tracing: fix record_all panic

tracing-core/src/field.rs

@@ -876,7 +876,7 @@ impl FieldSet {
     /// [`Identifier`]: super::callsite::Identifier
     /// [`Callsite`]: super::callsite::Callsite
     #[inline]
-    pub(crate) fn callsite(&self) -> callsite::Identifier {
+    pub(crate) const fn callsite(&self) -> callsite::Identifier {
         callsite::Identifier(self.callsite.0)
     }
 
@@ -949,6 +949,16 @@ impl FieldSet {
         }
     }
 
+    pub(crate) const fn fake_field(&self) -> Field {
+        Field {
+            i: usize::MAX,
+            fields: FieldSet {
+                names: self.names,
+                callsite: self.callsite(),
+            },
+        }
+    }
+
     /// Returns the number of fields in this `FieldSet`.
     #[inline]
     pub fn len(&self) -> usize {

tracing-core/src/metadata.rs

@@ -330,6 +330,14 @@ impl<'a> Metadata<'a> {
     pub fn is_span(&self) -> bool {
         self.kind.is_span()
     }
+
+    /// Generate a fake field that will never match a real field.
+    ///
+    /// Used via valueset to fill in for unknown fields.
+    #[doc(hidden)]
+    pub const fn private_fake_field(&self) -> field::Field {
+        self.fields.fake_field()
+    }
 }
 
 impl fmt::Debug for Metadata<'_> {

tracing/src/lib.rs

@@ -1127,6 +1127,18 @@ pub mod __macro_support {
                 .finish()
         }
     }
+
+    static CALLSITE: crate::callsite::DefaultCallsite =
+        crate::callsite::DefaultCallsite::new(&META);
+    static META: crate::Metadata<'static> = crate::metadata! {
+        name: "__fake_tracing_callsite",
+        target: module_path!(),
+        level: crate::Level::TRACE,
+        fields: crate::fieldset!(),
+        callsite: &CALLSITE,
+        kind: crate::metadata::Kind::SPAN,
+    };
+    pub static FAKE_FIELD: crate::field::Field = META.private_fake_field();
 }
 
 #[cfg(feature = "log")]