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

tracing-subscriber/src/filter/directive.rs

@@ -1,5 +1,9 @@
 use crate::filter::level::{self, LevelFilter};
-use std::{cmp::Ordering, error::Error, fmt, iter::FromIterator, str::FromStr};
+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 +39,21 @@ 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 +124,7 @@ 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()
@@ -353,6 +359,7 @@ impl FromStr for StaticDirective {
 // === impl ParseError ===
 
 impl ParseError {
+    #[cfg(feature = "std")]
     pub(crate) fn new() -> Self {
         ParseError {
             kind: ParseErrorKind::Other(None),
@@ -372,17 +379,19 @@ 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),
@@ -391,8 +400,9 @@ 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

@@ -102,7 +102,6 @@ use tracing_core::{
 /// [`Metadata`]: tracing_core::Metadata
 /// [`Targets`]: crate::filter::Targets
 #[cfg_attr(docsrs, doc(cfg(all(feature = "env-filter", feature = "std"))))]
-#[cfg(feature = "env-filter")]
 #[derive(Debug)]
 pub struct EnvFilter {
     statics: directive::Statics,

tracing-subscriber/src/filter/filter_fn.rs

@@ -1,5 +1,3 @@
-#[cfg(feature = "registry")]
-use crate::subscribe::Filter;
 use crate::{
     filter::LevelFilter,
     subscribe::{Context, Subscribe},
@@ -323,25 +321,6 @@ where
     }
 }
 
-#[cfg(feature = "registry")]
-#[cfg_attr(docsrs, doc(cfg(feature = "registry")))]
-impl<C, F> Filter<C> for FilterFn<F>
-where
-    F: Fn(&Metadata<'_>) -> bool,
-{
-    fn enabled(&self, metadata: &Metadata<'_>, _: &Context<'_, C>) -> bool {
-        self.is_enabled(metadata)
-    }
-
-    fn callsite_enabled(&self, metadata: &'static Metadata<'static>) -> Interest {
-        self.is_callsite_enabled(metadata)
-    }
-
-    fn max_level_hint(&self) -> Option<LevelFilter> {
-        self.max_level_hint
-    }
-}
-
 impl<C, F> Subscribe<C> for FilterFn<F>
 where
     F: Fn(&Metadata<'_>) -> bool + 'static,
@@ -661,26 +640,6 @@ where
     }
 }
 
-#[cfg(feature = "registry")]
-#[cfg_attr(docsrs, doc(cfg(feature = "registry")))]
-impl<C, F, R> Filter<C> for DynFilterFn<C, F, R>
-where
-    F: Fn(&Metadata<'_>, &Context<'_, C>) -> bool,
-    R: Fn(&'static Metadata<'static>) -> Interest,
-{
-    fn enabled(&self, metadata: &Metadata<'_>, cx: &Context<'_, C>) -> bool {
-        self.is_enabled(metadata, cx)
-    }
-
-    fn callsite_enabled(&self, metadata: &'static Metadata<'static>) -> Interest {
-        self.is_callsite_enabled(metadata)
-    }
-
-    fn max_level_hint(&self) -> Option<LevelFilter> {
-        self.max_level_hint
-    }
-}
-
 impl<C, F, R> Subscribe<C> for DynFilterFn<C, F, R>
 where
     F: Fn(&Metadata<'_>, &Context<'_, C>) -> bool + 'static,
@@ -741,6 +700,48 @@ where
     }
 }
 
+// === PLF impls ===
+
+feature! {
+    #![all(feature = "registry", feature = "std")]
+    use crate::subscribe::Filter;
+
+    impl<C, F> Filter<C> for FilterFn<F>
+    where
+        F: Fn(&Metadata<'_>) -> bool,
+    {
+        fn enabled(&self, metadata: &Metadata<'_>, _: &Context<'_, C>) -> bool {
+            self.is_enabled(metadata)
+        }
+
+        fn callsite_enabled(&self, metadata: &'static Metadata<'static>) -> Interest {
+            self.is_callsite_enabled(metadata)
+        }
+
+        fn max_level_hint(&self) -> Option<LevelFilter> {
+            self.max_level_hint
+        }
+    }
+
+    impl<C, F, R> Filter<C> for DynFilterFn<C, F, R>
+    where
+        F: Fn(&Metadata<'_>, &Context<'_, C>) -> bool,
+        R: Fn(&'static Metadata<'static>) -> Interest,
+    {
+        fn enabled(&self, metadata: &Metadata<'_>, cx: &Context<'_, C>) -> bool {
+            self.is_enabled(metadata, cx)
+        }
+
+        fn callsite_enabled(&self, metadata: &'static Metadata<'static>) -> Interest {
+            self.is_callsite_enabled(metadata)
+        }
+
+        fn max_level_hint(&self) -> Option<LevelFilter> {
+            self.max_level_hint
+        }
+    }
+}
+
 fn is_below_max_level(hint: &Option<LevelFilter>, metadata: &Metadata<'_>) -> bool {
     hint.as_ref()
         .map(|hint| metadata.level() <= hint)

tracing-subscriber/src/filter/mod.rs

@@ -8,37 +8,44 @@
 //!
 //! [`subscribe` module documentation]: crate::subscribe#filtering-with-subscribers
 //! [`Subscribe`]: crate::subscribe
-mod directive;
-#[cfg(feature = "env-filter")]
-mod env;
 mod filter_fn;
 mod level;
-#[cfg(feature = "registry")]
-mod subscriber_filters;
-pub mod targets;
 
