subscriber: add minimal #![no_std] support (#1648)

Depends on #1649 

## Motivation

Presently, the `tracing-subscriber` crate requires the Rust standard
library and doesn't build with `#![no_std]` targets. For the most part,
this is fine, as much of `tracing-subscriber` inherently depends on
`std` APIs.

However, `tracing-subscriber` also contains some key abstractions that
are necessary for interoperability: the `Subscriber` and `LookupSpan`
traits. Since these traits are in `tracing-subscriber`, `no-std` users
cannot currently access them.

Some of the other utilities in this crate, such as the field visitor
combinators, may also be useful for `#![no_std]` projects.

## Solution

This branch adds "std" and "alloc" feature flags to
`tracing-subscriber`, for conditionally enabling `libstd` and
`liballoc`, respectively. The `registry`, `fmt`, `EnvFilter`, and
`reload` APIs all require libstd, and cannot be implemented without it,
but the core `Subscribe` and `LookupSpan` traits are now available with
`#![no_std]`.

Fixes #999 

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

tracing-opentelemetry/Cargo.toml

@@ -25,7 +25,7 @@ default = ["tracing-log"]
 opentelemetry = { version = "0.16", default-features = false, features = ["trace"] }
 tracing = { path = "../tracing", version = "0.2", default-features = false, features = ["std"] }
 tracing-core = { path = "../tracing-core", version = "0.2" }
-tracing-subscriber = { path = "../tracing-subscriber", version = "0.3", default-features = false, features = ["registry"] }
+tracing-subscriber = { path = "../tracing-subscriber", version = "0.3", default-features = false, features = ["registry", "std"] }
 tracing-log = { path = "../tracing-log", version = "0.2", default-features = false, optional = true }
 
 [dev-dependencies]

tracing-subscriber/Cargo.toml

@@ -23,7 +23,9 @@ keywords = ["logging", "tracing", "metrics", "subscriber"]
 
 [features]
 
-default = ["smallvec", "fmt", "ansi", "tracing-log"]
+default = ["smallvec", "fmt", "ansi", "tracing-log", "std"]
+alloc = []
+std = ["alloc", "tracing-core/std"]
 env-filter = ["matchers", "regex", "lazy_static", "tracing"]
 fmt = ["registry"]
 ansi = ["fmt", "ansi_term"]
@@ -37,7 +39,7 @@ local-time = ["time/local-offset"]
 tracing-core = { path = "../tracing-core", version = "0.2" }
 
 # only required by the `env-filter` feature
-tracing = { optional = true, path = "../tracing", version = "0.2", default-features = false, features = ["std"] }
+tracing = { optional = true, path = "../tracing", version = "0.2", default-features = false }
 matchers = { optional = true, version = "0.1.0" }
 regex = { optional = true, version = "1", default-features = false, features = ["std"] }
 smallvec = { optional = true, version = "1" }

tracing-subscriber/src/field/debug.rs

@@ -1,8 +1,8 @@
 //! `MakeVisitor` wrappers for working with `fmt::Debug` fields.
-use super::{MakeVisitor, VisitFmt, VisitOutput, VisitWrite};
+use super::{MakeVisitor, VisitFmt, VisitOutput};
 use tracing_core::field::{Field, Visit};
 
-use std::{fmt, io};
+use core::fmt;
 
 /// A visitor wrapper that ensures any `fmt::Debug` fields are formatted using
 /// the alternate (`:#`) formatter.
@@ -84,13 +84,19 @@ where
     }
 }
 
-impl<V> VisitWrite for Alt<V>
-where
-    V: VisitWrite,
-{
-    #[inline]
-    fn writer(&mut self) -> &mut dyn io::Write {
-        self.0.writer()
+feature! {
+    #![feature = "std"]
+    use super::VisitWrite;
+    use std::io;
+
+    impl<V> VisitWrite for Alt<V>
+    where
+        V: VisitWrite,
+    {
+        #[inline]
+        fn writer(&mut self) -> &mut dyn io::Write {
+            self.0.writer()
+        }
     }
 }
 

tracing-subscriber/src/field/delimited.rs

@@ -1,7 +1,7 @@
 //! A `MakeVisitor` wrapper that separates formatted fields with a delimiter.
 use super::{MakeVisitor, VisitFmt, VisitOutput};
 
-use std::fmt;
+use core::fmt;
 use tracing_core::field::{Field, Visit};
 
 /// A `MakeVisitor` wrapper that wraps a visitor that writes formatted output so
@@ -133,6 +133,7 @@ where
 }
 
 #[cfg(test)]
+#[cfg(all(test, feature = "alloc"))]
 mod test {
     use super::*;
     use crate::field::test_util::*;

tracing-subscriber/src/field/display.rs

@@ -1,8 +1,8 @@
 //! `MakeVisitor` wrappers for working with `fmt::Display` fields.
-use super::{MakeVisitor, VisitFmt, VisitOutput, VisitWrite};
+use super::{MakeVisitor, VisitFmt, VisitOutput};
 use tracing_core::field::{Field, Visit};
 
