From 1e609ab5dd10a94ef3754ba39426d46e254f625e Mon Sep 17 00:00:00 2001
From: Taiki Endo <te316e89@gmail.com>
Date: Mon, 26 Oct 2020 05:53:03 +0900
Subject: [PATCH] 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   | 30 ++++++++++++++++++++++++++++++
 nested/src/lib.rs |  2 ++
 2 files changed, 32 insertions(+)

diff --git a/nested/build.rs b/nested/build.rs
index d451b74..a2659dd 100644
--- a/nested/build.rs
+++ b/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()
+}
diff --git a/nested/src/lib.rs b/nested/src/lib.rs
index 0cd8302..66d838b 100644
--- a/nested/src/lib.rs
+++ b/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 {