tracing: replace future names with spawn locations in task spans (#3074)

## Motivation

Currently, the per-task `tracing` spans generated by tokio's `tracing`
feature flag include the `std::any::type_name` of the future that was
spawned. When future combinators and/or libraries like Tower are in use,
these future names can get _quite_ long. Furthermore, when formatting
the `tracing` spans with their parent spans as context, any other task
spans in the span context where the future was spawned from can _also_
include extremely long future names.

In some cases, this can result in extremely high memory use just to
store the future names. For example, in Linkerd, when we enable
`tokio=trace` to enable the task spans, there's a spawned task whose
future name is _232990 characters long_. A proxy with only 14 spawned
tasks generates a task list that's over 690 KB. Enabling task spans
under load results in the process getting OOM killed very quickly.

## Solution

This branch removes future type names from the spans generated by
`spawn`. As a replacement, to allow identifying which `spawn` call a
span corresponds to, the task span now contains the source code location
where `spawn` was called, when the compiler supports the
`#[track_caller]` attribute. Since `track_caller` was stabilized in Rust
1.46.0, and our minimum supported Rust version is 1.45.0, we can't
assume that `#[track_caller]` is always available. Instead, we have a
RUSTFLAGS cfg, `tokio_track_caller`, that guards whether or not we use
it. I've also added a `build.rs` that detects the compiler minor
version, and sets the cfg flag automatically if the current compiler
version is >= 1.46. This means users shouldn't have to enable
`tokio_track_caller` manually.

Here's the trace output from the `chat` example, before this change:
![Screenshot_20201030_110157](https://user-images.githubusercontent.com/2796466/97741071-6d408800-1a9f-11eb-9ed6-b25e72f58c7b.png)
...and after:
![Screenshot_20201030_110303](https://user-images.githubusercontent.com/2796466/97741112-7e899480-1a9f-11eb-9197-c5a3f9ea1c05.png)

Closes #3073

Signed-off-by: Eliza Weisman <eliza@buoyant.io>

1 files changed, 22 insertions, 0 deletions

diff --git a/tokio/build.rs b/tokio/build.rs
new file mode 100644
index 00000000..fe5c8300
--- /dev/null
+++ b/tokio/build.rs
@@ -0,0 +1,22 @@
+use autocfg::AutoCfg;
+
+fn main() {
+    match AutoCfg::new() {
+        Ok(ac) => {
+            // The #[track_caller] attribute was stabilized in rustc 1.46.0.
+            if ac.probe_rustc_version(1, 46) {
+                autocfg::emit("tokio_track_caller")
+            }
+        }
+
+        Err(e) => {
+            // If we couldn't detect the compiler version and features, just
+            // print a warning. This isn't a fatal error: we can still build
+            // Tokio, we just can't enable cfgs automatically.
+            println!(
+                "cargo:warning=tokio: failed to detect compiler features: {}",
+                e
+            );
+        }
+    }
+}