-use std::{fmt, io};
+use core::fmt;
 
 /// A visitor wrapper that ensures any strings named "message" are formatted
 /// using `fmt::Display`
@@ -90,13 +90,19 @@ where
     }
 }
 
-impl<V> VisitWrite for Messages<V>
-where
-    V: VisitWrite,
-{
-    #[inline]
-    fn writer(&mut self) -> &mut dyn io::Write {
-        self.0.writer()
+feature! {
+    #![feature = "std"]
+    use super::VisitWrite;
+    use std::io;
+
+    impl<V> VisitWrite for Messages<V>
+    where
+        V: VisitWrite,
+    {
+        #[inline]
+        fn writer(&mut self) -> &mut dyn io::Write {
+            self.0.writer()
+        }
     }
 }
 

tracing-subscriber/src/field/mod.rs

@@ -2,7 +2,7 @@
 //!
 //! [fields]: tracing_core::field
 //! [field visitors]: tracing_core::field::Visit
-use std::{fmt, io};
+use core::{fmt, marker::PhantomData};
 pub use tracing_core::field::Visit;
 use tracing_core::{
     span::{Attributes, Record},
@@ -108,11 +108,16 @@ where
     }
 }
 
-/// Extension trait implemented by visitors to indicate that they write to an
-/// `io::Write` instance, and allow access to that writer.
-pub trait VisitWrite: VisitOutput<Result<(), io::Error>> {
-    /// Returns the writer that this visitor writes to.
-    fn writer(&mut self) -> &mut dyn io::Write;
+feature! {
+    #![feature = "std"]
+    use std::io;
+
+    /// Extension trait implemented by visitors to indicate that they write to an
+    /// `io::Write` instance, and allow access to that writer.
+    pub trait VisitWrite: VisitOutput<Result<(), io::Error>> {
+        /// Returns the writer that this visitor writes to.
+        fn writer(&mut self) -> &mut dyn io::Write;
+    }
 }
 
 /// Extension trait implemented by visitors to indicate that they write to a
@@ -223,7 +228,7 @@ where
 #[derive(Debug)]
 #[doc(hidden)]
 pub struct MakeExtMarker<T> {
-    _p: std::marker::PhantomData<T>,
+    _p: PhantomData<T>,
 }
 
 #[derive(Debug)]
@@ -232,10 +237,11 @@ pub struct RecordFieldsMarker {
     _p: (),
 }
 
-#[cfg(test)]
+#[cfg(all(test, feature = "alloc"))]
 #[macro_use]
 pub(in crate::field) mod test_util {
     use super::*;
+    pub(in crate::field) use alloc::string::String;
     use tracing_core::{
         callsite::Callsite,
         field::{Field, Value},

tracing-subscriber/src/filter/env/mod.rs

@@ -97,8 +97,7 @@ use tracing_core::{
 /// [`Event`]: tracing_core::Event
 /// [`level`]: tracing_core::Level
 /// [`Metadata`]: tracing_core::Metadata
-#[cfg(feature = "env-filter")]
-#[cfg_attr(docsrs, doc(cfg(feature = "env-filter")))]
+#[cfg_attr(docsrs, doc(cfg(all(feature = "env-filter", feature = "std"))))]
 #[derive(Debug)]
 pub struct EnvFilter {
     statics: directive::Statics,
@@ -121,7 +120,7 @@ type FilterVec<T> = Vec<T>;
 
 /// Indicates that an error occurred while parsing a `EnvFilter` from an
 /// environment variable.
-#[cfg_attr(docsrs, doc(cfg(feature = "env-filter")))]
+#[cfg_attr(docsrs, doc(cfg(all(feature = "env-filter", feature = "std"))))]
 #[derive(Debug)]
 pub struct FromEnvError {
     kind: ErrorKind,

tracing-subscriber/src/filter/mod.rs

@@ -2,12 +2,12 @@
 //! subscriber.
 //!
 //! [`Subscriber`]: crate::fmt::Subscriber
-#[cfg(feature = "env-filter")]
-mod env;
 mod level;
 
 pub use self::level::{LevelFilter, ParseError as LevelParseError};
 
