lib.rs

//! 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.
//!
//! ## Usage
//!
//! First, add this to your `Cargo.toml`:
//!
//! ```toml
//! [dependencies]
//! tracing-attributes = "0.1.9"
//! ```
//!
//! *Compiler support: requires rustc 1.39+*
//!
//! 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
//! [span]: https://docs.rs/tracing/latest/tracing/span/index.html
//! [instrument]: attr.instrument.html
#![doc(html_root_url = "https://docs.rs/tracing-attributes/0.1.9")]
#![doc(
    html_logo_url = "https://raw.githubusercontent.com/tokio-rs/tracing/master/assets/logo.svg",
    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,
    unused_allocation,
    unused_comparisons,
    unused_parens,
    while_true
)]
// TODO: once `tracing` bumps its MSRV to 1.42, remove this allow.
#![allow(unused)]
extern crate proc_macro;

use std::collections::{HashMap, HashSet};
use std::iter;

use proc_macro2::TokenStream;
use quote::{quote, quote_spanned, ToTokens, TokenStreamExt as _};
use syn::ext::IdentExt as _;
use syn::parse::{Parse, ParseStream};
use syn::{
    punctuated::Punctuated, spanned::Spanned, AttributeArgs, Block, Expr, ExprCall, FieldPat,
    FnArg, Ident, Item, ItemFn, Lit, LitInt, LitStr, Meta, MetaList, MetaNameValue, NestedMeta,
    Pat, PatIdent, PatReference, PatStruct, PatTuple, PatTupleStruct, PatType, Path, Signature,
    Stmt, Token,
};
/// Instruments a function to create and enter a `tracing` [span] every time
/// the function is called.
///
/// The generated span's name will be the name of the function. Any arguments
/// to that function will be recorded as fields using [`fmt::Debug`]. 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 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` 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(())
/// }
/// ```
///
/// If `tracing_futures` is specified as a dependency in `Cargo.toml`,
/// `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 async functions on traits,
/// something not currently possible with rustc alone),
/// 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, v: usize) -> ();
/// }
///
/// #[derive(Debug)]
/// struct FooImpl;
///
/// #[async_trait]
/// impl Foo for FooImpl {
///     #[instrument(skip(self))]
///     async fn foo(&self, v: usize) {}
/// }
/// ```

///
/// [span]: https://docs.rs/tracing/latest/tracing/span/index.html
/// [`tracing`]: https://github.com/tokio-rs/tracing
/// [`fmt::Debug`]: https://doc.rust-lang.org/std/fmt/trait.Debug.html
#[proc_macro_attribute]
pub fn instrument(
    args: proc_macro::TokenStream,
    item: proc_macro::TokenStream,
) -> proc_macro::TokenStream {
    let input: ItemFn = syn::parse_macro_input!(item as ItemFn);
    let args = syn::parse_macro_input!(args as InstrumentArgs);

    // check for async_trait-like patterns in the block and wrap the
    // internal function with Instrument instead of wrapping the
    // async_trait generated wrapper
    if let Some(internal_fun_name) =
        get_async_trait_name(&input.block, input.sig.asyncness.is_some())
    {
        // let's rewrite some statements!
        let mut stmts: Vec<Stmt> = input.block.stmts.to_vec();
        for stmt in &mut stmts {
            if let Stmt::Item(Item::Fn(fun)) = stmt {
                // instrument the function if we considered it as the one we truly want to trace
                if fun.sig.ident == internal_fun_name {
                    *stmt = syn::parse2(gen_body(fun, args, Some(input.sig.ident.to_string())))
                        .unwrap();
                    break;
                }
            }
        }

        let sig = &input.sig;
        let attrs = &input.attrs;
        quote!(
            #(#attrs) *
            #sig {
                #(#stmts) *
            }
        )
        .into()
    } else {
        gen_body(&input, args, None).into()
    }
}

fn gen_body(
    input: &ItemFn,
    args: InstrumentArgs,
    fun_name: Option<String>,
) -> proc_macro2::TokenStream {
    // these are needed ahead of time, as ItemFn contains the function body _and_
    // isn't representable inside a quote!/quote_spanned! macro
    // (Syn's ToTokens isn't implemented for ItemFn)
    let ItemFn {
        attrs,
        vis,
        block,
        sig,
        ..
    } = input;

    let Signature {
        output: return_type,
        inputs: params,
        unsafety,
        asyncness,
        constness,
        abi,
        ident,
        generics:
            syn::Generics {
                params: gen_params,
                where_clause,
                ..
            },
        ..
    } = sig;

    let err = args.err;
    let warnings = args.warnings();

