Make Level and LevelFilter Copy

[jyn514]

Motivation

This makes both structs easier to use because you no longer have to
worry about borrow errors while working with them. There's no downside
to making them Copy since both are wrappers around a usize.

Ideally, this would make Metadata::Level return Level instead of
&Level. However that's a breaking change, so I didn't make it here.

Solution

Derive Copy for both structs and fix various warnings that popped up as a result.

[jyn514]

This is split out from #990 (see #990 (comment)).

[jyn514]

I don't know how to fix this error :(

 ---- metadata::tests::level_filter_reprs stdout ----
thread 'metadata::tests::level_filter_reprs' panicked at 'assertion failed: `(left == right)`
  left: `5`,
 right: `140704621814760`: repr changed for LevelFilter::OFF', tracing-core/src/metadata.rs:869:13
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

[jyn514]

I tried this but it didn't work:

diff --git a/tracing-core/src/metadata.rs b/tracing-core/src/metadata.rs
index 8dd216f..d9ea459 100644
--- a/tracing-core/src/metadata.rs
+++ b/tracing-core/src/metadata.rs
@@ -432,7 +432,7 @@ impl LevelFilter {
     // `LevelFilter`. If this is the case, converting a `usize` value into a
     // `LevelFilter` (in `LevelFilter::current`) will be an identity conversion,
     // rather than generating a lookup table.
-    const OFF_USIZE: usize = LevelInner::Error as usize + 1;
+    const OFF_USIZE: usize = unsafe { core::mem::transmute(LevelFilter(None)) };
 
     /// Returns a `LevelFilter` that matches the most verbose [`Level`] that any
     /// currently active [`Subscriber`] will enable.
@@ -851,7 +851,7 @@ mod tests {
     #[test]
     fn level_filter_reprs() {
         let mapping = [
-            (LevelFilter::OFF, LevelInner::Error as usize + 1),
+            (LevelFilter::OFF, LevelFilter::OFF_USIZE),
             (LevelFilter::ERROR, LevelInner::Error as usize),
             (LevelFilter::WARN, LevelInner::Warn as usize),
             (LevelFilter::INFO, LevelInner::Info as usize),

thread 'metadata::tests::level_filter_reprs' panicked at 'assertion failed: `(left == right)`
  left: `5`,
 right: `140381994394536`: repr changed for LevelFilter::OFF', tracing-core/src/metadata.rs:869:13

[hawkw]

Historically, we've avoided adding Copy implementations for public API types, because this represents a potential breaking change if it ever becomes necessary to change the type's internal representation to one that is no longer trivially copyable. In this case, I don't think that's terribly likely, both it's worth weighing the cost before deciding whether to move forward with committing to Copy implementations for these types.

[jyn514]

Given that there's an explicit test for transmuting between usize and LevelFilter, I think it makes sense to add the Copy bound. Breaking Copy would also break the transmute.

[hawkw]

That's fair, in this particular case it's probably justified! :)

[hawkw]

Since this PR changes crates that depend on tracing-core to remove clone calls and rely on Level and LevelFilter being Copy, those crates will no longer compile with older versions of tracing-core. Can we bump the tracing-core version and update tracing, tracing-log, and tracing-subscriber to depend on the new tracing-core version as part of this PR? Thanks!

[jyn514]

Good idea, done!

[hawkw]

Okay, this looks good to me.

in re:

Ideally, this would make Metadata::Level return Level instead of
&Level. However that's a breaking change, so I didn't make it here.

We can make this change in tracing-core 0.2 --- we'll add it to the list of planned breaking changes in #922