Use #[track_caller] on Rust 1.46+

Author: taiki-e

build.rs

@@ -4,7 +4,7 @@ fn main() {
         Err(e) => {
             // If we couldn't detect the compiler version and features, just
             // print a warning. This isn't a fatal error: we can still build
-            // Tokio, we just can't enable cfgs automatically.
+            // Slab, we just can't enable cfgs automatically.
             println!(
                 "cargo:warning=slab: failed to detect compiler features: {}",
                 e
@@ -18,4 +18,7 @@ fn main() {
     if !cfg.probe_rustc_version(1, 39) {
         println!("cargo:rustc-cfg=slab_no_const_vec_new");
     }
+    if !cfg.probe_rustc_version(1, 46) {
+        println!("cargo:rustc-cfg=slab_no_track_caller");
+    }
 }

src/lib.rs

@@ -898,6 +898,7 @@ impl<T> Slab<T> {
     /// slab.key_of(bad); // this will panic
     /// unreachable!();
     /// ```
+    #[cfg_attr(not(slab_no_track_caller), track_caller)]
     pub fn key_of(&self, present_element: &T) -> usize {
         let element_ptr = present_element as *const T as usize;
         let base_ptr = self.entries.as_ptr() as usize;
@@ -1066,6 +1067,7 @@ impl<T> Slab<T> {
     /// assert_eq!(slab.remove(hello), "hello");
     /// assert!(!slab.contains(hello));
     /// ```
+    #[cfg_attr(not(slab_no_track_caller), track_caller)]
     pub fn remove(&mut self, key: usize) -> T {
         self.try_remove(key).expect("invalid key")
     }
@@ -1173,6 +1175,7 @@ impl<T> Slab<T> {
 impl<T> ops::Index<usize> for Slab<T> {
     type Output = T;
 
+    #[cfg_attr(not(slab_no_track_caller), track_caller)]
     fn index(&self, key: usize) -> &T {
         match self.entries.get(key) {
             Some(&Entry::Occupied(ref v)) => v,
@@ -1182,6 +1185,7 @@ impl<T> ops::Index<usize> for Slab<T> {
 }
 
 impl<T> ops::IndexMut<usize> for Slab<T> {
+    #[cfg_attr(not(slab_no_track_caller), track_caller)]
     fn index_mut(&mut self, key: usize) -> &mut T {
         match self.entries.get_mut(key) {
             Some(&mut Entry::Occupied(ref mut v)) => v,