    // generate the span's name
    let span_name = args
        // did the user override the span's name?
        .name
        .as_ref()
        .map(|name| quote!(#name))
        // are we overriding the name because the span is inside a function
        // generated by `async-trait`?
        .or_else(|| fun_name.as_ref().map(|name| quote!(#name)))
        // if neither override is present, use the parsed function's name.
        .unwrap_or_else(|| {
            let name = ident.to_string();
            quote!(#name)
        });

    // generate this inside a closure, so we can return early on errors.
    let span = (|| {
        // Pull out the arguments-to-be-skipped first, so we can filter results
        // below.
        let param_names: Vec<(Ident, Ident)> = params
            .clone()
            .into_iter()
            .flat_map(|param| match param {
                FnArg::Typed(PatType { pat, .. }) => param_names(*pat),
                FnArg::Receiver(_) => Box::new(iter::once(Ident::new("self", param.span()))),
            })
            // if we are inside a function generated by async-trait, we
            // should take care to rewrite "_self" as "self" for
            // 'user convenience'
            .map(|x| {
                if fun_name.is_some() && x == "_self" {
                    (Ident::new("self", x.span()), x)
                } else {
                    (x.clone(), x)
                }
            })
            .collect();

        // TODO: allow the user to rename fields at will (all the
        // machinery should be here)

        // Little dance with new (user-exposed) names and old (internal)
        // names of identifiers. That way, you can do the following
        // even though async_trait rewrite "self" as "_self":
        // ```
        // #[async_trait]
        // impl Foo for FooImpl {
        //     #[instrument(skip(self))]
        //     async fn foo(&self, v: usize) {}
        // }
        // ```

        for skip in &args.skips {
            if !param_names.iter().map(|(user, _)| user).any(|y| y == skip) {
                return quote_spanned! {skip.span()=>
                    compile_error!("attempting to skip non-existent parameter")
                };
            }
        }

        let level = args.level();
        let target = args.target();

        // filter out skipped fields
        let mut quoted_fields: Vec<_> = param_names
            .into_iter()
            .filter(|(param, _)| {
                if args.skips.contains(param) {
                    return false;
                }

                // If any parameters have the same name as a custom field, skip
                // and allow them to be formatted by the custom field.
                if let Some(ref fields) = args.fields {
                    fields.0.iter().all(|Field { ref name, .. }| {
                        let first = name.first();
                        first != name.last() || !first.iter().any(|name| name == &param)
                    })
                } else {
                    true
                }
            })
            .map(|(user_name, real_name)| quote!(#user_name = tracing::field::debug(&#real_name)))
            .collect();
        let custom_fields = &args.fields;
        let custom_fields = if quoted_fields.is_empty() {
            quote! { #custom_fields }
        } else {
            quote! {, #custom_fields }
        };

        quote!(tracing::span!(
            target: #target,
            #level,
            #span_name,
            #(#quoted_fields),*
            #custom_fields

        ))
    })();

