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 </pre><pre class="rust"><code><span class="kw">use </span>std::iter;

 <span class="kw">use </span>proc_macro2::TokenStream;
 <span class="kw">use </span>quote::{quote, quote_spanned, ToTokens};
 <span class="kw">use </span>syn::visit_mut::VisitMut;
 <span class="kw">use </span>syn::{
     punctuated::Punctuated, spanned::Spanned, Block, Expr, ExprAsync, ExprCall, FieldPat, FnArg,
     Ident, Item, ItemFn, Pat, PatIdent, PatReference, PatStruct, PatTuple, PatTupleStruct, PatType,
     Path, ReturnType, Signature, Stmt, Token, Type, TypePath,
 };

 <span class="kw">use crate</span>::{
     attr::{Field, Fields, FormatMode, InstrumentArgs, Level},
     MaybeItemFn, MaybeItemFnRef,
 };

 <span class="doccomment">/// Given an existing function, generate an instrumented version of that function
 </span><span class="kw">pub</span>(<span class="kw">crate</span>) <span class="kw">fn </span>gen_function&lt;<span class="lifetime">&#39;a</span>, B: ToTokens + <span class="lifetime">&#39;a</span>&gt;(
     input: MaybeItemFnRef&lt;<span class="lifetime">&#39;a</span>, B&gt;,
     args: InstrumentArgs,
     instrumented_function_name: <span class="kw-2">&amp;</span>str,
     self_type: <span class="prelude-ty">Option</span>&lt;<span class="kw-2">&amp;</span>TypePath&gt;,
 ) -&gt; proc_macro2::TokenStream {
     <span class="comment">// these are needed ahead of time, as ItemFn contains the function body _and_
     // isn&#39;t representable inside a quote!/quote_spanned! macro
     // (Syn&#39;s ToTokens isn&#39;t implemented for ItemFn)
     </span><span class="kw">let </span>MaybeItemFnRef {
         outer_attrs,
         inner_attrs,
         vis,
         sig,
         block,
     } = input;

     <span class="kw">let </span>Signature {
         output,
         inputs: params,
         unsafety,
         asyncness,
         constness,
         abi,
         ident,
         generics:
             syn::Generics {
                 params: gen_params,
                 where_clause,
                 ..
             },
         ..
     } = sig;

     <span class="kw">let </span>warnings = args.warnings();

     <span class="kw">let </span>(return_type, return_span) = <span class="kw">if let </span>ReturnType::Type(<span class="kw">_</span>, return_type) = <span class="kw-2">&amp;</span>output {
         (erase_impl_trait(return_type), return_type.span())
     } <span class="kw">else </span>{
         <span class="comment">// Point at function name if we don&#39;t have an explicit return type
         </span>(<span class="macro">syn::parse_quote! </span>{ () }, ident.span())
     };
     <span class="comment">// Install a fake return statement as the first thing in the function
     // body, so that we eagerly infer that the return type is what we
     // declared in the async fn signature.
     // The `#[allow(..)]` is given because the return statement is
     // unreachable, but does affect inference, so it needs to be written
     // exactly that way for it to do its magic.
     </span><span class="kw">let </span>fake_return_edge = <span class="macro">quote_spanned! </span>{return_span=&gt;
         <span class="attribute">#[allow(unreachable_code, clippy::diverging_sub_expression, clippy::let_unit_value, clippy::unreachable)]
         </span><span class="kw">if </span><span class="bool-val">false </span>{
             <span class="kw">let </span>__tracing_attr_fake_return: #return_type =
                 <span class="macro">unreachable!</span>(<span class="string">&quot;this is just for type inference, and is unreachable code&quot;</span>);
             <span class="kw">return </span>__tracing_attr_fake_return;
         }
     };
     <span class="kw">let </span>block = <span class="macro">quote! </span>{
         {
             #fake_return_edge
             #block
         }
     };

     <span class="kw">let </span>body = gen_block(
         <span class="kw-2">&amp;</span>block,
         params,
         asyncness.is_some(),
         args,
         instrumented_function_name,
         self_type,
     );

     <span class="macro">quote!</span>(
         #(#outer_attrs) *
         #vis #constness #unsafety #asyncness #abi <span class="kw">fn </span>#ident&lt;#gen_params&gt;(#params) #output
         #where_clause
         {
             #(#inner_attrs) *
             #warnings
             #body
         }
     )
 }

 <span class="doccomment">/// Instrument a block
 </span><span class="kw">fn </span>gen_block&lt;B: ToTokens&gt;(
     block: <span class="kw-2">&amp;</span>B,
     params: <span class="kw-2">&amp;</span>Punctuated&lt;FnArg, <span class="macro">Token!</span>[,]&gt;,
     async_context: bool,
     <span class="kw-2">mut </span>args: InstrumentArgs,
     instrumented_function_name: <span class="kw-2">&amp;</span>str,
     self_type: <span class="prelude-ty">Option</span>&lt;<span class="kw-2">&amp;</span>TypePath&gt;,
 ) -&gt; proc_macro2::TokenStream {
     <span class="comment">// generate the span&#39;s name
     </span><span class="kw">let </span>span_name = args
         <span class="comment">// did the user override the span&#39;s name?
         </span>.name
         .as_ref()
         .map(|name| <span class="macro">quote!</span>(#name))
         .unwrap_or_else(|| <span class="macro">quote!</span>(#instrumented_function_name));

