tracing, tracing-futures: instrument Future inside Drop (tokio-rs#2562)

## Motivation

Currently it is not possible to disable ANSI in `fmt::Subscriber`
without enabling the "ansi" crate feature. This makes it difficult for
users to implement interoperable settings that are controllable with
crate features without having to pull in the dependencies "ansi" does.

I hit this while writing an application with multiple logging options
set during compile-time and I wanted to cut down on dependencies if
possible.

## Solution

This changes `fmt::Subscriber::with_ansi()` to not require the "ansi"
feature flag. This way, `with_ansi(false)` can be called even when the
"ansi" feature is disabled. Calling `with_ansi(true)` when the "ansi"
feature is not enabled will panic in debug mode, or print a warning if
debug assertions are disabled.

Co-authored-by: daxpedda <[email protected]>
Co-authored-by: Eliza Weisman <[email protected]>

Author: 3 people

tracing-attributes/tests/async_fn.rs

@@ -124,6 +124,8 @@ fn async_fn_nested() {
         .exit(span2.clone())
         .enter(span2.clone())
         .exit(span2.clone())
+        .enter(span2.clone())
+        .exit(span2.clone())
         .drop_span(span2)
         .exit(span.clone())
         .enter(span.clone())
@@ -207,13 +209,17 @@ fn async_fn_with_async_trait() {
         .exit(span3.clone())
         .enter(span3.clone())
         .exit(span3.clone())
+        .enter(span3.clone())
+        .exit(span3.clone())
         .drop_span(span3)
         .new_span(span2.clone().with_fields(expect::field("self")))
         .enter(span2.clone())
         .event(expect::event().with_fields(expect::field("val").with_value(&5u64)))
         .exit(span2.clone())
         .enter(span2.clone())
         .exit(span2.clone())
+        .enter(span2.clone())
+        .exit(span2.clone())
         .drop_span(span2)
         .exit(span.clone())
         .enter(span.clone())

tracing-attributes/tests/err.rs

@@ -81,6 +81,8 @@ fn test_async() {
         .exit(span.clone())
         .enter(span.clone())
         .exit(span.clone())
+        .enter(span.clone())
+        .exit(span.clone())
         .drop_span(span)
         .only()
         .run_with_handle();
@@ -137,6 +139,8 @@ fn test_mut_async() {
         .exit(span.clone())
         .enter(span.clone())
         .exit(span.clone())
+        .enter(span.clone())
+        .exit(span.clone())
         .drop_span(span)
         .only()
         .run_with_handle();

tracing/src/instrument.rs

@@ -6,6 +6,13 @@ use core::{
     pin::Pin,
     task::{Context, Poll},
 };
+use core::{
+    future::Future,
+    marker::Sized,
+    mem::{self, ManuallyDrop},
+    pin::Pin,
+    task::{Context, Poll},
+};
 use pin_project_lite::pin_project;
 
 #[cfg(feature = "std")]
@@ -451,14 +458,25 @@ impl<T> WithDispatch<T> {
 
     /// Get a pinned mutable reference to the wrapped type.
     pub fn inner_pin_mut(self: Pin<&mut Self>) -> Pin<&mut T> {
-        self.project().inner
+        self.project().span_and_inner_pin_mut().1
     }
 
     /// Consumes the `Instrumented`, returning the wrapped type.
     ///
     /// Note that this drops the span.
     pub fn into_inner(self) -> T {
-        self.inner
+        // To manually destructure `Instrumented` without `Drop`, we save
+        // pointers to the fields and use `mem::forget` to leave those pointers
+        // valid.
+        let span: *const Span = &self.span;
+        let inner: *const ManuallyDrop<T> = &self.inner;
+        mem::forget(self);
+        // SAFETY: Those pointers are valid for reads, because `Drop` didn't
+        //         run, and properly aligned, because `Instrumented` isn't
+        //         `#[repr(packed)]`.
+        let _span = unsafe { span.read() };
+        let inner = unsafe { inner.read() };
+        ManuallyDrop::into_inner(inner)
     }
 }