Make proc-macro-nested empty crate at Rust 1.45+

proc-macro-hack does nothing in these versions, so proc-macro-nested
doesn't need to do anything either.

nested/build.rs

@@ -2,6 +2,8 @@ use std::env;
 use std::fs;
 use std::iter;
 use std::path::Path;
+use std::process::Command;
+use std::str;
 
 /*
 #[doc(hidden)]
@@ -14,10 +16,27 @@ macro_rules! count {
 }
 */
 
+// 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() {
  // Tell Cargo not to rerun on src/lib.rs changes.
  println!("cargo:rerun-if-changed=build.rs");
 
+ let minor = match rustc_minor_version() {
+ Some(minor) => minor,
+ None => return,
+ };
+
+ // Function like procedural macros in expressions patterns statements stabilized in Rust 1.45:
+ // https://blog.rust-lang.org/2020/07/16/Rust-1.45.0.html#stabilizing-function-like-procedural-macros-in-expressions-patterns-and-statements
+ if minor >= 45 {
+ println!("cargo:rustc-cfg=fn_like_proc_macro");
+ // proc-macro-hack does nothing in these versions, so proc-macro-nested
+ // doesn't need to do anything either.
+ return;
+ }
+
  let mut content = String::new();
  content += "#[doc(hidden)]\n";
  content += "#[macro_export]\n";
@@ -46,3 +65,14 @@ fn main() {
  fs::write(dest_path, content).unwrap();
  }
 }
+
+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()
+}

nested/src/lib.rs

@@ -40,8 +40,10 @@
 
 #![no_std]
 
+#[cfg(not(fn_like_proc_macro))]
 include!(concat!(env!("OUT_DIR"), "/count.rs"));
 
+#[cfg(not(fn_like_proc_macro))]
 #[doc(hidden)]
 #[macro_export]
 macro_rules! dispatch {