-#[cfg(feature = "env-filter")]
-#[cfg_attr(docsrs, doc(cfg(feature = "env-filter")))]
-pub use self::env::*;
+feature! {
+    #![all(feature = "env-filter", feature = "std")]
+    mod env;
+    pub use self::env::*;
+}

tracing-subscriber/src/fmt/fmt_subscriber.rs

@@ -61,6 +61,7 @@ use tracing_core::{
 ///
 /// [`Subscriber`]: subscribe::Subscribe
 #[derive(Debug)]
+#[cfg_attr(docsrs, doc(cfg(all(feature = "fmt", feature = "std"))))]
 pub struct Subscriber<C, N = format::DefaultFields, E = format::Format, W = fn() -> io::Stdout> {
     make_writer: W,
     fmt_fields: N,

tracing-subscriber/src/fmt/mod.rs

@@ -288,8 +288,11 @@ use std::{any::TypeId, error::Error, io, ptr::NonNull};
 use tracing_core::{collect::Interest, span, Event, Metadata};
 
 mod fmt_subscriber;
+#[cfg_attr(docsrs, doc(cfg(all(feature = "fmt", feature = "std"))))]
 pub mod format;
+#[cfg_attr(docsrs, doc(cfg(all(feature = "fmt", feature = "std"))))]
 pub mod time;
+#[cfg_attr(docsrs, doc(cfg(all(feature = "fmt", feature = "std"))))]
 pub mod writer;
 pub use fmt_subscriber::{FmtContext, FormattedFields, Subscriber};
 
@@ -311,6 +314,7 @@ pub use self::{
 ///
 /// This consists of an inner `Formatter` wrapped in a subscriber that performs filtering.
 #[derive(Debug)]
+#[cfg_attr(docsrs, doc(cfg(all(feature = "fmt", feature = "std"))))]
 pub struct Collector<
     N = format::DefaultFields,
     E = format::Format,
@@ -322,11 +326,13 @@ pub struct Collector<
 
 /// A collector that logs formatted representations of `tracing` events.
 /// This type only logs formatted events; it does not perform any filtering.
+#[cfg_attr(docsrs, doc(cfg(all(feature = "fmt", feature = "std"))))]
 pub type Formatter<N = format::DefaultFields, E = format::Format, W = fn() -> io::Stdout> =
     subscribe::Layered<fmt_subscriber::Subscriber<Registry, N, E, W>, Registry>;
 
 /// Configures and constructs `Collector`s.
 #[derive(Debug)]
+#[cfg_attr(docsrs, doc(cfg(all(feature = "fmt", feature = "std"))))]
 pub struct CollectorBuilder<
     N = format::DefaultFields,
     E = format::Format,
@@ -400,6 +406,7 @@ pub struct CollectorBuilder<
 /// [`init`]: CollectorBuilder::init()
 /// [`try_init`]: CollectorBuilder::try_init()
 /// [`finish`]: CollectorBuilder::finish()
+#[cfg_attr(docsrs, doc(cfg(all(feature = "fmt", feature = "std"))))]
 pub fn fmt() -> CollectorBuilder {
     CollectorBuilder::default()
 }
@@ -411,6 +418,7 @@ pub fn fmt() -> CollectorBuilder {
 ///
 /// [formatting subscriber]: Subscriber
 /// [composed]: super::subscribe
+#[cfg_attr(docsrs, doc(cfg(all(feature = "fmt", feature = "std"))))]
 pub fn subscriber<C>() -> Subscriber<C> {
     Subscriber::default()
 }

tracing-subscriber/src/fmt/writer.rs

@@ -3,7 +3,7 @@
 //! [`io::Write`]: std::io::Write
 
 use std::{
-    fmt::{self, Debug},
+    fmt,
     io::{self, Write},
     sync::{Mutex, MutexGuard},
 };
@@ -642,6 +642,7 @@ pub struct Tee<A, B> {
 /// `tracing-subscriber`'s `FormatEvent`/`FormatTime` traits expect a
 /// `fmt::Write` implementation, while `serde_json::Serializer` and `time`'s
 /// `format_into` methods expect an `io::Write`.
+#[cfg(any(feature = "json", feature = "time"))]
 pub(in crate::fmt) struct WriteAdaptor<'a> {
     fmt_write: &'a mut dyn fmt::Write,
 }
@@ -710,8 +711,8 @@ impl BoxMakeWriter {
     }
 }
 
-impl Debug for BoxMakeWriter {
-    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+impl fmt::Debug for BoxMakeWriter {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
         f.debug_tuple("BoxMakeWriter")
             .field(&format_args!("<{}>", self.name))
             .finish()
@@ -1097,12 +1098,13 @@ where
 
 // === impl WriteAdaptor ===
 
+#[cfg(any(feature = "json", feature = "time"))]
 impl<'a> WriteAdaptor<'a> {
     pub(in crate::fmt) fn new(fmt_write: &'a mut dyn fmt::Write) -> Self {
         Self { fmt_write }
     }
 }
-
+#[cfg(any(feature = "json", feature = "time"))]
 impl<'a> io::Write for WriteAdaptor<'a> {
     fn write(&mut self, buf: &[u8]) -> io::Result<usize> {
         let s =
@@ -1120,6 +1122,7 @@ impl<'a> io::Write for WriteAdaptor<'a> {
     }
 }
 
+#[cfg(any(feature = "json", feature = "time"))]
 impl<'a> fmt::Debug for WriteAdaptor<'a> {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
         f.pad("WriteAdaptor { .. }")