     <span class="kw">let </span>args_level = args.level();
     <span class="kw">let </span>level = args_level.clone();

     <span class="kw">let </span>follows_from = args.follows_from.iter();
     <span class="kw">let </span>follows_from = <span class="macro">quote! </span>{
         #(<span class="kw">for </span>cause <span class="kw">in </span>#follows_from {
             __tracing_attr_span.follows_from(cause);
         })*
     };

     <span class="comment">// generate this inside a closure, so we can return early on errors.
     </span><span class="kw">let </span>span = (|| {
         <span class="comment">// Pull out the arguments-to-be-skipped first, so we can filter results
         // below.
         </span><span class="kw">let </span>param_names: Vec&lt;(Ident, (Ident, RecordType))&gt; = params
             .clone()
             .into_iter()
             .flat_map(|param| <span class="kw">match </span>param {
                 FnArg::Typed(PatType { pat, ty, .. }) =&gt; {
                     param_names(<span class="kw-2">*</span>pat, RecordType::parse_from_ty(<span class="kw-2">&amp;</span>ty))
                 }
                 FnArg::Receiver(<span class="kw">_</span>) =&gt; Box::new(iter::once((
                     Ident::new(<span class="string">&quot;self&quot;</span>, param.span()),
                     RecordType::Debug,
                 ))),
             })
             <span class="comment">// Little dance with new (user-exposed) names and old (internal)
             // names of identifiers. That way, we could do the following
             // even though async_trait (&lt;=0.1.43) rewrites &quot;self&quot; as &quot;_self&quot;:
             // ```
             // #[async_trait]
             // impl Foo for FooImpl {
             //     #[instrument(skip(self))]
             //     async fn foo(&amp;self, v: usize) {}
             // }
             // ```
             </span>.map(|(x, record_type)| {
                 <span class="comment">// if we are inside a function generated by async-trait &lt;=0.1.43, we need to
                 // take care to rewrite &quot;_self&quot; as &quot;self&quot; for &#39;user convenience&#39;
                 </span><span class="kw">if </span>self_type.is_some() &amp;&amp; x == <span class="string">&quot;_self&quot; </span>{
                     (Ident::new(<span class="string">&quot;self&quot;</span>, x.span()), (x, record_type))
                 } <span class="kw">else </span>{
                     (x.clone(), (x, record_type))
                 }
             })
             .collect();

         <span class="kw">for </span>skip <span class="kw">in </span><span class="kw-2">&amp;</span>args.skips {
             <span class="kw">if </span>!param_names.iter().map(|(user, <span class="kw">_</span>)| user).any(|y| y == skip) {
                 <span class="kw">return </span><span class="macro">quote_spanned! </span>{skip.span()=&gt;
                     <span class="macro">compile_error!</span>(<span class="string">&quot;attempting to skip non-existent parameter&quot;</span>)
                 };
             }
         }

         <span class="kw">let </span>target = args.target();

         <span class="kw">let </span>parent = args.parent.iter();

         <span class="comment">// filter out skipped fields
         </span><span class="kw">let </span>quoted_fields: Vec&lt;<span class="kw">_</span>&gt; = param_names
             .iter()
             .filter(|(param, <span class="kw">_</span>)| {
                 <span class="kw">if </span>args.skip_all || args.skips.contains(param) {
                     <span class="kw">return </span><span class="bool-val">false</span>;
                 }

                 <span class="comment">// If any parameters have the same name as a custom field, skip
                 // and allow them to be formatted by the custom field.
                 </span><span class="kw">if let </span><span class="prelude-val">Some</span>(<span class="kw-2">ref </span>fields) = args.fields {
                     fields.<span class="number">0</span>.iter().all(|Field { <span class="kw-2">ref </span>name, .. }| {
                         <span class="kw">let </span>first = name.first();
                         first != name.last() || !first.iter().any(|name| name == <span class="kw-2">&amp;</span>param)
                     })
                 } <span class="kw">else </span>{
                     <span class="bool-val">true
                 </span>}
             })
             .map(|(user_name, (real_name, record_type))| <span class="kw">match </span>record_type {
                 RecordType::Value =&gt; <span class="macro">quote!</span>(#user_name = #real_name),
                 RecordType::Debug =&gt; <span class="macro">quote!</span>(#user_name = tracing::field::debug(<span class="kw-2">&amp;</span>#real_name)),
             })
             .collect();

         <span class="comment">// replace every use of a variable with its original name
         </span><span class="kw">if let </span><span class="prelude-val">Some</span>(Fields(<span class="kw-2">ref mut </span>fields)) = args.fields {
             <span class="kw">let </span><span class="kw-2">mut </span>replacer = IdentAndTypesRenamer {
                 idents: param_names.into_iter().map(|(a, (b, <span class="kw">_</span>))| (a, b)).collect(),
                 types: Vec::new(),
             };

             <span class="comment">// when async-trait &lt;=0.1.43 is in use, replace instances
             // of the &quot;Self&quot; type inside the fields values
             </span><span class="kw">if let </span><span class="prelude-val">Some</span>(self_type) = self_type {
                 replacer.types.push((<span class="string">&quot;Self&quot;</span>, self_type.clone()));
             }

             <span class="kw">for </span>e <span class="kw">in </span>fields.iter_mut().filter_map(|f| f.value.as_mut()) {
                 syn::visit_mut::visit_expr_mut(<span class="kw-2">&amp;mut </span>replacer, e);
             }
         }