    // Generate the instrumented function body.
    // If the function is an `async fn`, this will wrap it in an async block,
    // which is `instrument`ed using `tracing-futures`. Otherwise, this will
    // enter the span and then perform the rest of the body.
    // If `err` is in args, instrument any resulting `Err`s.
    let body = if asyncness.is_some() {
        if err {
            quote_spanned! {block.span()=>
                let __tracing_attr_span = #span;
                tracing_futures::Instrument::instrument(async move {
                    match async move { #block }.await {
                        Ok(x) => Ok(x),
                        Err(e) => {
                            tracing::error!(error = %e);
                            Err(e)
                        }
                    }
                }, __tracing_attr_span).await
            }
        } else {
            quote_spanned!(block.span()=>
                let __tracing_attr_span = #span;
                    tracing_futures::Instrument::instrument(
                        async move { #block },
                        __tracing_attr_span
                    )
                    .await
            )
        }
    } else if err {
        quote_spanned!(block.span()=>
            let __tracing_attr_span = #span;
            let __tracing_attr_guard = __tracing_attr_span.enter();
            match { #block } {
                Ok(x) => Ok(x),
                Err(e) => {
                    tracing::error!(error = %e);
                    Err(e)
                }
            }
        )
    } else {
        quote_spanned!(block.span()=>
            let __tracing_attr_span = #span;
            let __tracing_attr_guard = __tracing_attr_span.enter();
            #block
        )
    };

    quote!(
        #(#attrs) *
        #vis #constness #unsafety #asyncness #abi fn #ident<#gen_params>(#params) #return_type
        #where_clause
        {
            #warnings
            #body
        }
    )
}

#[derive(Default, Debug)]
struct InstrumentArgs {
    level: Option<Level>,
    name: Option<LitStr>,
    target: Option<LitStr>,
    skips: HashSet<Ident>,
    fields: Option<Fields>,
    err: bool,
    /// Errors describing any unrecognized parse inputs that we skipped.
    parse_warnings: Vec<syn::Error>,
}

impl InstrumentArgs {
    fn level(&self) -> impl ToTokens {
        fn is_level(lit: &LitInt, expected: u64) -> bool {
            match lit.base10_parse::<u64>() {
                Ok(value) => value == expected,
                Err(_) => false,
            }
        }

        match &self.level {
            Some(Level::Str(ref lit)) if lit.value().eq_ignore_ascii_case("trace") => {
                quote!(tracing::Level::TRACE)
            }
            Some(Level::Str(ref lit)) if lit.value().eq_ignore_ascii_case("debug") => {
                quote!(tracing::Level::DEBUG)
            }
            Some(Level::Str(ref lit)) if lit.value().eq_ignore_ascii_case("info") => {
                quote!(tracing::Level::INFO)
            }
            Some(Level::Str(ref lit)) if lit.value().eq_ignore_ascii_case("warn") => {
                quote!(tracing::Level::WARN)
            }
            Some(Level::Str(ref lit)) if lit.value().eq_ignore_ascii_case("error") => {
                quote!(tracing::Level::ERROR)
            }
            Some(Level::Int(ref lit)) if is_level(lit, 1) => quote!(tracing::Level::TRACE),
            Some(Level::Int(ref lit)) if is_level(lit, 2) => quote!(tracing::Level::DEBUG),
            Some(Level::Int(ref lit)) if is_level(lit, 3) => quote!(tracing::Level::INFO),
            Some(Level::Int(ref lit)) if is_level(lit, 4) => quote!(tracing::Level::WARN),
            Some(Level::Int(ref lit)) if is_level(lit, 5) => quote!(tracing::Level::ERROR),
            Some(Level::Path(ref pat)) => quote!(#pat),
            Some(lit) => quote! {
                compile_error!(
                    "unknown verbosity level, expected one of \"trace\", \
                     \"debug\", \"info\", \"warn\", or \"error\", or a number 1-5"
                )
            },
            None => quote!(tracing::Level::INFO),
        }
    }

    fn target(&self) -> impl ToTokens {
        if let Some(ref target) = self.target {
            quote!(#target)
        } else {
            quote!(module_path!())
        }
    }

