Rework structured value casting (#396)

Also adds a const-fn based mechanism for pulling concrete values out of generic ones

Author: KodrAus

benches/value.rs

@@ -0,0 +1,30 @@
+#![cfg(feature = "kv_unstable")]
+#![feature(test)]
+
+extern crate log;
+extern crate test;
+
+use log::kv::Value;
+
+#[bench]
+fn u8_to_value(b: &mut test::Bencher) {
+    b.iter(|| Value::from(1u8))
+}
+
+#[bench]
+fn u8_to_value_debug(b: &mut test::Bencher) {
+    b.iter(|| Value::from_debug(&1u8))
+}
+
+#[bench]
+fn str_to_value_debug(b: &mut test::Bencher) {
+    b.iter(|| Value::from_debug(&"a string"))
+}
+
+#[bench]
+fn custom_to_value_debug(b: &mut test::Bencher) {
+    #[derive(Debug)]
+    struct A;
+
+    b.iter(|| Value::from_debug(&A))
+}

build.rs

@@ -2,13 +2,72 @@
 //! atomics and sets `cfg` flags accordingly.
 
 use std::env;
+use std::process::Command;
+use std::str::{self, FromStr};
+
+#[cfg(feature = "kv_unstable")]
+#[path = "src/kv/value/internal/cast/primitive.rs"]
+mod primitive;
 
 fn main() {
-    let target = env::var("TARGET").unwrap();
+    let minor = match rustc_minor_version() {
+        Some(minor) => minor,
+        None => return,
+    };
+
+    let target = match rustc_target() {
+        Some(target) => target,
+        None => return,
+    };
 
+    // If the target isn't thumbv6 then we can use atomic CAS
     if !target.starts_with("thumbv6") {
         println!("cargo:rustc-cfg=atomic_cas");
     }
 
+    // If the Rust version is at least 1.46.0 then we can use type ids at compile time
+    if minor >= 47 {
+        println!("cargo:rustc-cfg=const_type_id");
+    }
+
+    // Generate sorted type id lookup
+    #[cfg(feature = "kv_unstable")]
+    primitive::generate();
+
+    println!("cargo:rustc-cfg=srcbuild");
     println!("cargo:rerun-if-changed=build.rs");
 }
+
+fn rustc_target() -> Option<String> {
+    env::var("TARGET").ok()
+}
+
+// From the `serde` build script
+fn rustc_minor_version() -> Option<u32> {
+    let rustc = match env::var_os("RUSTC") {
+        Some(rustc) => rustc,
+        None => return None,
+    };
+
+    let output = match Command::new(rustc).arg("--version").output() {
+        Ok(output) => output,
+        Err(_) => return None,
+    };
+
+    let version = match str::from_utf8(&output.stdout) {
+        Ok(version) => version,
+        Err(_) => return None,
+    };
+
+    let mut pieces = version.split('.');
+    if pieces.next() != Some("rustc 1") {
+        return None;
+    }
+
+    let next = match pieces.next() {
+        Some(next) => next,
+        None => return None,
+    };
+
+    u32::from_str(next).ok()
+}

src/kv/value/fill.rs

@@ -2,17 +2,17 @@
 
 use std::fmt;
 
-use super::internal::{Erased, Inner, Visitor};
+use super::internal::{Inner, Visitor};
 use super::{Error, Value};
 
 impl<'v> Value<'v> {
     /// Get a value from a fillable slot.
     pub fn from_fill<T>(value: &'v T) -> Self
     where
-        T: Fill + 'static,
+        T: Fill,
     {
         Value {
-            inner: Inner::Fill(unsafe { Erased::new_unchecked::<T>(value) }),
+            inner: Inner::Fill(value),
         }
     }
 }

src/kv/value/impls.rs

@@ -7,24 +7,6 @@ use std::fmt;
 
 use super::{Primitive, ToValue, Value};
 
-macro_rules! impl_into_owned {
-    ($($into_ty:ty => $convert:ident,)*) => {
-        $(
-            impl ToValue for $into_ty {
-                fn to_value(&self) -> Value {
-                    Value::from(*self)
-                }
-            }
-
-            impl<'v> From<$into_ty> for Value<'v> {
-                fn from(value: $into_ty) -> Self {
-                    Value::from_primitive(value as $convert)
-                }
-            }
-        )*
-    };
-}
-
 impl<'v> ToValue for &'v str {
     fn to_value(&self) -> Value {
         Value::from(*self)
@@ -67,25 +49,25 @@ where
     }
 }
 
-impl_into_owned! [
-    usize => u64,
-    u8 => u64,
-    u16 => u64,
-    u32 => u64,
-    u64 => u64,
-
-    isize => i64,
-    i8 => i64,
-    i16 => i64,
-    i32 => i64,
-    i64 => i64,
-
-    f32 => f64,
-    f64 => f64,
-
-    char => char,
-    bool => bool,
-];
+macro_rules! impl_to_value_primitive {
+    ($($into_ty:ty,)*) => {
+        $(
+            impl ToValue for $into_ty {
+                fn to_value(&self) -> Value {
+                    Value::from(*self)
+                }
+            }
+
+            impl<'v> From<$into_ty> for Value<'v> {
+                fn from(value: $into_ty) -> Self {
+                    Value::from_primitive(value)
+                }
+            }
+        )*
+    };
+}
+
+impl_to_value_primitive![usize, u8, u16, u32, u64, isize, i8, i16, i32, i64, f32, f64, char, bool,];
 
 #[cfg(feature = "std")]
 mod std_support {

src/kv/value/internal/cast.rs renamed to src/kv/value/internal/cast/mod.rs

@@ -4,12 +4,23 @@
 //! but may end up executing arbitrary caller code if the value is complex.
 //! They will also attempt to downcast erased types into a primitive where possible.
 
