Specialize target and level arguments that can be statically determined

src/__private_api.rs

@@ -1,7 +1,8 @@
 //! WARNING: this is not part of the crate's public API and is subject to change at any time
 
-use self::sealed::KVs;
-use crate::{Level, Metadata, Record};
+use self::sealed::{KVs, Level as LevelTrait, Target};
+use crate::{Level, LevelFilter, Metadata, Record};
+use std::cmp::Ordering;
 use std::fmt::Arguments;
 pub use std::{file, format_args, line, module_path, stringify};
 
@@ -12,36 +13,122 @@ pub type Value<'a> = dyn crate::kv::value::ToValue + 'a;
 pub type Value<'a> = str;
 
 mod sealed {
+    /// Types for the `target` argument.
+    pub trait Target<'a>: Copy {
+        fn into_target(self, module_path: &'a &'a str) -> &'a &'a str;
+    }
+
+    /// Types for the `level` argument.
+    pub trait Level: Copy {
+        fn into_level(self) -> crate::Level;
+    }
+
     /// Types for the `kv` argument.
     pub trait KVs<'a> {
-        fn into_kvs(self) -> Option<&'a [(&'a str, &'a super::Value<'a>)]>;
+        fn into_kvs(self) -> Option<&'a &'a [(&'a str, &'a super::Value<'a>)]>;
+    }
+}
+
+// Types for the `target` argument.
+
+/// Caller specified target explicitly.
+impl<'a> Target<'a> for &'a &'a str {
+    #[inline]
+    fn into_target(self, _module_path: &'a &'a str) -> &'a &'a str {
+        self
     }
 }
 
-// Types for the `kv` argument.
+/// Caller did not specified target.
+impl<'a> Target<'a> for () {
+    #[inline]
+    fn into_target(self, module_path: &'a &'a str) -> &'a &'a str {
+        module_path
+    }
+}
+
+// Types for the `kvs` argument.
 
-impl<'a> KVs<'a> for &'a [(&'a str, &'a Value<'a>)] {
+/// Caller specified key-value data explicitly.
+impl<'a> KVs<'a> for &'a &'a [(&'a str, &'a Value<'a>)] {
     #[inline]
-    fn into_kvs(self) -> Option<&'a [(&'a str, &'a Value<'a>)]> {
+    fn into_kvs(self) -> Option<&'a &'a [(&'a str, &'a Value<'a>)]> {
         Some(self)
     }
 }
 
+/// Caller did not specify key-value data.
 impl<'a> KVs<'a> for () {
     #[inline]
-    fn into_kvs(self) -> Option<&'a [(&'a str, &'a Value<'a>)]> {
+    fn into_kvs(self) -> Option<&'a &'a [(&'a str, &'a Value<'a>)]> {
         None
     }
 }
 
+// Types for the `level` argument.
+
+/// The log level is dynamically determined.
+impl LevelTrait for Level {
+    #[inline]
+    fn into_level(self) -> Level {
+        self
+    }
+}
+
+macro_rules! define_static_levels {
+    ($(($name:ident, $level:ident),)*) => {$(
+        #[derive(Clone, Copy, Debug)]
+        pub struct $name;
+
+        /// The log level is statically determined.
+        impl LevelTrait for $name {
+            #[inline]
+            fn into_level(self) -> Level {
+                Level::$level
+            }
+        }
+
+        impl PartialEq<LevelFilter> for $name {
+            #[inline]
+            fn eq(&self, other: &LevelFilter) -> bool {
+                self.into_level().eq(other)
+            }
+        }
+
+        impl PartialOrd<LevelFilter> for $name {
+            #[inline]
+            fn partial_cmp(&self, other: &LevelFilter) -> Option<Ordering> {
+                self.into_level().partial_cmp(other)
+            }
+        }
+    )*};
+}
+
+define_static_levels![
+    (LevelError, Error),
+    (LevelWarn, Warn),
+    (LevelInfo, Info),
+    (LevelDebug, Debug),
+    (LevelTrace, Trace),
+];
+
 // Log implementation.
 
+/// Log arguments that are not generic.
+#[derive(Debug)]
+pub struct NonGenericArgs<'a> {
+    pub module_path_and_file: &'static (&'static str, &'static str),
+    pub line: u32,
+    pub args: Arguments<'a>,
+}
+
+// Note that all argument types are selected to have sizes less than or equal to a single pointer size, which allows
+// tail call optimizations to be applied.
 fn log_impl(
-    args: Arguments,
+    non_generic_args: &NonGenericArgs,
+    target: &&str,
     level: Level,
-    &(target, module_path, file): &(&str, &'static str, &'static str),
-    line: u32,
-    kvs: Option<&[(&str, &Value)]>,
+    kvs: Option<&&[(&str, &Value)]>,
 ) {
     #[cfg(not(feature = "kv_unstable"))]
     if kvs.is_some() {
@@ -53,33 +140,34 @@ fn log_impl(
     let mut builder = Record::builder();
 
     builder
-        .args(args)
+        .args(non_generic_args.args)
         .level(level)
         .target(target)
-        .module_path_static(Some(module_path))
-        .file_static(Some(file))
-        .line(Some(line));
+        .module_path_static(Some(non_generic_args.module_path_and_file.0))
+        .file_static(Some(non_generic_args.module_path_and_file.1))
+        .line(Some(non_generic_args.line));
 
     #[cfg(feature = "kv_unstable")]
     builder.key_values(&kvs);
 
     crate::logger().log(&builder.build());
 }
 
-pub fn log<'a, K>(
-    args: Arguments,
-    level: Level,
-    target_module_path_and_file: &(&str, &'static str, &'static str),
-    line: u32,
-    kvs: K,
-) where
+// `#[inline(never)]` is used to prevent compiler from inlining this function so that the binary size could be kept as
+// small as possible. Also, the argument types are carefully selected so that the performance cost of this function
+// could be kept as low as possible.
+#[inline(never)]
+pub fn log<'a, T, L, K>(non_generic_args: &NonGenericArgs, target: T, level: L, kvs: K)
+where
+    T: Target<'a>,
+    L: LevelTrait,
     K: KVs<'a>,
 {
+    // For all possible generic arguments combinations, tail call optimization can be applied to this function call.
     log_impl(
-        args,
-        level,
-        target_module_path_and_file,
-        line,
+        non_generic_args,
+        target.into_target(&non_generic_args.module_path_and_file.0),
+        level.into_level(),
         kvs.into_kvs(),
     )
 }