tracing: Switch to unconditional no_std

… and remove stdlib module.

Author: jplatte

tracing/src/lib.rs

@@ -912,7 +912,8 @@
 //! [static verbosity level]: level_filters#compile-time-filters
 //! [instrument]: https://docs.rs/tracing-attributes/latest/tracing_attributes/attr.instrument.html
 //! [flags]: #crate-feature-flags
-#![cfg_attr(not(feature = "std"), no_std)]
+
+#![no_std]
 #![cfg_attr(docsrs, feature(doc_cfg), deny(rustdoc::broken_intra_doc_links))]
 #![doc(
     html_logo_url = "https://raw.githubusercontent.com/tokio-rs/tracing/main/assets/logo-type.png",
@@ -941,8 +942,8 @@
     while_true
 )]
 
-#[cfg(not(feature = "std"))]
-extern crate alloc;
+#[cfg(feature = "std")]
+extern crate std;
 
 // Somehow this `use` statement is necessary for us to re-export the `core`
 // macros on Rust 1.26.0. I'm not sure how this makes it work, but it does.
@@ -980,7 +981,6 @@ pub mod field;
 pub mod instrument;
 pub mod level_filters;
 pub mod span;
-pub(crate) mod stdlib;
 pub mod subscriber;
 
 #[doc(hidden)]

tracing/src/span.rs

@@ -317,19 +317,20 @@
 //! [`follows_from`]: Span::follows_from()
 //! [guard]: Entered
 //! [parent]: #span-relationships
+
 pub use tracing_core::span::{Attributes, Id, Record};
 
-use crate::stdlib::{
+use crate::{
+    dispatcher::{self, Dispatch},
+    field, Metadata,
+};
+use core::{
     cmp, fmt,
     hash::{Hash, Hasher},
     marker::PhantomData,
     mem,
     ops::Deref,
 };
-use crate::{
-    dispatcher::{self, Dispatch},
-    field, Metadata,
-};
 
 /// Trait implemented by types which have a span `Id`.
 pub trait AsId: crate::sealed::Sealed {