-use std::any::TypeId;
 use std::fmt;
 
-use super::{Erased, Inner, Primitive, Visitor};
+use super::{Inner, Primitive, Visitor};
 use crate::kv::value::{Error, Value};
 
+mod primitive;
+
+/// Attempt to capture a primitive from some generic value.
+///
+/// If the value is a primitive type, then cast it here, avoiding needing to erase its value
+/// This makes `Value`s produced by `Value::from_*` more useful
+pub(super) fn try_from_primitive<'v, T: 'static>(value: &'v T) -> Option<Value<'v>> {
+    primitive::from_any(value).map(|primitive| Value {
+        inner: Inner::Primitive(primitive),
+    })
+}
+
 impl<'v> Value<'v> {
     /// Try get a `usize` from this value.
     ///
@@ -203,8 +214,9 @@ impl<'v> Inner<'v> {
                 Ok(())
             }
 
-            fn borrowed_str(&mut self, v: &'v str) -> Result<(), Error> {
-                self.0 = Cast::Primitive(Primitive::Str(v));
+            #[cfg(feature = "std")]
+            fn str(&mut self, s: &str) -> Result<(), Error> {
+                self.0 = Cast::String(s.to_owned());
                 Ok(())
             }
 
@@ -213,9 +225,8 @@ impl<'v> Inner<'v> {
                 Ok(())
             }
 
-            #[cfg(feature = "std")]
-            fn str(&mut self, v: &str) -> Result<(), Error> {
-                self.0 = Cast::String(v.into());
+            fn borrowed_str(&mut self, v: &'v str) -> Result<(), Error> {
+                self.0 = Cast::Primitive(Primitive::Str(v));
                 Ok(())
             }
 
@@ -231,24 +242,14 @@ impl<'v> Inner<'v> {
             }
         }
 
-        // Try downcast an erased value first
-        // It also lets us avoid the Visitor infrastructure for simple primitives
-        let primitive = match self {
-            Inner::Primitive(value) => Some(value),
-            Inner::Fill(value) => value.downcast_primitive(),
-            Inner::Debug(value) => value.downcast_primitive(),
-            Inner::Display(value) => value.downcast_primitive(),
-
-            #[cfg(feature = "sval")]
-            Inner::Sval(value) => value.downcast_primitive(),
-        };
-
-        primitive.map(Cast::Primitive).unwrap_or_else(|| {
+        if let Inner::Primitive(value) = self {
+            Cast::Primitive(value)
+        } else {
             // If the erased value isn't a primitive then we visit it
             let mut cast = CastVisitor(Cast::Primitive(Primitive::None));
             let _ = self.visit(&mut cast);
             cast.0
-        })
+        }
     }
 }
 
@@ -321,57 +322,6 @@ impl<'v> Primitive<'v> {
     }
 }
 
-impl<'v, T: ?Sized + 'static> Erased<'v, T> {
-    // NOTE: This function is a perfect candidate for memoization
-    // The outcome could be stored in a `Cell<Primitive>`
-    fn downcast_primitive(self) -> Option<Primitive<'v>> {
-        macro_rules! type_ids {
-            ($($value:ident : $ty:ty => $cast:expr,)*) => {{
-                struct TypeIds;
-
-                impl TypeIds {
-                    fn downcast_primitive<'v, T: ?Sized>(&self, value: Erased<'v, T>) -> Option<Primitive<'v>> {
-                        $(
-                            if TypeId::of::<$ty>() == value.type_id {
-                                let $value = unsafe { value.downcast_unchecked::<$ty>() };
-                                return Some(Primitive::from($cast));
-                            }
-                        )*
-
-                        None
-                    }
-                }
-
-                TypeIds
-            }};
-        }
-
-        let type_ids = type_ids![
-            value: usize => *value as u64,
-            value: u8 => *value as u64,
-            value: u16 => *value as u64,
-            value: u32 => *value as u64,
-            value: u64 => *value,
-
-            value: isize => *value as i64,
-            value: i8 => *value as i64,
-            value: i16 => *value as i64,
-            value: i32 => *value as i64,
-            value: i64 => *value,
-
-            value: f32 => *value as f64,
-            value: f64 => *value,
-
-            value: char => *value,
-            value: bool => *value,
-
-            value: &str => *value,
-        ];
-
-        type_ids.downcast_primitive(self)
-    }
-}
-
 #[cfg(feature = "std")]
 mod std_support {
     use super::*;