diff --git a/build.rs b/build.rs
index cd6df23..a175711 100644
--- a/build.rs
+++ b/build.rs
@@ -1,3 +1,37 @@
+// rustc-cfg emitted by the build script:
+//
+// "u128"
+//     Include u128 and i128 constructors for proc_macro2::Literal. Enabled on
+//     any compiler 1.26+.
+//
+// "use_proc_macro"
+//     Link to extern crate proc_macro. Available on any compiler and any target
+//     except wasm32. Requires "proc-macro" Cargo cfg to be enabled (default is
+//     enabled). On wasm32 we never link to proc_macro even if "proc-macro" cfg
+//     is enabled.
+//
+// "wrap_proc_macro"
+//     Wrap types from libproc_macro rather than polyfilling the whole API.
+//     Enabled on rustc 1.29+ as long as procmacro2_semver_exempt is not set,
+//     because we can't emulate the unstable API without emulating everything
+//     else. Also enabled unconditionally on nightly, in which case the
+//     procmacro2_semver_exempt surface area is implemented by using the
+//     nightly-only proc_macro API.
+//
+// "slow_extend"
+//     Fallback when `impl Extend for TokenStream` is not available. These impls
+//     were added one version later than the rest of the proc_macro token API.
+//     Enabled on rustc 1.29 only.
+//
+// "nightly"
+//     Enable the Span::unstable method. This is to support proc_macro_span and
+//     proc_macro_diagnostic use on the nightly channel without requiring the
+//     semver exemption opt-in. Enabled when building with nightly.
+//
+// "super_unstable"
+//     Implement the semver exempt API in terms of the nightly-only proc_macro
+//     API. Enabled when using procmacro2_semver_exempt on a nightly compiler.
+
 use std::env;
 use std::process::Command;
 use std::str;
@@ -7,12 +41,12 @@
 
     let target = env::var("TARGET").unwrap();
 
-    let minor = match rustc_minor_version() {
-        Some(n) => n,
+    let version = match rustc_version() {
+        Some(version) => version,
         None => return,
     };
 
-    if minor >= 26 {
+    if version.minor >= 26 {
         println!("cargo:rustc-cfg=u128");
     }
 
@@ -21,20 +55,28 @@
     }
     println!("cargo:rustc-cfg=use_proc_macro");
 
-    // Rust 1.29 stabilized the necessary APIs in the `proc_macro` crate
-    if (minor >= 29 && !cfg!(procmacro2_semver_exempt)) || cfg!(feature = "nightly") {
-        println!("cargo:rustc-cfg=wrap_proc_macro");
-
-        if cfg!(procmacro2_semver_exempt) {
-            println!("cargo:rustc-cfg=super_unstable");
-            // https://github.com/alexcrichton/proc-macro2/issues/147
-            println!("cargo:rustc-cfg=procmacro2_semver_exempt");
-        }
+    let semver_exempt = cfg!(procmacro2_semver_exempt);
+    if semver_exempt {
+        // https://github.com/alexcrichton/proc-macro2/issues/147
+        println!("cargo:rustc-cfg=procmacro2_semver_exempt");
     }
 
-    if minor == 29 {
+    // Rust 1.29 stabilized the necessary APIs in the `proc_macro` crate
+    if version.nightly || version.minor >= 29 && !semver_exempt {
+        println!("cargo:rustc-cfg=wrap_proc_macro");
+    }
+
+    if version.minor == 29 {
         println!("cargo:rustc-cfg=slow_extend");
     }
+
+    if version.nightly {
+        println!("cargo:rustc-cfg=nightly");
+    }
+
+    if semver_exempt && version.nightly {
+        println!("cargo:rustc-cfg=super_unstable");
+    }
 }
 
 fn enable_use_proc_macro(target: &str) -> bool {
@@ -47,7 +89,12 @@
     cfg!(feature = "proc-macro")
 }
 
-fn rustc_minor_version() -> Option<u32> {
+struct RustcVersion {
+    minor: u32,
+    nightly: bool,
+}
+
+fn rustc_version() -> Option<RustcVersion> {
     macro_rules! otry {
         ($e:expr) => {
             match $e {
@@ -56,12 +103,20 @@
             }
         };
     }
+
     let rustc = otry!(env::var_os("RUSTC"));
     let output = otry!(Command::new(rustc).arg("--version").output().ok());
     let version = otry!(str::from_utf8(&output.stdout).ok());
+    let nightly = version.contains("nightly");
     let mut pieces = version.split('.');
     if pieces.next() != Some("rustc 1") {
         return None;
     }
-    otry!(pieces.next()).parse().ok()
+    let minor = otry!(pieces.next());
+    let minor = otry!(minor.parse().ok());
+
+    Some(RustcVersion {
+        minor: minor,
+        nightly: nightly,
+    })
 }