         <span class="kw">let </span>custom_fields = <span class="kw-2">&amp;</span>args.fields;

         <span class="macro">quote!</span>(<span class="macro">tracing::span!</span>(
             target: #target,
             #(parent: #parent,)*
             #level,
             #span_name,
             #(#quoted_fields,)*
             #custom_fields

         ))
     })();

     <span class="kw">let </span>target = args.target();

     <span class="kw">let </span>err_event = <span class="kw">match </span>args.err_args {
         <span class="prelude-val">Some</span>(event_args) =&gt; {
             <span class="kw">let </span>level_tokens = event_args.level(Level::Error);
             <span class="kw">match </span>event_args.mode {
                 FormatMode::Default | FormatMode::Display =&gt; <span class="prelude-val">Some</span>(<span class="macro">quote!</span>(
                     <span class="macro">tracing::event!</span>(target: #target, #level_tokens, error = %e)
                 )),
                 FormatMode::Debug =&gt; <span class="prelude-val">Some</span>(<span class="macro">quote!</span>(
                     <span class="macro">tracing::event!</span>(target: #target, #level_tokens, error = <span class="question-mark">?</span>e)
                 )),
             }
         }
         <span class="kw">_ </span>=&gt; <span class="prelude-val">None</span>,
     };

     <span class="kw">let </span>ret_event = <span class="kw">match </span>args.ret_args {
         <span class="prelude-val">Some</span>(event_args) =&gt; {
             <span class="kw">let </span>level_tokens = event_args.level(args_level);
             <span class="kw">match </span>event_args.mode {
                 FormatMode::Display =&gt; <span class="prelude-val">Some</span>(<span class="macro">quote!</span>(
                     <span class="macro">tracing::event!</span>(target: #target, #level_tokens, <span class="kw">return </span>= %x)
                 )),
                 FormatMode::Default | FormatMode::Debug =&gt; <span class="prelude-val">Some</span>(<span class="macro">quote!</span>(
                     <span class="macro">tracing::event!</span>(target: #target, #level_tokens, <span class="kw">return </span>= <span class="question-mark">?</span>x)
                 )),
             }
         }
         <span class="kw">_ </span>=&gt; <span class="prelude-val">None</span>,
     };

     <span class="comment">// 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.
     // If `ret` is in args, instrument any resulting `Ok`s when the function
     // returns `Result`s, otherwise instrument any resulting values.
     </span><span class="kw">if </span>async_context {
         <span class="kw">let </span>mk_fut = <span class="kw">match </span>(err_event, ret_event) {
             (<span class="prelude-val">Some</span>(err_event), <span class="prelude-val">Some</span>(ret_event)) =&gt; <span class="macro">quote_spanned!</span>(block.span()=&gt;
                 <span class="kw">async move </span>{
                     <span class="kw">match async move </span>#block.<span class="kw">await </span>{
                         <span class="attribute">#[allow(clippy::unit_arg)]
                         </span><span class="prelude-val">Ok</span>(x) =&gt; {
                             #ret_event;
                             <span class="prelude-val">Ok</span>(x)
                         },
                         <span class="prelude-val">Err</span>(e) =&gt; {
                             #err_event;
                             <span class="prelude-val">Err</span>(e)
                         }
                     }
                 }
             ),
             (<span class="prelude-val">Some</span>(err_event), <span class="prelude-val">None</span>) =&gt; <span class="macro">quote_spanned!</span>(block.span()=&gt;
                 <span class="kw">async move </span>{
                     <span class="kw">match async move </span>#block.<span class="kw">await </span>{
                         <span class="attribute">#[allow(clippy::unit_arg)]
                         </span><span class="prelude-val">Ok</span>(x) =&gt; <span class="prelude-val">Ok</span>(x),
                         <span class="prelude-val">Err</span>(e) =&gt; {
                             #err_event;
                             <span class="prelude-val">Err</span>(e)
                         }
                     }
                 }
             ),
             (<span class="prelude-val">None</span>, <span class="prelude-val">Some</span>(ret_event)) =&gt; <span class="macro">quote_spanned!</span>(block.span()=&gt;
                 <span class="kw">async move </span>{
                     <span class="kw">let </span>x = <span class="kw">async move </span>#block.<span class="kw">await</span>;
                     #ret_event;
                     x
                 }
             ),
             (<span class="prelude-val">None</span>, <span class="prelude-val">None</span>) =&gt; <span class="macro">quote_spanned!</span>(block.span()=&gt;
                 <span class="kw">async move </span>#block
             ),
         };

         <span class="kw">return </span><span class="macro">quote!</span>(
             <span class="kw">let </span>__tracing_attr_span = #span;
             <span class="kw">let </span>__tracing_instrument_future = #mk_fut;
             <span class="kw">if </span>!__tracing_attr_span.is_disabled() {
                 #follows_from
                 tracing::Instrument::instrument(
                     __tracing_instrument_future,
                     __tracing_attr_span
                 )
                 .<span class="kw">await
             </span>} <span class="kw">else </span>{
                 __tracing_instrument_future.<span class="kw">await
             </span>}
         );
     }

