Use underscore consts on Rust 1.37+

serde_derive/Cargo.toml

@@ -9,7 +9,7 @@ repository = "https://github.com/serde-rs/serde"
 documentation = "https://serde.rs/derive.html"
 keywords = ["serde", "serialization", "no_std"]
 readme = "crates-io.md"
-include = ["src/**/*.rs", "crates-io.md", "README.md", "LICENSE-APACHE", "LICENSE-MIT"]
+include = ["build.rs", "src/**/*.rs", "crates-io.md", "README.md", "LICENSE-APACHE", "LICENSE-MIT"]
 
 [features]
 default = []

serde_derive/build.rs

@@ -0,0 +1,30 @@
+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,
+    };
+
+    // Underscore const names stabilized in Rust 1.37:
+    // https://github.com/rust-lang/rust/pull/61347
+    if minor >= 37 {
+        println!("cargo:rustc-cfg=underscore_consts");
+    }
+}
+
+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()
+}

serde_derive/src/dummy.rs

@@ -1,4 +1,5 @@
-use proc_macro2::{Ident, Span, TokenStream};
+use proc_macro2::{Ident, TokenStream};
+use quote::format_ident;
 
 use syn;
 use try;
@@ -11,10 +12,11 @@ pub fn wrap_in_const(
 ) -> TokenStream {
     let try_replacement = try::replacement();
 
-    let dummy_const = Ident::new(
-        &format!("_IMPL_{}_FOR_{}", trait_, unraw(ty)),
-        Span::call_site(),
-    );
+    let dummy_const = if cfg!(underscore_consts) {
+        format_ident!("_")
+    } else {
+        format_ident!("_IMPL_{}_FOR_{}", trait_, unraw(ty))
+    };
 
     let use_serde = match serde_path {
         Some(path) => quote! {