tracing: allow constant field names in macros (tokio-rs#2617)

I've found myself in the case where I wanted to have customized event field name
for different trait implementations. In fact, these implementations are
completely unrelated (in separate applications), so, in this use case, I find
more readable to have `foo="some_id"` and `bar=16` instead of `resource="foo"
value="some_id"` and `resource=bar value=16`

Because events only accept identifier or literal as field name, this is quite
cumbersome/impossible to do. A simple solution could be to make events accept
constant expression too; in my use case, I could then add a associated constant
to my trait.

This PR proposes a new syntax for using constant field names:
```rust
tracing::debug!({ CONSTANT_EXPR } = "foo");
```
This is the same syntax than constant expression, so it should be quite intuitive.

To avoid constant expression names conflict, internal variables of macro
expansion have been prefixed with `__`, e.g. `__CALLSITE`.

Co-authored-by: Joseph Perez <joseph.perez@numberly.com>
Co-authored-by: Eliza Weisman <eliza@buoyant.io>

tracing/src/macros.rs

@@ -2959,6 +2959,47 @@ macro_rules! valueset {
         )
     };
 
+    // Handle constant names
+    (@ { $(,)* $($out:expr),* }, $next:expr, { $k:expr } = ?$val:expr, $($rest:tt)*) => {
+        $crate::valueset!(
+            @ { $($out),*, (&$next, Some(&debug(&$val) as &dyn Value)) },
+            $next,
+            $($rest)*
+        )
+    };
+    (@ { $(,)* $($out:expr),* }, $next:expr, { $k:expr } = %$val:expr, $($rest:tt)*) => {
+        $crate::valueset!(
+            @ { $($out),*, (&$next, Some(&display(&$val) as &dyn Value)) },
+            $next,
+            $($rest)*
+        )
+    };
+    (@ { $(,)* $($out:expr),* }, $next:expr, { $k:expr } = $val:expr, $($rest:tt)*) => {
+        $crate::valueset!(
+            @ { $($out),*, (&$next, Some(&$val as &dyn Value)) },
+            $next,
+            $($rest)*
+        )
+    };
+    (@ { $(,)* $($out:expr),* }, $next:expr, { $k:expr } = ?$val:expr) => {
+        $crate::valueset!(
+            @ { $($out),*, (&$next, Some(&debug(&$val) as &dyn Value)) },
+            $next,
+        )
+    };
+    (@ { $(,)* $($out:expr),* }, $next:expr, { $k:expr } = %$val:expr) => {
+        $crate::valueset!(
+            @ { $($out),*, (&$next, Some(&display(&$val) as &dyn Value)) },
+            $next,
+        )
+    };
+    (@ { $(,)* $($out:expr),* }, $next:expr, { $k:expr } = $val:expr) => {
+        $crate::valueset!(
+            @ { $($out),*, (&$next, Some(&$val as &dyn Value)) },
+            $next,
+        )
+    };
+
     // Remainder is unparsable, but exists --- must be format args!
     (@ { $(,)* $($out:expr),* }, $next:expr, $($rest:tt)+) => {
         $crate::valueset!(@ { (&$next, $crate::__macro_support::Option::Some(&$crate::__macro_support::format_args!($($rest)+) as &dyn Value)), $($out),* }, $next, )

tracing/tests/event.rs

@@ -525,3 +525,45 @@ fn string_field() {
 
     handle.assert_finished();
 }
+
+#[cfg_attr(target_arch = "wasm32", wasm_bindgen_test::wasm_bindgen_test)]
+#[test]
+fn constant_field_name() {
+    let expect_event = || {
+        expect::event().with_fields(
+            expect::field("foo")
+                .with_value(&"bar")
+                .and(expect::field("constant string").with_value(&"also works"))
+                .and(expect::field("foo.bar").with_value(&"baz"))
+                .and(expect::field("message").with_value(&debug(format_args!("quux"))))
+                .only(),
+        )
+    };
+    let (subscriber, handle) = subscriber::mock()
+        .event(expect_event())
+        .event(expect_event())
+        .only()
+        .run_with_handle();
+
+    with_default(subscriber, || {
+        const FOO: &str = "foo";
+        tracing::event!(
+            Level::INFO,
+            { std::convert::identity(FOO) } = "bar",
+            { "constant string" } = "also works",
+            foo.bar = "baz",
+            "quux"
+        );
+        tracing::event!(
+            Level::INFO,
+            {
+                { std::convert::identity(FOO) } = "bar",
+                { "constant string" } = "also works",
+                foo.bar = "baz",
+            },
+            "quux"
+        );
+    });
+
+    handle.assert_finished();
+}