     <span class="kw">let </span>span = <span class="macro">quote!</span>(
         <span class="comment">// These variables are left uninitialized and initialized only
         // if the tracing level is statically enabled at this point.
         // While the tracing level is also checked at span creation
         // time, that will still create a dummy span, and a dummy guard
         // and drop the dummy guard later. By lazily initializing these
         // variables, Rust will generate a drop flag for them and thus
         // only drop the guard if it was created. This creates code that
         // is very straightforward for LLVM to optimize out if the tracing
         // level is statically disabled, while not causing any performance
         // regression in case the level is enabled.
         </span><span class="kw">let </span>__tracing_attr_span;
         <span class="kw">let </span>__tracing_attr_guard;
         <span class="kw">if </span><span class="macro">tracing::level_enabled!</span>(#level) {
             __tracing_attr_span = #span;
             #follows_from
             __tracing_attr_guard = __tracing_attr_span.enter();
         }
     );

     <span class="kw">match </span>(err_event, ret_event) {
         (<span class="prelude-val">Some</span>(err_event), <span class="prelude-val">Some</span>(ret_event)) =&gt; <span class="macro">quote_spanned! </span>{block.span()=&gt;
             #span
             <span class="attribute">#[allow(clippy::redundant_closure_call)]
             </span><span class="kw">match </span>(<span class="kw">move </span>|| #block)() {
                 <span class="attribute">#[allow(clippy::unit_arg)]
                 </span><span class="prelude-val">Ok</span>(x) =&gt; {
                     #ret_event;
                     <span class="prelude-val">Ok</span>(x)
                 },
                 <span class="prelude-val">Err</span>(e) =&gt; {
                     #err_event;
                     <span class="prelude-val">Err</span>(e)
                 }
             }
         },
         (<span class="prelude-val">Some</span>(err_event), <span class="prelude-val">None</span>) =&gt; <span class="macro">quote_spanned!</span>(block.span()=&gt;
             #span
             <span class="attribute">#[allow(clippy::redundant_closure_call)]
             </span><span class="kw">match </span>(<span class="kw">move </span>|| #block)() {
                 <span class="attribute">#[allow(clippy::unit_arg)]
                 </span><span class="prelude-val">Ok</span>(x) =&gt; <span class="prelude-val">Ok</span>(x),
                 <span class="prelude-val">Err</span>(e) =&gt; {
                     #err_event;
                     <span class="prelude-val">Err</span>(e)
                 }
             }
         ),
         (<span class="prelude-val">None</span>, <span class="prelude-val">Some</span>(ret_event)) =&gt; <span class="macro">quote_spanned!</span>(block.span()=&gt;
             #span
             <span class="attribute">#[allow(clippy::redundant_closure_call)]
             </span><span class="kw">let </span>x = (<span class="kw">move </span>|| #block)();
             #ret_event;
             x
         ),
         (<span class="prelude-val">None</span>, <span class="prelude-val">None</span>) =&gt; <span class="macro">quote_spanned!</span>(block.span() =&gt;
             <span class="comment">// Because `quote` produces a stream of tokens _without_ whitespace, the
             // `if` and the block will appear directly next to each other. This
             // generates a clippy lint about suspicious `if/else` formatting.
             // Therefore, suppress the lint inside the generated code...
             </span><span class="attribute">#[allow(clippy::suspicious_else_formatting)]
             </span>{
                 #span
                 <span class="comment">// ...but turn the lint back on inside the function body.
                 </span><span class="attribute">#[warn(clippy::suspicious_else_formatting)]
                 </span>#block
             }
         ),
     }
 }

 <span class="doccomment">/// Indicates whether a field should be recorded as `Value` or `Debug`.
 </span><span class="kw">enum </span>RecordType {
     <span class="doccomment">/// The field should be recorded using its `Value` implementation.
     </span>Value,
     <span class="doccomment">/// The field should be recorded using `tracing::field::debug()`.
     </span>Debug,
 }

 <span class="kw">impl </span>RecordType {
     <span class="doccomment">/// Array of primitive types which should be recorded as [RecordType::Value].
     </span><span class="kw">const </span>TYPES_FOR_VALUE: <span class="kw-2">&amp;</span><span class="lifetime">&#39;static </span>[<span class="kw-2">&amp;</span><span class="lifetime">&#39;static </span>str] = <span class="kw-2">&amp;</span>[
         <span class="string">&quot;bool&quot;</span>,
         <span class="string">&quot;str&quot;</span>,
         <span class="string">&quot;u8&quot;</span>,
         <span class="string">&quot;i8&quot;</span>,
         <span class="string">&quot;u16&quot;</span>,
         <span class="string">&quot;i16&quot;</span>,
         <span class="string">&quot;u32&quot;</span>,
         <span class="string">&quot;i32&quot;</span>,
         <span class="string">&quot;u64&quot;</span>,
         <span class="string">&quot;i64&quot;</span>,
         <span class="string">&quot;f32&quot;</span>,
         <span class="string">&quot;f64&quot;</span>,
         <span class="string">&quot;usize&quot;</span>,
         <span class="string">&quot;isize&quot;</span>,
         <span class="string">&quot;NonZeroU8&quot;</span>,
         <span class="string">&quot;NonZeroI8&quot;</span>,
         <span class="string">&quot;NonZeroU16&quot;</span>,
         <span class="string">&quot;NonZeroI16&quot;</span>,
         <span class="string">&quot;NonZeroU32&quot;</span>,
         <span class="string">&quot;NonZeroI32&quot;</span>,
         <span class="string">&quot;NonZeroU64&quot;</span>,
         <span class="string">&quot;NonZeroI64&quot;</span>,
         <span class="string">&quot;NonZeroUsize&quot;</span>,
         <span class="string">&quot;NonZeroIsize&quot;</span>,
         <span class="string">&quot;Wrapping&quot;</span>,
     ];

