1 files changed, 44 insertions, 0 deletions

diff --git a/vendor/tracing-core/src/field.rs b/vendor/tracing-core/src/field.rs
index c9869ce2c..e103c75a9 100644
--- a/vendor/tracing-core/src/field.rs
+++ b/vendor/tracing-core/src/field.rs
@@ -145,6 +145,16 @@ pub struct Field {
 pub struct Empty;
 
 /// Describes the fields present on a span.
+///
+/// ## Equality
+///
+/// In well-behaved applications, two `FieldSet`s [initialized] with equal
+/// [callsite identifiers] will have identical fields. Consequently, in release
+/// builds, [`FieldSet::eq`] *only* checks that its arguments have equal
+/// callsites. However, the equality of field names is checked in debug builds.
+///
+/// [initialized]: Self::new
+/// [callsite identifiers]: callsite::Identifier
 pub struct FieldSet {
     /// The names of each field on the described span.
     names: &'static [&'static str],
@@ -911,6 +921,40 @@ impl fmt::Display for FieldSet {
     }
 }
 
+impl Eq for FieldSet {}
+
+impl PartialEq for FieldSet {
+    fn eq(&self, other: &Self) -> bool {
+        if core::ptr::eq(&self, &other) {
+            true
+        } else if cfg!(not(debug_assertions)) {
+            // In a well-behaving application, two `FieldSet`s can be assumed to
+            // be totally equal so long as they share the same callsite.
+            self.callsite == other.callsite
+        } else {
+            // However, when debug-assertions are enabled, do NOT assume that
+            // the application is well-behaving; check every the field names of
+            // each `FieldSet` for equality.
+
+            // `FieldSet` is destructured here to ensure a compile-error if the
+            // fields of `FieldSet` change.
+            let Self {
+                names: lhs_names,
+                callsite: lhs_callsite,
+            } = self;
+
+            let Self {
+                names: rhs_names,
+                callsite: rhs_callsite,
+            } = &other;
+
+            // Check callsite equality first, as it is probably cheaper to do
+            // than str equality.
+            lhs_callsite == rhs_callsite && lhs_names == rhs_names
+        }
+    }
+}
+
 // ===== impl Iter =====
 
 impl Iterator for Iter {