }
///
/// #[derive(Debug)]
/// struct FooImpl(usize);
///
/// #[async_trait]
/// impl Foo for FooImpl {
///     #[instrument(fields(value = self.0, tmp = std::any::type_name::<Self>()))]
///     async fn foo(&self, arg: usize) {}
/// }
/// ```
///
/// Note than on `async-trait` <= 0.1.43, references to the `Self`
/// type inside the `fields` argument were only allowed when the instrumented
/// function is a method (i.e., the function receives `self` as an argument).
/// For example, this *used to not work* because the instrument function
/// didn't receive `self`:
/// ```
/// # use tracing::instrument;
/// use async_trait::async_trait;
///
/// #[async_trait]
/// pub trait Bar {
///     async fn bar();
/// }
///
/// #[derive(Debug)]
/// struct BarImpl(usize);
///
/// #[async_trait]
/// impl Bar for BarImpl {
///     #[instrument(fields(tmp = std::any::type_name::<Self>()))]
///     async fn bar() {}
/// }
/// ```
/// Instead, you should manually rewrite any `Self` types as the type for
/// which you implement the trait: `#[instrument(fields(tmp = std::any::type_name::<Bar>()))]`
/// (or maybe you can just bump `async-trait`).
///
/// [span]: https://docs.rs/tracing/latest/tracing/span/index.html
/// [`tracing`]: https://github.com/tokio-rs/tracing
/// [`fmt::Debug`]: std::fmt::Debug
#[proc_macro_attribute]
pub fn instrument(
    args: proc_macro::TokenStream,
    item: proc_macro::TokenStream,
) -> proc_macro::TokenStream {
    let args = syn::parse_macro_input!(args as attr::InstrumentArgs);
    // Cloning a `TokenStream` is cheap since it's reference counted internally.
    instrument_precise(args.clone(), item.clone())
        .unwrap_or_else(|_err| instrument_speculative(args, item))
}

/// Instrument the function, without parsing the function body (instead using the raw tokens).
fn instrument_speculative(
    args: attr::InstrumentArgs,
    item: proc_macro::TokenStream,
) -> proc_macro::TokenStream {
    let input = syn::parse_macro_input!(item as MaybeItemFn);
    let instrumented_function_name = input.sig.ident.to_string();
    expand::gen_function(
        input.as_ref(),
        args,
        instrumented_function_name.as_str(),
        None,
    )
    .into()
}

/// Instrument the function, by fully parsing the function body,
/// which allows us to rewrite some statements related to async_trait-like patterns.
fn instrument_precise(
    args: attr::InstrumentArgs,
    item: proc_macro::TokenStream,
) -> Result<proc_macro::TokenStream, syn::Error> {
    let input = syn::parse::<ItemFn>(item)?;
    let instrumented_function_name = input.sig.ident.to_string();

    // check for async_trait-like patterns in the block, and instrument
    // the future instead of the wrapper
    if let Some(async_trait) = expand::AsyncTraitInfo::from_fn(&input) {
        return Ok(async_trait.gen_async_trait(args, instrumented_function_name.as_str()));
    }

    Ok(expand::gen_function(
        (&input).into(),
        args,
        instrumented_function_name.as_str(),
        None,
    )
    .into())
}

/// This is a more flexible/imprecise `ItemFn` type,
/// which's block is just a `TokenStream` (it may contain invalid code).
#[derive(Debug, Clone)]
struct MaybeItemFn {
    attrs: Vec<Attribute>,
    vis: Visibility,
    sig: Signature,
    block: TokenStream,
}

impl MaybeItemFn {
    fn as_ref(&self) -> MaybeItemFnRef<'_, TokenStream> {
        MaybeItemFnRef {
            attrs: &self.attrs,
            vis: &self.vis,
            sig: &self.sig,
            block: &self.block,
        }
    }
}

/// This parses a `TokenStream` into a `MaybeItemFn`
/// (just like `ItemFn`, but skips parsing the body).
impl Parse for MaybeItemFn {
    fn parse(input: ParseStream<'_>) -> syn::Result<Self> {
        let attrs = input.call(syn::Attribute::parse_outer)?;
        let vis: Visibility = input.parse()?;
        let sig: Signature = input.parse()?;
        let block: TokenStream = input.parse()?;
        Ok(Self {
            attrs,
            vis,
            sig,
            block,
        })
    }
}

/// A generic reference type for `MaybeItemFn`,
/// that takes a generic block type `B` that implements `ToTokens` (eg. `TokenStream`, `Block`).
#[derive(Debug, Clone)]
struct MaybeItemFnRef<'a, B: ToTokens> {
    attrs: &'a Vec<Attribute>,
    vis: &'a Visibility,
    sig: &'a Signature,
    block: &'a B,
}

impl<'a> From<&'a ItemFn> for MaybeItemFnRef<'a, Box<Block>> {
    fn from(val: &'a ItemFn) -> Self {
        MaybeItemFnRef {
            attrs: &val.attrs,
            vis: &val.vis,
            sig: &val.sig,
            block: &val.block,
        }
    }
}