Use #[proc_macro] at Rust 1.45+

futures-macro/Cargo.toml

@@ -16,6 +16,9 @@ proc-macro = true
 
 [features]
 
+[build-dependencies]
+autocfg = "1"
+
 [dependencies]
 proc-macro2 = "1.0"
 proc-macro-hack = "0.5.19"

futures-macro/build.rs

@@ -0,0 +1,26 @@
+#![warn(rust_2018_idioms, single_use_lifetimes)]
+
+use autocfg::AutoCfg;
+
+// 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 cfg = match AutoCfg::new() {
+        Ok(cfg) => cfg,
+        Err(e) => {
+            println!(
+                "cargo:warning={}: unable to determine rustc version: {}",
+                env!("CARGO_PKG_NAME"),
+                e
+            );
+            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 cfg.probe_rustc_version(1, 45) {
+        println!("cargo:rustc-cfg=fn_like_proc_macro");
+    }
+}

futures-macro/src/lib.rs

@@ -13,31 +13,34 @@
 extern crate proc_macro;
 
 use proc_macro::TokenStream;
-use proc_macro_hack::proc_macro_hack;
 
 mod join;
 mod select;
 
 /// The `join!` macro.
-#[proc_macro_hack]
+#[cfg_attr(fn_like_proc_macro, proc_macro)]
+#[cfg_attr(not(fn_like_proc_macro), proc_macro_hack::proc_macro_hack)]
 pub fn join_internal(input: TokenStream) -> TokenStream {
     crate::join::join(input)
 }
 
 /// The `try_join!` macro.
-#[proc_macro_hack]
+#[cfg_attr(fn_like_proc_macro, proc_macro)]
+#[cfg_attr(not(fn_like_proc_macro), proc_macro_hack::proc_macro_hack)]
 pub fn try_join_internal(input: TokenStream) -> TokenStream {
     crate::join::try_join(input)
 }
 
 /// The `select!` macro.
-#[proc_macro_hack]
+#[cfg_attr(fn_like_proc_macro, proc_macro)]
+#[cfg_attr(not(fn_like_proc_macro), proc_macro_hack::proc_macro_hack)]
 pub fn select_internal(input: TokenStream) -> TokenStream {
     crate::select::select(input)
 }
 
 /// The `select_biased!` macro.
-#[proc_macro_hack]
+#[cfg_attr(fn_like_proc_macro, proc_macro)]
+#[cfg_attr(not(fn_like_proc_macro), proc_macro_hack::proc_macro_hack)]
 pub fn select_biased_internal(input: TokenStream) -> TokenStream {
     crate::select::select_biased(input)
 }

futures-util/Cargo.toml

@@ -32,6 +32,9 @@ bilock = []
 read-initializer = ["io", "futures-io/read-initializer", "futures-io/unstable"]
 write-all-vectored = ["io"]
 
+[build-dependencies]
+autocfg = "1"
+
 [dependencies]
 futures-core = { path = "../futures-core", version = "=1.0.0-alpha.0", default-features = false }
 futures-task = { path = "../futures-task", version = "=0.4.0-alpha.0", default-features = false }

futures-util/build.rs

@@ -0,0 +1,26 @@
+#![warn(rust_2018_idioms, single_use_lifetimes)]
+
+use autocfg::AutoCfg;
+
+// 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 cfg = match AutoCfg::new() {
+        Ok(cfg) => cfg,
+        Err(e) => {
+            println!(
+                "cargo:warning={}: unable to determine rustc version: {}",
+                env!("CARGO_PKG_NAME"),
+                e
+            );
+            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 cfg.probe_rustc_version(1, 45) {
+        println!("cargo:rustc-cfg=fn_like_proc_macro");
+    }
+}

futures-util/src/async_await/join_mod.rs

@@ -1,7 +1,5 @@
 //! The `join` macro.
 
-use proc_macro_hack::proc_macro_hack;
-
 macro_rules! document_join_macro {
     ($join:item $try_join:item) => {
         /// Polls multiple futures simultaneously, returning a tuple
@@ -81,12 +79,14 @@ macro_rules! document_join_macro {
     }
 }
 
+#[allow(unreachable_pub)]
 #[doc(hidden)]
-#[proc_macro_hack(support_nested, only_hack_old_rustc)]
+#[cfg_attr(not(fn_like_proc_macro), proc_macro_hack::proc_macro_hack(support_nested))]
 pub use futures_macro::join_internal;
 
+#[allow(unreachable_pub)]
 #[doc(hidden)]
-#[proc_macro_hack(support_nested, only_hack_old_rustc)]
+#[cfg_attr(not(fn_like_proc_macro), proc_macro_hack::proc_macro_hack(support_nested))]
 pub use futures_macro::try_join_internal;
 
 document_join_macro! {

futures-util/src/async_await/select_mod.rs

@@ -1,7 +1,5 @@
 //! The `select` macro.
 
-use proc_macro_hack::proc_macro_hack;
-
 macro_rules! document_select_macro {
     // This branch is required for `futures 0.3.1`, from before select_biased was introduced
     ($select:item) => {
@@ -309,12 +307,14 @@ macro_rules! document_select_macro {
 }
 
 #[cfg(feature = "std")]
+#[allow(unreachable_pub)]
 #[doc(hidden)]
-#[proc_macro_hack(support_nested, only_hack_old_rustc)]
+#[cfg_attr(not(fn_like_proc_macro), proc_macro_hack::proc_macro_hack(support_nested))]
 pub use futures_macro::select_internal;
 
+#[allow(unreachable_pub)]
 #[doc(hidden)]
-#[proc_macro_hack(support_nested, only_hack_old_rustc)]
+#[cfg_attr(not(fn_like_proc_macro), proc_macro_hack::proc_macro_hack(support_nested))]
 pub use futures_macro::select_biased_internal;
 
 document_select_macro! {