-pub use self::directive::ParseError;
+feature! {
+    #![all(feature = "env-filter", feature = "std")]
+    mod env;
+    pub use self::env::*;
+}
+
+feature! {
+    #![all(feature = "registry", feature = "std")]
+    mod subscriber_filters;
+    pub use self::subscriber_filters::*;
+}
+
 pub use self::filter_fn::*;
 #[cfg(not(feature = "registry"))]
 pub(crate) use self::has_plf_stubs::*;
 
-#[cfg(feature = "registry")]
-#[cfg_attr(docsrs, doc(cfg(feature = "registry")))]
-pub use self::subscriber_filters::*;
-
 pub use self::level::{LevelFilter, ParseError as LevelParseError};
 
-#[cfg(feature = "env-filter")]
-#[cfg_attr(docsrs, doc(cfg(feature = "env-filter")))]
-pub use self::env::*;
+#[cfg(not(all(feature = "registry", feature = "std")))]
+pub(crate) use self::has_plf_stubs::*;
+
+feature! {
+    #![any(feature = "std", feature = "alloc")]
+    pub mod targets;
+    pub use self::targets::Targets;
 
-pub use self::targets::Targets;
+    mod directive;
+    pub use self::directive::ParseError;
+}
 
 /// Stub implementations of the per-layer-fitler detection functions for when the
 /// `registry` feature is disabled.
-#[cfg(not(feature = "registry"))]
+#[cfg(not(all(feature = "registry", feature = "std")))]
 mod has_plf_stubs {
-    pub(crate) fn is_plf_downcast_marker(_: std::any::TypeId) -> bool {
+    pub(crate) fn is_plf_downcast_marker(_: core::any::TypeId) -> bool {
         false
     }
 

tracing-subscriber/src/filter/targets.rs

@@ -13,8 +13,11 @@ use crate::{
     },
     subscribe,
 };
-use std::{
+#[cfg(not(feature = "std"))]
+use alloc::string::String;
+use core::{
     iter::{Extend, FilterMap, FromIterator},
+    slice,
     str::FromStr,
 };
 use tracing_core::{Collect, Interest, Metadata};
@@ -462,7 +465,7 @@ impl Iterator for IntoIter {
 #[derive(Debug)]
 pub struct Iter<'a>(
     FilterMap<
-        std::slice::Iter<'a, StaticDirective>,
+        slice::Iter<'a, StaticDirective>,
         fn(&'a StaticDirective) -> Option<(&'a str, LevelFilter)>,
     >,
 );
@@ -494,6 +497,17 @@ impl<'a> Iterator for Iter<'a> {
 mod tests {
     use super::*;
 
+    feature! {
+        #![not(feature = "std")]
+        use alloc::{vec, vec::Vec, string::ToString};
+
+        // `dbg!` is only available with `libstd`; just nop it out when testing
+        // with alloc only.
+        macro_rules! dbg {
+            ($x:expr) => { $x }
+        }
+    }
+
     fn expect_parse(s: &str) -> Targets {
         match dbg!(s).parse::<Targets>() {
             Err(e) => panic!("string {:?} did not parse successfully: {}", s, e),
@@ -643,6 +657,8 @@ mod tests {
     }
 
     #[test]
+    // `println!` is only available with `libstd`.
+    #[cfg(feature = "std")]
     fn size_of_filters() {
         fn print_sz(s: &str) {
             let filter = s.parse::<Targets>().expect("filter should parse");

tracing-subscriber/src/lib.rs

@@ -129,7 +129,7 @@
 //! [`time` crate]: https://crates.io/crates/time
 //! [`liballoc`]: https://doc.rust-lang.org/alloc/index.html
 //! [`libstd`]: https://doc.rust-lang.org/std/index.html
-#![doc(html_root_url = "https://docs.rs/tracing-subscriber/0.2.12")]
+#![doc(html_root_url = "https://docs.rs/tracing-subscriber/0.2.25")]
 #![doc(
     html_logo_url = "https://raw.githubusercontent.com/tokio-rs/tracing/master/assets/logo-type.png",
     html_favicon_url = "https://raw.githubusercontent.com/tokio-rs/tracing/master/assets/favicon.ico",