core: allow ValueSets of any length (#2508)

## Motivation

Fixes: #1566

## Solution

This change removes the maximum of 32 fields limitation using const
generics.

Having this arbitrary restriction in place to prevent stack overflows
feels a little misplaced to me since stack size varies between
environments.

tracing-core/src/field.rs

@@ -879,9 +879,6 @@ impl FieldSet {
     }
 
     /// Returns a new `ValueSet` with entries for this `FieldSet`'s values.
-    ///
-    /// Note that a `ValueSet` may not be constructed with arrays of over 32
-    /// elements.
     #[doc(hidden)]
     pub fn value_set<'v, V>(&'v self, values: &'v V) -> ValueSet<'v>
     where
@@ -1080,28 +1077,10 @@ impl<'a> fmt::Display for ValueSet<'a> {
 mod private {
     use super::*;
 
-    /// Marker trait implemented by arrays which are of valid length to
-    /// construct a `ValueSet`.
-    ///
-    /// `ValueSet`s may only be constructed from arrays containing 32 or fewer
-    /// elements, to ensure the array is small enough to always be allocated on the
-    /// stack. This trait is only implemented by arrays of an appropriate length,
-    /// ensuring that the correct size arrays are used at compile-time.
+    /// Restrictions on `ValueSet` lengths were removed in #2508 but this type remains for backwards compatibility.
     pub trait ValidLen<'a>: Borrow<[(&'a Field, Option<&'a (dyn Value + 'a)>)]> {}
-}
 
-macro_rules! impl_valid_len {
-    ( $( $len:tt ),+ ) => {
-        $(
-            impl<'a> private::ValidLen<'a> for
-                [(&'a Field, ::core::option::Option<&'a (dyn Value + 'a)>); $len] {}
-        )+
-    }
-}
-
-impl_valid_len! {
-    0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20,
-    21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32
+    impl<'a, const N: usize> ValidLen<'a> for [(&'a Field, Option<&'a (dyn Value + 'a)>); N] {}
 }
 
 #[cfg(test)]

tracing/src/lib.rs

@@ -386,22 +386,6 @@
 //! span.record("parting", &"goodbye world!");
 //! ```
 //!
-//! Note that a span may have up to 32 fields. The following will not compile:
-//!
-//! ```rust,compile_fail
-//! # use tracing::Level;
-//! # fn main() {
-//! let bad_span = span!(
-//!     Level::TRACE,
-//!     "too many fields!",
-//!     a = 1, b = 2, c = 3, d = 4, e = 5, f = 6, g = 7, h = 8, i = 9,
-//!     j = 10, k = 11, l = 12, m = 13, n = 14, o = 15, p = 16, q = 17,
-//!     r = 18, s = 19, t = 20, u = 21, v = 22, w = 23, x = 24, y = 25,
-//!     z = 26, aa = 27, bb = 28, cc = 29, dd = 30, ee = 31, ff = 32, gg = 33
-//! );
-//! # }
-//! ```
-//!
 //! Finally, events may also include human-readable messages, in the form of a
 //! [format string][fmt] and (optional) arguments, **after** the event's
 //! key-value fields. If a format string and arguments are provided,

tracing/src/span.rs

@@ -22,7 +22,7 @@
 //!   override their default values.
 //! - The span's [verbosity level]
 //! - A string literal providing the span's name.
-//! - Finally, between zero and 32 arbitrary key/value fields.
+//! - Finally, zero or more arbitrary key/value fields.
 //!
 //! [`target`]: super::Metadata::target
 //!

tracing/tests/macros.rs

@@ -308,6 +308,48 @@ fn span_with_non_rust_symbol() {
     span!(Level::TRACE, "non-rust", "guid:x-request-id" = "abcdef");
 }
 
+#[cfg_attr(target_arch = "wasm32", wasm_bindgen_test::wasm_bindgen_test)]
+#[test]
+fn large_span() {
+    span!(
+        Level::TRACE,
+        "spans with more than 32 fields have been supported since #2508",
+        a = 1,
+        b = 2,
+        c = 3,
+        d = 4,
+        e = 5,
+        f = 6,
+        g = 7,
+        h = 8,
+        i = 9,
+        j = 10,
+        k = 11,
+        l = 12,
+        m = 13,
+        n = 14,
+        o = 15,
+        p = 16,
+        q = 17,
+        r = 18,
+        s = 19,
+        t = 20,
+        u = 21,
+        v = 22,
+        w = 23,
+        x = 24,
+        y = 25,
+        z = 26,
+        aa = 27,
+        bb = 28,
+        cc = 29,
+        dd = 30,
+        ee = 31,
+        ff = 32,
+        gg = 33
+    );
+}
+
 #[cfg_attr(target_arch = "wasm32", wasm_bindgen_test::wasm_bindgen_test)]
 #[test]
 fn event() {