    /// Generate "deprecation" warnings for any unrecognized attribute inputs
    /// that we skipped.
    ///
    /// For backwards compatibility, we need to emit compiler warnings rather
    /// than errors for unrecognized inputs. Generating a fake deprecation is
    /// the only way to do this on stable Rust right now.
    fn warnings(&self) -> impl ToTokens {
        let warnings = self.parse_warnings.iter().map(|err| {
            let msg = format!("found unrecognized input, {}", err);
            let msg = LitStr::new(&msg, err.span());
            // TODO(eliza): This is a bit of a hack, but it's just about the
            // only way to emit warnings from a proc macro on stable Rust.
            // Eventually, when the `proc_macro::Diagnostic` API stabilizes, we
            // should definitely use that instead.
            quote_spanned! {err.span()=>
                #[warn(deprecated)]
                {
                    #[deprecated(since = "not actually deprecated", note = #msg)]
                    const TRACING_INSTRUMENT_WARNING: () = ();
                    let _ = TRACING_INSTRUMENT_WARNING;
                }
            }
        });
        quote! {
            { #(#warnings)* }
        }
    }
}

impl Parse for InstrumentArgs {
    fn parse(input: ParseStream<'_>) -> syn::Result<Self> {
        let mut args = Self::default();
        while !input.is_empty() {
            let lookahead = input.lookahead1();
            if lookahead.peek(kw::name) {
                if args.name.is_some() {
                    return Err(input.error("expected only a single `name` argument"));
                }
                let name = input.parse::<StrArg<kw::name>>()?.value;
                args.name = Some(name);
            } else if lookahead.peek(LitStr) {
                // XXX: apparently we support names as either named args with an
                // sign, _or_ as unnamed string literals. That's weird, but
                // changing it is apparently breaking.
                if args.name.is_some() {
                    return Err(input.error("expected only a single `name` argument"));
                }
                args.name = Some(input.parse()?);
            } else if lookahead.peek(kw::target) {
                if args.target.is_some() {
                    return Err(input.error("expected only a single `target` argument"));
                }
                let target = input.parse::<StrArg<kw::target>>()?.value;
                args.target = Some(target);
            } else if lookahead.peek(kw::level) {
                if args.level.is_some() {
                    return Err(input.error("expected only a single `level` argument"));
                }
                args.level = Some(input.parse()?);
            } else if lookahead.peek(kw::skip) {
                if !args.skips.is_empty() {
                    return Err(input.error("expected only a single `skip` argument"));
                }
                let Skips(skips) = input.parse()?;
                args.skips = skips;
            } else if lookahead.peek(kw::fields) {
                if args.fields.is_some() {
                    return Err(input.error("expected only a single `fields` argument"));
                }
                args.fields = Some(input.parse()?);
            } else if lookahead.peek(kw::err) {
                let _ = input.parse::<kw::err>()?;
                args.err = true;
            } else if lookahead.peek(Token![,]) {
                let _ = input.parse::<Token![,]>()?;
            } else {
                // We found a token that we didn't expect!
                // We want to emit warnings for these, rather than errors, so
                // we'll add it to the list of unrecognized inputs we've seen so
                // far and keep going.
                args.parse_warnings.push(lookahead.error());
                // Parse the unrecognized token tree to advance the parse
                // stream, and throw it away so we can keep parsing.
                let _ = input.parse::<proc_macro2::TokenTree>();
            }
        }
        Ok(args)
    }
}

struct StrArg<T> {
    value: LitStr,
    _p: std::marker::PhantomData<T>,
}

impl<T: Parse> Parse for StrArg<T> {
    fn parse(input: ParseStream<'_>) -> syn::Result<Self> {
        let _ = input.parse::<T>()?;
        let _ = input.parse::<Token![=]>()?;
        let value = input.parse()?;
        Ok(Self {
            value,
            _p: std::marker::PhantomData,
        })
    }
}

struct Skips(HashSet<Ident>);

impl Parse for Skips {
    fn parse(input: ParseStream<'_>) -> syn::Result<Self> {
        let _ = input.parse::<kw::skip>();
        let content;
        let _ = syn::parenthesized!(content in input);
        let names: Punctuated<Ident, Token![,]> = content.parse_terminated(Ident::parse_any)?;
        let mut skips = HashSet::new();
        for name in names {
            if skips.contains(&name) {
                return Err(syn::Error::new(
                    name.span(),
                    "tried to skip the same field twice",
                ));
            } else {
                skips.insert(name);
            }
        }
        Ok(Self(skips))
    }
}

