From d331d9592e4584c8b222671dcc4845addb259512 Mon Sep 17 00:00:00 2001
From: Tamme Schichler <tamme@schichler.dev>
Date: Sat, 8 Jan 2022 16:16:39 +0100
Subject: [PATCH] implement `tracing` override for `#[instrument]`

---
 tracing-attributes/src/attr.rs   | 43 ++++++++++++++++++++++++--------
 tracing-attributes/src/expand.rs |  2 +-
 2 files changed, 33 insertions(+), 12 deletions(-)

diff --git a/tracing-attributes/src/attr.rs b/tracing-attributes/src/attr.rs
index 9992e608b0..ce718fd660 100644
--- a/tracing-attributes/src/attr.rs
+++ b/tracing-attributes/src/attr.rs
@@ -275,6 +275,11 @@ pub(crate) enum FieldKind {
     Value,
 }
 
+pub(crate) struct WithArgs<'a, T> {
+    value: &'a T,
+    args: &'a InstrumentArgs,
+}
+
 impl Parse for Fields {
     fn parse(input: ParseStream<'_>) -> syn::Result<Self> {
         let _ = input.parse::<kw::fields>();
@@ -285,9 +290,17 @@ impl Parse for Fields {
     }
 }
 
-impl ToTokens for Fields {
+impl Fields {
+    pub(crate) fn with_args<'a>(&'a self, args: &'a InstrumentArgs) -> WithArgs<'a, Self> {
+        WithArgs { value: self, args }
+    }
+}
+
+impl ToTokens for WithArgs<'_, Fields> {
     fn to_tokens(&self, tokens: &mut TokenStream) {
-        self.0.to_tokens(tokens)
+        for field in &self.value.0 {
+            field.with_args(self.args).to_tokens(tokens)
+        }
     }
 }
 
@@ -319,26 +332,34 @@ impl Parse for Field {
     }
 }
 
-impl ToTokens for Field {
+impl Field {
+    fn with_args<'a>(&'a self, args: &'a InstrumentArgs) -> WithArgs<'a, Self> {
+        WithArgs { value: self, args }
+    }
+}
+
+impl ToTokens for WithArgs<'_, Field> {
     fn to_tokens(&self, tokens: &mut TokenStream) {
-        if let Some(ref value) = self.value {
-            let name = &self.name;
-            let kind = &self.kind;
+        let field = self.value;
+
+        if let Some(ref value) = field.value {
+            let name = &field.name;
+            let kind = &field.kind;
             tokens.extend(quote! {
                 #name = #kind#value
             })
-        } else if self.kind == FieldKind::Value {
+        } else if field.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;
-            let tracing = todo!();
+            let name = &field.name;
+            let tracing = self.args.tracing();
             tokens.extend(quote!(#name = #tracing::field::Empty))
         } else {
-            self.kind.to_tokens(tokens);
-            self.name.to_tokens(tokens);
+            field.kind.to_tokens(tokens);
+            field.name.to_tokens(tokens);
         }
     }
 }
diff --git a/tracing-attributes/src/expand.rs b/tracing-attributes/src/expand.rs
index 81498b6507..caf21f6119 100644
--- a/tracing-attributes/src/expand.rs
+++ b/tracing-attributes/src/expand.rs
@@ -179,7 +179,7 @@ fn gen_block<B: ToTokens>(
             }
         }
 
-        let custom_fields = &args.fields;
+        let custom_fields = args.fields.as_ref().map(|fields| fields.with_args(&args));
 
         quote!(#tracing::span!(
             target: #target,