     <span class="doccomment">/// Parse `RecordType` from [Type] by looking up
     /// the [RecordType::TYPES_FOR_VALUE] array.
     </span><span class="kw">fn </span>parse_from_ty(ty: <span class="kw-2">&amp;</span>Type) -&gt; <span class="self">Self </span>{
         <span class="kw">match </span>ty {
             Type::Path(TypePath { path, .. })
                 <span class="kw">if </span>path
                     .segments
                     .iter()
                     .last()
                     .map(|path_segment| {
                         <span class="kw">let </span>ident = path_segment.ident.to_string();
                         <span class="self">Self</span>::TYPES_FOR_VALUE.iter().any(|<span class="kw-2">&amp;</span>t| t == ident)
                     })
                     .unwrap_or(<span class="bool-val">false</span>) =&gt;
             {
                 RecordType::Value
             }
             Type::Reference(syn::TypeReference { elem, .. }) =&gt; RecordType::parse_from_ty(elem),
             <span class="kw">_ </span>=&gt; RecordType::Debug,
         }
     }
 }

 <span class="kw">fn </span>param_names(pat: Pat, record_type: RecordType) -&gt; Box&lt;<span class="kw">dyn </span>Iterator&lt;Item = (Ident, RecordType)&gt;&gt; {
     <span class="kw">match </span>pat {
         Pat::Ident(PatIdent { ident, .. }) =&gt; Box::new(iter::once((ident, record_type))),
         Pat::Reference(PatReference { pat, .. }) =&gt; param_names(<span class="kw-2">*</span>pat, record_type),
         <span class="comment">// We can&#39;t get the concrete type of fields in the struct/tuple
         // patterns by using `syn`. e.g. `fn foo(Foo { x, y }: Foo) {}`.
         // Therefore, the struct/tuple patterns in the arguments will just
         // always be recorded as `RecordType::Debug`.
         </span>Pat::Struct(PatStruct { fields, .. }) =&gt; Box::new(
             fields
                 .into_iter()
                 .flat_map(|FieldPat { pat, .. }| param_names(<span class="kw-2">*</span>pat, RecordType::Debug)),
         ),
         Pat::Tuple(PatTuple { elems, .. }) =&gt; Box::new(
             elems
                 .into_iter()
                 .flat_map(|p| param_names(p, RecordType::Debug)),
         ),
         Pat::TupleStruct(PatTupleStruct { elems, .. }) =&gt; Box::new(
             elems
                 .into_iter()
                 .flat_map(|p| param_names(p, RecordType::Debug)),
         ),

         <span class="comment">// The above *should* cover all cases of irrefutable patterns,
         // but we purposefully don&#39;t do any funny business here
         // (such as panicking) because that would obscure rustc&#39;s
         // much more informative error message.
         </span><span class="kw">_ </span>=&gt; Box::new(iter::empty()),
     }
 }

 <span class="doccomment">/// The specific async code pattern that was detected
 </span><span class="kw">enum </span>AsyncKind&lt;<span class="lifetime">&#39;a</span>&gt; {
     <span class="doccomment">/// Immediately-invoked async fn, as generated by `async-trait &lt;= 0.1.43`:
     /// `async fn foo&lt;...&gt;(...) {...}; Box::pin(foo&lt;...&gt;(...))`
     </span>Function(<span class="kw-2">&amp;</span><span class="lifetime">&#39;a </span>ItemFn),
     <span class="doccomment">/// A function returning an async (move) block, optionally `Box::pin`-ed,
     /// as generated by `async-trait &gt;= 0.1.44`:
     /// `Box::pin(async move { ... })`
     </span>Async {
         async_expr: <span class="kw-2">&amp;</span><span class="lifetime">&#39;a </span>ExprAsync,
         pinned_box: bool,
     },
 }

 <span class="kw">pub</span>(<span class="kw">crate</span>) <span class="kw">struct </span>AsyncInfo&lt;<span class="lifetime">&#39;block</span>&gt; {
     <span class="comment">// statement that must be patched
     </span>source_stmt: <span class="kw-2">&amp;</span><span class="lifetime">&#39;block </span>Stmt,
     kind: AsyncKind&lt;<span class="lifetime">&#39;block</span>&gt;,
     self_type: <span class="prelude-ty">Option</span>&lt;TypePath&gt;,
     input: <span class="kw-2">&amp;</span><span class="lifetime">&#39;block </span>ItemFn,
 }