#[derive(Debug)]
struct Fields(Punctuated<Field, Token![,]>);

#[derive(Debug)]
struct Field {
    name: Punctuated<Ident, Token![.]>,
    value: Option<Expr>,
    kind: FieldKind,
}

#[derive(Debug, Eq, PartialEq)]
enum FieldKind {
    Debug,
    Display,
    Value,
}

impl Parse for Fields {
    fn parse(input: ParseStream<'_>) -> syn::Result<Self> {
        let _ = input.parse::<kw::fields>();
        let content;
        let _ = syn::parenthesized!(content in input);
        let fields: Punctuated<_, Token![,]> = content.parse_terminated(Field::parse)?;
        Ok(Self(fields))
    }
}

impl ToTokens for Fields {
    fn to_tokens(&self, tokens: &mut TokenStream) {
        self.0.to_tokens(tokens)
    }
}

impl Parse for Field {
    fn parse(input: ParseStream<'_>) -> syn::Result<Self> {
        let mut kind = FieldKind::Value;
        if input.peek(Token![%]) {
            input.parse::<Token![%]>()?;
            kind = FieldKind::Display;
        } else if input.peek(Token![?]) {
            input.parse::<Token![?]>()?;
            kind = FieldKind::Debug;
        };
        let name = Punctuated::parse_separated_nonempty_with(input, Ident::parse_any)?;
        let value = if input.peek(Token![=]) {
            input.parse::<Token![=]>()?;
            if input.peek(Token![%]) {
                input.parse::<Token![%]>()?;
                kind = FieldKind::Display;
            } else if input.peek(Token![?]) {
                input.parse::<Token![?]>()?;
                kind = FieldKind::Debug;
            };
            Some(input.parse()?)
        } else {
            None
        };
        Ok(Self { name, kind, value })
    }
}

impl ToTokens for Field {
    fn to_tokens(&self, tokens: &mut TokenStream) {
        if let Some(ref value) = self.value {
            let name = &self.name;
            let kind = &self.kind;
            tokens.extend(quote! {
                #name = #kind#value
            })
        } else if self.kind == FieldKind::Value {
            // XXX(eliza): I don't like that fields without values produce
            // empty fields rather than local variable shorthand...but,
            // we've released a version where field names without values in
            // `instrument` produce empty field values, so changing it now
            // is a breaking change. agh.
            let name = &self.name;
            tokens.extend(quote!(#name = tracing::field::Empty))
        } else {
            self.kind.to_tokens(tokens);
            self.name.to_tokens(tokens);
        }
    }
}

impl ToTokens for FieldKind {
    fn to_tokens(&self, tokens: &mut TokenStream) {
        match self {
            FieldKind::Debug => tokens.extend(quote! { ? }),
            FieldKind::Display => tokens.extend(quote! { % }),
            _ => {}
        }
    }
}

#[derive(Debug)]
enum Level {
    Str(LitStr),
    Int(LitInt),
    Path(Path),
}

impl Parse for Level {
    fn parse(input: ParseStream<'_>) -> syn::Result<Self> {
        let _ = input.parse::<kw::level>()?;
        let _ = input.parse::<Token![=]>()?;
        let lookahead = input.lookahead1();
        if lookahead.peek(LitStr) {
            Ok(Self::Str(input.parse()?))
        } else if lookahead.peek(LitInt) {
            Ok(Self::Int(input.parse()?))
        } else if lookahead.peek(Ident) {
            Ok(Self::Path(input.parse()?))
        } else {
            Err(lookahead.error())
        }
    }
}

fn param_names(pat: Pat) -> Box<dyn Iterator<Item = Ident>> {
    match pat {
        Pat::Ident(PatIdent { ident, .. }) => Box::new(iter::once(ident)),
        Pat::Reference(PatReference { pat, .. }) => param_names(*pat),
        Pat::Struct(PatStruct { fields, .. }) => Box::new(
            fields
                .into_iter()
                .flat_map(|FieldPat { pat, .. }| param_names(*pat)),
        ),
        Pat::Tuple(PatTuple { elems, .. }) => Box::new(elems.into_iter().flat_map(param_names)),
        Pat::TupleStruct(PatTupleStruct {
            pat: PatTuple { elems, .. },
            ..
        }) => Box::new(elems.into_iter().flat_map(param_names)),

