use std::env;
use std::process::Command;
use std::str::{self, FromStr};

fn main() {
    // Decide ideal limb width for arithmetic in the float parser. Refer to
    // src/lexical/math.rs for where this has an effect.
    let target_arch = env::var("CARGO_CFG_TARGET_ARCH").unwrap();
    match target_arch.as_str() {
        "aarch64" | "mips64" | "powerpc64" | "x86_64" => {
            println!("cargo:rustc-cfg=limb_width_64");
        }
        _ => {
            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=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=btreemap_remove_entry");
    }
}

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()
}

use std::env;

fn main() {
    // Decide ideal limb width for arithmetic in the float parser. Refer to
    // src/lexical/math.rs for where this has an effect.
    let target_arch = env::var("CARGO_CFG_TARGET_ARCH").unwrap();
    match target_arch.as_str() {
        "aarch64" | "mips64" | "powerpc64" | "x86_64" => {
            println!("cargo:rustc-cfg=limb_width_64");
        }
        _ => {
            println!("cargo:rustc-cfg=limb_width_32");
        }
    }
}

--- 
+++ 
@@ -1,4 +1,6 @@
 use std::env;
+use std::process::Command;
+use std::str::{self, FromStr};
 
 fn main() {
     // Decide ideal limb width for arithmetic in the float parser. Refer to
@@ -12,4 +14,33 @@
             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=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=btreemap_remove_entry");
+    }
 }
+
+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()
+}
Add compiler version detection for BTreeMap api
