From c09b799f45ff0e6bec9bfd044ab69ab2f6c117f0 Mon Sep 17 00:00:00 2001
From: Eliza Weisman <eliza@buoyant.io>
Date: Wed, 27 Jul 2022 14:52:52 -0700
Subject: [PATCH] core: give `SetGlobalDefaultError` a useful `Debug` impl
 (#2250)

## Motivation

When a global default dispatcher has already been set, the
`dispatch::set_global_default` function fails with a
`SetGlobalDefaultError`. The `fmt::Display` impl for this type includes
a message explaining that the error indicates that a global default has
already been set, but the `fmt::Debug` impl is derived, and just looks
like this:
```
SetGlobalDefaultError { _no_construct: () }
```
which isn't particularly helpful.

Unfortunately, when `unwrap()`ping or `expect()`ing a `Result`, the
`fmt::Debug` implementation for the error type is used, rather than
`fmt::Display`. This means that the error message will not explain to
the user why setting the global default dispatcher failed, which is a
bummer.

## Solution

This branch replaces the derived `Debug` impl with a manually
implemented one that prints the same message as the `Display` impl, but
formatted like a tuple struct with a single string field. This avoids
emitting `Debug` output that's *just* a textual human-readable message,
rather than looking like Rust code, but ensures the message is visible
to the user when writing code like
```rust
tracing::dispatch::set_global_default(foo).unwrap();
```

The mesasge now looks like:
```
SetGlobalDefaultError("a global default trace dispatcher has already been set")
```
which should be a bit easier to debug.
---
 tracing-core/src/dispatcher.rs | 15 +++++++++++++--
 1 file changed, 13 insertions(+), 2 deletions(-)

diff --git a/tracing-core/src/dispatcher.rs b/tracing-core/src/dispatcher.rs
index 041722c366..a3661817c0 100644
--- a/tracing-core/src/dispatcher.rs
+++ b/tracing-core/src/dispatcher.rs
@@ -293,14 +293,21 @@ pub fn has_been_set() -> bool {
 }
 
 /// Returned if setting the global dispatcher fails.
-#[derive(Debug)]
 pub struct SetGlobalDefaultError {
     _no_construct: (),
 }
 
+impl fmt::Debug for SetGlobalDefaultError {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        f.debug_tuple("SetGlobalDefaultError")
+            .field(&Self::MESSAGE)
+            .finish()
+    }
+}
+
 impl fmt::Display for SetGlobalDefaultError {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
-        f.pad("a global default trace dispatcher has already been set")
+        f.pad(Self::MESSAGE)
     }
 }
 
@@ -308,6 +315,10 @@ impl fmt::Display for SetGlobalDefaultError {
 #[cfg_attr(docsrs, doc(cfg(feature = "std")))]
 impl error::Error for SetGlobalDefaultError {}
 
+impl SetGlobalDefaultError {
+    const MESSAGE: &'static str = "a global default trace dispatcher has already been set";
+}
+
 /// Executes a closure with a reference to this thread's current [dispatcher].
 ///
 /// Note that calls to `get_default` should not be nested; if this function is