        // The above *should* cover all cases of irrefutable patterns,
        // but we purposefully don't do any funny business here
        // (such as panicking) because that would obscure rustc's
        // much more informative error message.
        _ => Box::new(iter::empty()),
    }
}

mod kw {
    syn::custom_keyword!(fields);
    syn::custom_keyword!(skip);
    syn::custom_keyword!(level);
    syn::custom_keyword!(target);
    syn::custom_keyword!(name);
    syn::custom_keyword!(err);
}

// Get the name of the inner function we need to hook, if the function
// was generated by async-trait.
// When we are given a function generated by async-trait, that function
// is only a "temporary" one that returns a pinned future, and it is
// that pinned future that needs to be instrumented, otherwise we will
// only collect information on the moment the future was "built",
// and not its true span of execution.
// So we inspect the block of the function to find if we can find the
// pattern `async fn foo<...>(...) {...}; Box::pin(foo<...>(...))` and
// return the name `foo` if that is the case. Our caller will then be
// able to use that information to instrument the proper function.
// (this follows the approach suggested in
// https://github.com/dtolnay/async-trait/issues/45#issuecomment-571245673)
fn get_async_trait_name(block: &Block, block_is_async: bool) -> Option<String> {
    // are we in an async context? If yes, this isn't a async_trait-like pattern
    if block_is_async {
        return None;
    }

    // list of async functions declared inside the block
    let mut inside_funs = Vec::new();
    // last expression declared in the block (it determines the return
    // value of the block, so that if we are working on a function
    // whose `trait` or `impl` declaration is annotated by async_trait,
    // this is quite likely the point where the future is pinned)
    let mut last_expr = None;

    // obtain the list of direct internal functions and the last
    // expression of the block
    for stmt in &block.stmts {
        if let Stmt::Item(Item::Fn(fun)) = &stmt {
            // is the function declared as async? If so, this is a good
            // candidate, let's keep it in hand
            if fun.sig.asyncness.is_some() {
                inside_funs.push(fun.sig.ident.to_string());
            }
        } else if let Stmt::Expr(e) = &stmt {
            last_expr = Some(e);
        }
    }

    // let's play with (too much) pattern matching
    // is the last expression a function call?
    if let Some(Expr::Call(ExprCall {
        func: outside_func,
        args: outside_args,
        ..
    })) = last_expr
    {
        if let Expr::Path(path) = outside_func.as_ref() {
            // is it a call to `Box::pin()`?
            if "Box::pin" == path_to_string(&path.path) {
                // does it takes at least an argument? (if it doesn't,
                // it's not gonna compile anyway, but that's no reason
                // to (try to) perform an out of bounds access)
                if outside_args.is_empty() {
                    return None;
                }
                // is the argument to Box::pin a function call itself?
                if let Expr::Call(ExprCall { func, args, .. }) = &outside_args[0] {
                    if let Expr::Path(inside_path) = func.as_ref() {
                        // "stringify" the path of the function called
                        let func_name = path_to_string(&inside_path.path);
                        // is this function directly defined insided the current block?
                        if inside_funs.contains(&func_name) {
                            // we must hook this function now
                            return Some(func_name);
                        }
                    }
                }
            }
        }
    }
    None
}

// Return a path as a String
fn path_to_string(path: &Path) -> String {
    use std::fmt::Write;
    // some heuristic to prevent too many allocations
    let mut res = String::with_capacity(path.segments.len() * 5);
    for i in 0..path.segments.len() {
        write!(&mut res, "{}", path.segments[i].ident)
            .expect("writing to a String should never fail");
        if i < path.segments.len() - 1 {
            res.push_str("::");
        }
    }
    res
}