Restore support for compilers without alloc

build.rs

@@ -0,0 +1,28 @@
+use std::env;
+use std::process::Command;
+use std::str;
+
+// The rustc-cfg strings below are *not* public API. Please let us know by
+// opening a GitHub issue if your build environment requires some way to enable
+// these cfgs other than by executing our build script.
+fn main() {
+    let minor = match rustc_minor_version() {
+        Some(minor) => minor,
+        None => return,
+    };
+
+    if minor < 36 {
+        println!("cargo:rustc-cfg=inventory_require_std");
+    }
+}
+
+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;
+    }
+    pieces.next()?.parse().ok()
+}

src/lib.rs

@@ -1,4 +1,3 @@
-#![no_std]
 //! **Typed distributed plugin registration.**
 //!
 //! This crate provides a way to set up a plugin registry into which plugins
@@ -95,7 +94,11 @@
 //! There is no guarantee about the order that plugins of the same type are
 //! visited by the iterator. They may be visited in any order.
 
+#![cfg_attr(not(inventory_require_std), no_std)]
+
+#[cfg(not(inventory_require_std))]
 extern crate alloc;
+#[cfg(not(inventory_require_std))]
 use alloc::boxed::Box;
 
 // Not public API.