Annotate temporary result type when possible

This change is necessary to make spez work, otherwise the compiler gets
confused about the specialization to choose in the spez::Match<i>
generated traits. The issue is not spez-specific, as the macro is simple
sugar over the auto-deref specialization trick in general.

This commit is also the "maximalist" solution to #74, as it simply
ignores the return type if it has syntax incompatible with the context
we want to use.

Closes: #74
Co-Authored-By: Anthony Deschamps <[email protected]>

autometrics-macros/src/lib.rs

@@ -3,7 +3,7 @@ use percent_encoding::{utf8_percent_encode, NON_ALPHANUMERIC};
 use proc_macro2::TokenStream;
 use quote::quote;
 use std::env;
-use syn::{parse_macro_input, ImplItem, ItemFn, ItemImpl, Result};
+use syn::{parse_macro_input, ImplItem, ItemFn, ItemImpl, Result, ReturnType, Type};
 
 mod parse;
 mod result_labels;
@@ -60,6 +60,13 @@ fn instrument_function(args: &AutometricsArgs, item: ItemFn) -> Result<TokenStre
     // Build the documentation we'll add to the function's RustDocs
     let metrics_docs = create_metrics_docs(&prometheus_url, &function_name, args.track_concurrency);
 
+    // Type annotation to allow type inference to work on return expressions (such as `.collect()`).
+    let return_type = match sig.output {
+        ReturnType::Default => quote! { : () },
+        ReturnType::Type(_, ref t) if matches!(t.as_ref(), &Type::ImplTrait(_)) => quote! {},
+        ReturnType::Type(_, ref t) => quote! { : #t },
+    };
+
     // Wrap the body of the original function, using a slightly different approach based on whether the function is async
     let call_function = if sig.asyncness.is_some() {
         quote! {
@@ -144,7 +151,7 @@ fn instrument_function(args: &AutometricsArgs, item: ItemFn) -> Result<TokenStre
                 AutometricsTracker::start(#gauge_labels)
             };
 
-            let result = #call_function;
+            let result #return_type = #call_function;
 
             {
                 use autometrics::__private::{HistogramLabels, TrackMetrics};

autometrics/src/labels.rs

@@ -237,7 +237,7 @@ macro_rules! result_labels {
         $crate::__private::spez! {
             for val = $e;
 
-            match<T: GetLabels, E: GetLabels> &Result<T, E> -> Option<ResultAndReturnTypeLabels> {
+            match<T, E> &::std::result::Result<T, E> where T: GetLabels, E: GetLabels -> ::std::option::Option<ResultAndReturnTypeLabels> {
                 match val {
                     Ok(ok) => Some((
                         ok.__autometrics_get_labels().unwrap_or(OK_KEY),
@@ -250,7 +250,7 @@ macro_rules! result_labels {
                 }
             }
 
-            match<T, E: GetLabels> &Result<T, E> -> Option<ResultAndReturnTypeLabels> {
+            match<T, E> &::std::result::Result<T, E> where E: GetLabels -> ::std::option::Option<ResultAndReturnTypeLabels> {
                 match val {
                     Ok(ok) => Some((
                         OK_KEY,
@@ -263,7 +263,7 @@ macro_rules! result_labels {
                 }
             }
 
-            match<T: GetLabels, E> &Result<T, E> -> Option<ResultAndReturnTypeLabels> {
+            match<T, E> &::std::result::Result<T, E> where T: GetLabels -> ::std::option::Option<ResultAndReturnTypeLabels> {
                 match val {
                     Ok(ok) => Some((
                         ok.__autometrics_get_labels().unwrap_or(OK_KEY),
@@ -276,7 +276,7 @@ macro_rules! result_labels {
                 }
             }
 
-            match<T, E> &Result<T, E> -> Option<ResultAndReturnTypeLabels> {
+            match<T, E> &::std::result::Result<T, E> -> ::std::option::Option<ResultAndReturnTypeLabels> {
                 match val {
                     Ok(ok) => Some((
                         OK_KEY,
@@ -289,11 +289,11 @@ macro_rules! result_labels {
                 }
             }
 
-            match<T: GetLabels> &T -> Option<ResultAndReturnTypeLabels> {
+            match<T> &T where T: GetLabels -> ::std::option::Option<ResultAndReturnTypeLabels> {
                 val.__autometrics_get_labels().map(|label| (label, val.__autometrics_static_str()))
             }
 
-            match<T> T -> Option<ResultAndReturnTypeLabels> {
+            match<T> T -> ::std::option::Option<ResultAndReturnTypeLabels> {
                 None
             }
         }