Simplify/remove build.rs following the bump to 2021 edition

build.rs

@@ -1,6 +1,4 @@
 use std::env;
-use std::process::Command;
-use std::str::{self, FromStr};
 
 fn main() {
     println!("cargo:rerun-if-changed=build.rs");
@@ -16,39 +14,4 @@ fn main() {
             println!("cargo:rustc-cfg=limb_width_32");
         }
     }
-
-    let minor = match rustc_minor_version() {
-        Some(minor) => minor,
-        None => return,
-    };
-
-    // BTreeMap::get_key_value
-    // https://blog.rust-lang.org/2019/12/19/Rust-1.40.0.html#additions-to-the-standard-library
-    if minor < 40 {
-        println!("cargo:rustc-cfg=no_btreemap_get_key_value");
-    }
-
-    // BTreeMap::remove_entry
-    // https://blog.rust-lang.org/2020/07/16/Rust-1.45.0.html#library-changes
-    if minor < 45 {
-        println!("cargo:rustc-cfg=no_btreemap_remove_entry");
-    }
-
-    // BTreeMap::retain
-    // https://blog.rust-lang.org/2021/06/17/Rust-1.53.0.html#stabilized-apis
-    if minor < 53 {
-        println!("cargo:rustc-cfg=no_btreemap_retain");
-    }
-}
-
-fn rustc_minor_version() -> Option<u32> {
-    let rustc = env::var_os("RUSTC")?;
-    let output = Command::new(rustc).arg("--version").output().ok()?;
-    let version = str::from_utf8(&output.stdout).ok()?;
-    let mut pieces = version.split('.');
-    if pieces.next() != Some("rustc 1") {
-        return None;
-    }
-    let next = pieces.next()?;
-    u32::from_str(next).ok()
 }

src/map.rs

@@ -106,7 +106,6 @@ impl Map<String, Value> {
     /// The key may be any borrowed form of the map's key type, but the ordering
     /// on the borrowed form *must* match the ordering on the key type.
     #[inline]
-    #[cfg(any(feature = "preserve_order", not(no_btreemap_get_key_value)))]
     pub fn get_key_value<Q>(&self, key: &Q) -> Option<(&String, &Value)>
     where
         String: Borrow<Q>,
@@ -153,44 +152,7 @@ impl Map<String, Value> {
         String: Borrow<Q>,
         Q: ?Sized + Ord + Eq + Hash,
     {
-        #[cfg(any(feature = "preserve_order", not(no_btreemap_remove_entry)))]
-        return self.map.remove_entry(key);
-        #[cfg(all(
-            not(feature = "preserve_order"),
-            no_btreemap_remove_entry,
-            not(no_btreemap_get_key_value),
-        ))]
-        {
-            let (key, _value) = self.map.get_key_value(key)?;
-            let key = key.clone();
-            let value = self.map.remove::<String>(&key)?;
-            Some((key, value))
-        }
-        #[cfg(all(
-            not(feature = "preserve_order"),
-            no_btreemap_remove_entry,
-            no_btreemap_get_key_value,
-        ))]
-        {
-            use core::ops::{Bound, RangeBounds};
-
-            struct Key<'a, Q: ?Sized>(&'a Q);
-
-            impl<'a, Q: ?Sized> RangeBounds<Q> for Key<'a, Q> {
-                fn start_bound(&self) -> Bound<&Q> {
-                    Bound::Included(self.0)
-                }
-                fn end_bound(&self) -> Bound<&Q> {
-                    Bound::Included(self.0)
-                }
-            }
-
-            let mut range = self.map.range(Key(key));
-            let (key, _value) = range.next()?;
-            let key = key.clone();
-            let value = self.map.remove::<String>(&key)?;
-            Some((key, value))
-        }
+        self.map.remove_entry(key)
     }
 
     /// Moves all elements from other into self, leaving other empty.
@@ -276,7 +238,6 @@ impl Map<String, Value> {
     ///
     /// In other words, remove all pairs `(k, v)` such that `f(&k, &mut v)`
     /// returns `false`.
-    #[cfg(not(no_btreemap_retain))]
     #[inline]
     pub fn retain<F>(&mut self, f: F)
     where

tests/map.rs

@@ -35,7 +35,6 @@ fn test_append() {
     assert!(val.is_empty());
 }
 
-#[cfg(not(no_btreemap_retain))]
 #[test]
 fn test_retain() {
     let mut v: Value = from_str(r#"{"b":null,"a":null,"c":null}"#).unwrap();