 <span class="kw">impl</span>&lt;<span class="lifetime">&#39;block</span>&gt; AsyncInfo&lt;<span class="lifetime">&#39;block</span>&gt; {
     <span class="doccomment">/// Get the AST of the inner function we need to hook, if it looks like a
     /// manual future implementation.
     ///
     /// When we are given a function that returns a (pinned) future containing the
     /// user logic, it is that (pinned) future that needs to be instrumented.
     /// Were we to instrument its parent, we would only collect information
     /// regarding the allocation of that future, and not its own span of execution.
     ///
     /// We inspect the block of the function to find if it matches any of the
     /// following patterns:
     ///
     /// - Immediately-invoked async fn, as generated by `async-trait &lt;= 0.1.43`:
     ///   `async fn foo&lt;...&gt;(...) {...}; Box::pin(foo&lt;...&gt;(...))`
     ///
     /// - A function returning an async (move) block, optionally `Box::pin`-ed,
     ///   as generated by `async-trait &gt;= 0.1.44`:
     ///   `Box::pin(async move { ... })`
     ///
     /// We the return the statement that must be instrumented, along with some
     /// other information.
     /// &#39;gen_body&#39; will then be able to use that information to instrument the
     /// proper function/future.
     ///
     /// (this follows the approach suggested in
     /// https://github.com/dtolnay/async-trait/issues/45#issuecomment-571245673)
     </span><span class="kw">pub</span>(<span class="kw">crate</span>) <span class="kw">fn </span>from_fn(input: <span class="kw-2">&amp;</span><span class="lifetime">&#39;block </span>ItemFn) -&gt; <span class="prelude-ty">Option</span>&lt;<span class="self">Self</span>&gt; {
         <span class="comment">// are we in an async context? If yes, this isn&#39;t a manual async-like pattern
         </span><span class="kw">if </span>input.sig.asyncness.is_some() {
             <span class="kw">return </span><span class="prelude-val">None</span>;
         }

         <span class="kw">let </span>block = <span class="kw-2">&amp;</span>input.block;

         <span class="comment">// list of async functions declared inside the block
         </span><span class="kw">let </span>inside_funs = block.stmts.iter().filter_map(|stmt| {
             <span class="kw">if let </span>Stmt::Item(Item::Fn(fun)) = <span class="kw-2">&amp;</span>stmt {
                 <span class="comment">// If the function is async, this is a candidate
                 </span><span class="kw">if </span>fun.sig.asyncness.is_some() {
                     <span class="kw">return </span><span class="prelude-val">Some</span>((stmt, fun));
                 }
             }
             <span class="prelude-val">None
         </span>});

         <span class="comment">// last expression of the block: it determines the return value of the
         // block, this is quite likely a `Box::pin` statement or an async block
         </span><span class="kw">let </span>(last_expr_stmt, last_expr) = block.stmts.iter().rev().find_map(|stmt| {
             <span class="kw">if let </span>Stmt::Expr(expr, _semi) = stmt {
                 <span class="prelude-val">Some</span>((stmt, expr))
             } <span class="kw">else </span>{
                 <span class="prelude-val">None
             </span>}
         })<span class="question-mark">?</span>;

         <span class="comment">// is the last expression an async block?
         </span><span class="kw">if let </span>Expr::Async(async_expr) = last_expr {
             <span class="kw">return </span><span class="prelude-val">Some</span>(AsyncInfo {
                 source_stmt: last_expr_stmt,
                 kind: AsyncKind::Async {
                     async_expr,
                     pinned_box: <span class="bool-val">false</span>,
                 },
                 self_type: <span class="prelude-val">None</span>,
                 input,
             });
         }

         <span class="comment">// is the last expression a function call?
         </span><span class="kw">let </span>(outside_func, outside_args) = <span class="kw">match </span>last_expr {
             Expr::Call(ExprCall { func, args, .. }) =&gt; (func, args),
             <span class="kw">_ </span>=&gt; <span class="kw">return </span><span class="prelude-val">None</span>,
         };

         <span class="comment">// is it a call to `Box::pin()`?
         </span><span class="kw">let </span>path = <span class="kw">match </span>outside_func.as_ref() {
             Expr::Path(path) =&gt; <span class="kw-2">&amp;</span>path.path,
             <span class="kw">_ </span>=&gt; <span class="kw">return </span><span class="prelude-val">None</span>,
         };
         <span class="kw">if </span>!path_to_string(path).ends_with(<span class="string">&quot;Box::pin&quot;</span>) {
             <span class="kw">return </span><span class="prelude-val">None</span>;
         }

         <span class="comment">// Does the call take an argument? If it doesn&#39;t,
         // it&#39;s not gonna compile anyway, but that&#39;s no reason
         // to (try to) perform an out of bounds access
         </span><span class="kw">if </span>outside_args.is_empty() {
             <span class="kw">return </span><span class="prelude-val">None</span>;
         }

         <span class="comment">// Is the argument to Box::pin an async block that
         // captures its arguments?
         </span><span class="kw">if let </span>Expr::Async(async_expr) = <span class="kw-2">&amp;</span>outside_args[<span class="number">0</span>] {
             <span class="kw">return </span><span class="prelude-val">Some</span>(AsyncInfo {
                 source_stmt: last_expr_stmt,
                 kind: AsyncKind::Async {
                     async_expr,
                     pinned_box: <span class="bool-val">true</span>,
                 },
                 self_type: <span class="prelude-val">None</span>,
                 input,
             });
         }

         <span class="comment">// Is the argument to Box::pin a function call itself?
         </span><span class="kw">let </span>func = <span class="kw">match </span><span class="kw-2">&amp;</span>outside_args[<span class="number">0</span>] {
             Expr::Call(ExprCall { func, .. }) =&gt; func,
             <span class="kw">_ </span>=&gt; <span class="kw">return </span><span class="prelude-val">None</span>,
         };

         <span class="comment">// &quot;stringify&quot; the path of the function called
         </span><span class="kw">let </span>func_name = <span class="kw">match </span><span class="kw-2">**</span>func {
             Expr::Path(<span class="kw-2">ref </span>func_path) =&gt; path_to_string(<span class="kw-2">&amp;</span>func_path.path),
             <span class="kw">_ </span>=&gt; <span class="kw">return </span><span class="prelude-val">None</span>,
         };

