1 files changed, 38 insertions, 1 deletions
diff --git a/build.rs b/build.rs
index dd09e62..0e12602 100644
--- a/build.rs
+++ b/build.rs
@@ -1,4 +1,6 @@
 use std::env;
+use std::process::Command;
+use std::str::{self, FromStr};
 
 fn main() {
     println!("cargo:rerun-if-changed=build.rs");
@@ -7,11 +9,46 @@ fn main() {
     // 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" | "loongarch64" => {
+        "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=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()
 }