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

Backports #1648 from `master`.

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 `Layer` 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 `Layer` and `LookupSpan` traits are now available with
`#![no_std]`.

Fixes #999

Signed-off-by: Eliza Weisman <[email protected]>

.github/workflows/CI.yml

@@ -243,9 +243,15 @@ jobs:
       working-directory: tracing
       # this skips running doctests under the `--no-default-features` flag,
       # as rustdoc isn't aware of cargo's feature flags.
-    - name: "Test tracing-subscriber with all features disabled"
+    - name: "Test tracing-subscriber no-std support"
       run: cargo test --lib --tests --no-default-features
       working-directory: tracing-subscriber
+    - name: "Test tracing-subscriber with liballoc only"
+      run: cargo test --lib --tests --no-default-features --features "alloc"
+      working-directory: tracing-subscriber
+    - name: "Test tracing-subscriber with no default features"
+      run: cargo test --lib --tests --no-default-features --features "std"
+      working-directory: tracing-subscriber
 
   style:
     # Check style.

tracing-opentelemetry/Cargo.toml

@@ -25,7 +25,7 @@ default = ["tracing-log"]
 opentelemetry = { version = "0.16", default-features = false, features = ["trace"] }
 tracing = { path = "../tracing", version = "0.1", default-features = false, features = ["std"] }
 tracing-core = { path = "../tracing-core", version = "0.1" }
-tracing-subscriber = { path = "../tracing-subscriber", version = "0.2", default-features = false, features = ["registry"] }
+tracing-subscriber = { path = "../tracing-subscriber", version = "0.2", default-features = false, features = ["registry", "std"] }
 tracing-log = { path = "../tracing-log", version = "0.1", 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 = "0.1.20" }
 
 # only required by the filter feature
-tracing = { optional = true, path = "../tracing", version = "0.1", default-features = false, features = ["std"] }
+tracing = { optional = true, path = "../tracing", version = "0.1", 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

@@ -1,8 +1,8 @@
 //! Utilities for working with [fields] and [field visitors].
 //!
-//! [fields]: https://docs.rs/tracing-core/latest/tracing_core/field/index.html
-//! [field visitors]: https://docs.rs/tracing-core/latest/tracing_core/field/trait.Visit.html
-use std::{fmt, io};
+//! [fields]: tracing_core::field
+//! [field visitors]: tracing_core::field::Visit
+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/directive.rs

@@ -1,5 +1,10 @@
 use crate::filter::level::{self, LevelFilter};
-use std::{cmp::Ordering, error::Error, fmt, iter::FromIterator, str::FromStr};
+#[cfg(not(feature = "std"))]
+use alloc::{
+    string::String,
+    vec::{self, Vec},
+};
+use core::{cmp::Ordering, fmt, iter::FromIterator, slice, str::FromStr};
 use tracing_core::Metadata;
 /// Indicates that a string could not be parsed as a filtering directive.
 #[derive(Debug)]
@@ -35,19 +40,21 @@ pub(in crate::filter) trait Match {
 
 #[derive(Debug)]
 enum ParseErrorKind {
-    Field(Box<dyn Error + Send + Sync>),
+    #[cfg(feature = "std")]
+    Field(Box<dyn std::error::Error + Send + Sync>),
     Level(level::ParseError),
     Other(Option<&'static str>),
 }
 
 // === impl DirectiveSet ===
 
 impl<T> DirectiveSet<T> {
+    #[cfg(feature = "std")]
     pub(crate) fn is_empty(&self) -> bool {
         self.directives.is_empty()
     }
 
-    pub(crate) fn iter(&self) -> std::slice::Iter<'_, T> {
+    pub(crate) fn iter(&self) -> slice::Iter<'_, T> {
         self.directives.iter()
     }
 }
@@ -118,7 +125,7 @@ impl<T> IntoIterator for DirectiveSet<T> {
     #[cfg(feature = "smallvec")]
     type IntoIter = smallvec::IntoIter<[T; 8]>;
     #[cfg(not(feature = "smallvec"))]
-    type IntoIter = std::vec::IntoIter<T>;
+    type IntoIter = vec::IntoIter<T>;
 
     fn into_iter(self) -> Self::IntoIter {
         self.directives.into_iter()
@@ -358,6 +365,7 @@ impl FromStr for StaticDirective {
 // === impl ParseError ===
 
 impl ParseError {
+    #[cfg(feature = "std")]
     pub(crate) fn new() -> Self {
         ParseError {
             kind: ParseErrorKind::Other(None),
@@ -377,17 +385,19 @@ impl fmt::Display for ParseError {
             ParseErrorKind::Other(None) => f.pad("invalid filter directive"),
             ParseErrorKind::Other(Some(msg)) => write!(f, "invalid filter directive: {}", msg),
             ParseErrorKind::Level(ref l) => l.fmt(f),
+            #[cfg(feature = "std")]
             ParseErrorKind::Field(ref e) => write!(f, "invalid field filter: {}", e),
         }
     }
 }
 
-impl Error for ParseError {
+#[cfg(feature = "std")]
+impl std::error::Error for ParseError {
     fn description(&self) -> &str {
         "invalid filter directive"
     }
 
-    fn source(&self) -> Option<&(dyn Error + 'static)> {
+    fn source(&self) -> Option<&(dyn std::error::Error + 'static)> {
         match self.kind {
             ParseErrorKind::Other(_) => None,
             ParseErrorKind::Level(ref l) => Some(l),
@@ -396,8 +406,9 @@ impl Error for ParseError {
     }
 }
 
-impl From<Box<dyn Error + Send + Sync>> for ParseError {
-    fn from(e: Box<dyn Error + Send + Sync>) -> Self {
+#[cfg(feature = "std")]
+impl From<Box<dyn std::error::Error + Send + Sync>> for ParseError {
+    fn from(e: Box<dyn std::error::Error + Send + Sync>) -> Self {
         Self {
             kind: ParseErrorKind::Field(e),
         }

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

@@ -94,16 +94,13 @@ use tracing_core::{
 /// without filtering on field values. When these features are not required,
 /// [`Targets`] provides a lighter-weight alternative to [`EnvFilter`].
 ///
-/// [`Layer`]: ../layer/trait.Layer.html
-/// [`env_logger`]: https://docs.rs/env_logger/0.7.1/env_logger/#enabling-logging
-/// [`Span`]: https://docs.rs/tracing-core/latest/tracing_core/span/index.html
-/// [fields]: https://docs.rs/tracing-core/latest/tracing_core/struct.Field.html
-/// [`Event`]: https://docs.rs/tracing-core/latest/tracing_core/struct.Event.html
-/// [`level`]: https://docs.rs/tracing-core/latest/tracing_core/struct.Level.html
-/// [`Metadata`]: https://docs.rs/tracing-core/latest/tracing_core/struct.Metadata.html
+/// [`Span`]: tracing_core::span
+/// [fields]: tracing_core::Field
+/// [`Event`]: tracing_core::Event
+/// [`level`]: tracing_core::Level
+/// [`Metadata`]: tracing_core::Metadata
 /// [`Targets`]: crate::filter::Targets
-#[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 +118,7 @@ type FieldMap<T> = HashMap<Field, 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,