         <span class="comment">// Was that function defined inside of the current block?
         // If so, retrieve the statement where it was declared and the function itself
         </span><span class="kw">let </span>(stmt_func_declaration, func) = inside_funs
             .into_iter()
             .find(|(<span class="kw">_</span>, fun)| fun.sig.ident == func_name)<span class="question-mark">?</span>;

         <span class="comment">// If &quot;_self&quot; is present as an argument, we store its type to be able to rewrite &quot;Self&quot; (the
         // parameter type) with the type of &quot;_self&quot;
         </span><span class="kw">let </span><span class="kw-2">mut </span>self_type = <span class="prelude-val">None</span>;
         <span class="kw">for </span>arg <span class="kw">in </span><span class="kw-2">&amp;</span>func.sig.inputs {
             <span class="kw">if let </span>FnArg::Typed(ty) = arg {
                 <span class="kw">if let </span>Pat::Ident(PatIdent { <span class="kw-2">ref </span>ident, .. }) = <span class="kw-2">*</span>ty.pat {
                     <span class="kw">if </span>ident == <span class="string">&quot;_self&quot; </span>{
                         <span class="kw">let </span><span class="kw-2">mut </span>ty = <span class="kw-2">*</span>ty.ty.clone();
                         <span class="comment">// extract the inner type if the argument is &quot;&amp;self&quot; or &quot;&amp;mut self&quot;
                         </span><span class="kw">if let </span>Type::Reference(syn::TypeReference { elem, .. }) = ty {
                             ty = <span class="kw-2">*</span>elem;
                         }

                         <span class="kw">if let </span>Type::Path(tp) = ty {
                             self_type = <span class="prelude-val">Some</span>(tp);
                             <span class="kw">break</span>;
                         }
                     }
                 }
             }
         }

         <span class="prelude-val">Some</span>(AsyncInfo {
             source_stmt: stmt_func_declaration,
             kind: AsyncKind::Function(func),
             self_type,
             input,
         })
     }

     <span class="kw">pub</span>(<span class="kw">crate</span>) <span class="kw">fn </span>gen_async(
         <span class="self">self</span>,
         args: InstrumentArgs,
         instrumented_function_name: <span class="kw-2">&amp;</span>str,
     ) -&gt; <span class="prelude-ty">Result</span>&lt;proc_macro::TokenStream, syn::Error&gt; {
         <span class="comment">// let&#39;s rewrite some statements!
         </span><span class="kw">let </span><span class="kw-2">mut </span>out_stmts: Vec&lt;TokenStream&gt; = <span class="self">self
             </span>.input
             .block
             .stmts
             .iter()
             .map(|stmt| stmt.to_token_stream())
             .collect();

         <span class="kw">if let </span><span class="prelude-val">Some</span>((iter, _stmt)) = <span class="self">self
             </span>.input
             .block
             .stmts
             .iter()
             .enumerate()
             .find(|(_iter, stmt)| <span class="kw-2">*</span>stmt == <span class="self">self</span>.source_stmt)
         {
             <span class="comment">// instrument the future by rewriting the corresponding statement
             </span>out_stmts[iter] = <span class="kw">match </span><span class="self">self</span>.kind {
                 <span class="comment">// `Box::pin(immediately_invoked_async_fn())`
                 </span>AsyncKind::Function(fun) =&gt; {
                     <span class="kw">let </span>fun = MaybeItemFn::from(fun.clone());
                     gen_function(
                         fun.as_ref(),
                         args,
                         instrumented_function_name,
                         <span class="self">self</span>.self_type.as_ref(),
                     )
                 }
                 <span class="comment">// `async move { ... }`, optionally pinned
                 </span>AsyncKind::Async {
                     async_expr,
                     pinned_box,
                 } =&gt; {
                     <span class="kw">let </span>instrumented_block = gen_block(
                         <span class="kw-2">&amp;</span>async_expr.block,
                         <span class="kw-2">&amp;</span><span class="self">self</span>.input.sig.inputs,
                         <span class="bool-val">true</span>,
                         args,
                         instrumented_function_name,
                         <span class="prelude-val">None</span>,
                     );
                     <span class="kw">let </span>async_attrs = <span class="kw-2">&amp;</span>async_expr.attrs;
                     <span class="kw">if </span>pinned_box {
                         <span class="macro">quote! </span>{
                             Box::pin(#(#async_attrs) * <span class="kw">async move </span>{ #instrumented_block })
                         }
                     } <span class="kw">else </span>{
                         <span class="macro">quote! </span>{
                             #(#async_attrs) * <span class="kw">async move </span>{ #instrumented_block }
                         }
                     }
                 }
             };
         }

         <span class="kw">let </span>vis = <span class="kw-2">&amp;</span><span class="self">self</span>.input.vis;
         <span class="kw">let </span>sig = <span class="kw-2">&amp;</span><span class="self">self</span>.input.sig;
         <span class="kw">let </span>attrs = <span class="kw-2">&amp;</span><span class="self">self</span>.input.attrs;
         <span class="prelude-val">Ok</span>(<span class="macro">quote!</span>(
             #(#attrs) *
             #vis #sig {
                 #(#out_stmts) *
             }
         )
         .into())
     }
 }

 <span class="comment">// Return a path as a String
 </span><span class="kw">fn </span>path_to_string(path: <span class="kw-2">&amp;</span>Path) -&gt; String {
     <span class="kw">use </span>std::fmt::Write;
     <span class="comment">// some heuristic to prevent too many allocations
     </span><span class="kw">let </span><span class="kw-2">mut </span>res = String::with_capacity(path.segments.len() * <span class="number">5</span>);
     <span class="kw">for </span>i <span class="kw">in </span><span class="number">0</span>..path.segments.len() {
         <span class="macro">write!</span>(<span class="kw-2">&amp;mut </span>res, <span class="string">&quot;{}&quot;</span>, path.segments[i].ident)
             .expect(<span class="string">&quot;writing to a String should never fail&quot;</span>);
         <span class="kw">if </span>i &lt; path.segments.len() - <span class="number">1 </span>{
             res.push_str(<span class="string">&quot;::&quot;</span>);
         }
     }
     res
 }

 <span class="doccomment">/// A visitor struct to replace idents and types in some piece
 /// of code (e.g. the &quot;self&quot; and &quot;Self&quot; tokens in user-supplied
 /// fields expressions when the function is generated by an old
 /// version of async-trait).
 </span><span class="kw">struct </span>IdentAndTypesRenamer&lt;<span class="lifetime">&#39;a</span>&gt; {
     types: Vec&lt;(<span class="kw-2">&amp;</span><span class="lifetime">&#39;a </span>str, TypePath)&gt;,
     idents: Vec&lt;(Ident, Ident)&gt;,
 }

 <span class="kw">impl</span>&lt;<span class="lifetime">&#39;a</span>&gt; VisitMut <span class="kw">for </span>IdentAndTypesRenamer&lt;<span class="lifetime">&#39;a</span>&gt; {
     <span class="comment">// we deliberately compare strings because we want to ignore the spans
     // If we apply clippy&#39;s lint, the behavior changes
     </span><span class="attribute">#[allow(clippy::cmp_owned)]
     </span><span class="kw">fn </span>visit_ident_mut(<span class="kw-2">&amp;mut </span><span class="self">self</span>, id: <span class="kw-2">&amp;mut </span>Ident) {
         <span class="kw">for </span>(old_ident, new_ident) <span class="kw">in </span><span class="kw-2">&amp;</span><span class="self">self</span>.idents {
             <span class="kw">if </span>id.to_string() == old_ident.to_string() {
                 <span class="kw-2">*</span>id = new_ident.clone();
             }
         }
     }

     <span class="kw">fn </span>visit_type_mut(<span class="kw-2">&amp;mut </span><span class="self">self</span>, ty: <span class="kw-2">&amp;mut </span>Type) {
         <span class="kw">for </span>(type_name, new_type) <span class="kw">in </span><span class="kw-2">&amp;</span><span class="self">self</span>.types {
             <span class="kw">if let </span>Type::Path(TypePath { path, .. }) = ty {
                 <span class="kw">if </span>path_to_string(path) == <span class="kw-2">*</span>type_name {
                     <span class="kw-2">*</span>ty = Type::Path(new_type.clone());
                 }
             }
         }
     }
 }

 <span class="comment">// A visitor struct that replace an async block by its patched version
 </span><span class="kw">struct </span>AsyncTraitBlockReplacer&lt;<span class="lifetime">&#39;a</span>&gt; {
     block: <span class="kw-2">&amp;</span><span class="lifetime">&#39;a </span>Block,
     patched_block: Block,
 }

 <span class="kw">impl</span>&lt;<span class="lifetime">&#39;a</span>&gt; VisitMut <span class="kw">for </span>AsyncTraitBlockReplacer&lt;<span class="lifetime">&#39;a</span>&gt; {
     <span class="kw">fn </span>visit_block_mut(<span class="kw-2">&amp;mut </span><span class="self">self</span>, i: <span class="kw-2">&amp;mut </span>Block) {
         <span class="kw">if </span>i == <span class="self">self</span>.block {
             <span class="kw-2">*</span>i = <span class="self">self</span>.patched_block.clone();
         }
     }
 }

 <span class="comment">// Replaces any `impl Trait` with `_` so it can be used as the type in
 // a `let` statement&#39;s LHS.
 </span><span class="kw">struct </span>ImplTraitEraser;

 <span class="kw">impl </span>VisitMut <span class="kw">for </span>ImplTraitEraser {
     <span class="kw">fn </span>visit_type_mut(<span class="kw-2">&amp;mut </span><span class="self">self</span>, t: <span class="kw-2">&amp;mut </span>Type) {
         <span class="kw">if let </span>Type::ImplTrait(..) = t {
             <span class="kw-2">*</span>t = syn::TypeInfer {
                 underscore_token: <span class="macro">Token!</span>[<span class="kw">_</span>](t.span()),
             }
             .into();
         } <span class="kw">else </span>{
             syn::visit_mut::visit_type_mut(<span class="self">self</span>, t);
         }
     }
 }

 <span class="kw">fn </span>erase_impl_trait(ty: <span class="kw-2">&amp;</span>Type) -&gt; Type {
     <span class="kw">let </span><span class="kw-2">mut </span>ty = ty.clone();
     ImplTraitEraser.visit_type_mut(<span class="kw-2">&amp;mut </span>ty);
     ty
 }
 </code></pre></div>
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