simplify and fixup check for nightly Rust

The try_reserve_2 feature is enabled on stable and beta Rust 1.63, and
nightly Rust 1.64 and above.

build.rs

@@ -24,18 +24,25 @@ fn main() {
     if compiler.minor >= 56 {
         println!("cargo:rustc-cfg=shrink_to");
     }
-    if compiler.minor >= 63
-        && (!compiler.beta || compiler.minor >= 64)
-        && (!compiler.nightly || compiler.minor >= 65)
+    // Stable and beta 1.63 have a stable try_reserve_2.
+    if (compiler.minor >= 63
+        && (compiler.channel == ReleaseChannel::Stable || compiler.channel == ReleaseChannel::Beta))
+        || compiler.minor >= 64
     {
         println!("cargo:rustc-cfg=try_reserve_2");
     }
 }
 
 struct Compiler {
     minor: u32,
-    beta: bool,
-    nightly: bool,
+    channel: ReleaseChannel,
+}
+
+#[derive(Clone, Copy, Debug, Eq, PartialEq)]
+enum ReleaseChannel {
+    Stable,
+    Beta,
+    Nightly,
 }
 
 fn rustc_version() -> Option<Compiler> {
@@ -47,11 +54,12 @@ fn rustc_version() -> Option<Compiler> {
         return None;
     }
     let minor = pieces.next()?.parse().ok()?;
-    let beta = version.contains("beta");
-    let nightly = version.contains("nightly");
-    Some(Compiler {
-        minor,
-        beta,
-        nightly,
-    })
+    let channel = if version.contains("nightly") {
+        ReleaseChannel::Nightly
+    } else if version.contains("beta") {
+        ReleaseChannel::Beta
+    } else {
+        ReleaseChannel::Stable
+    };
+    Some(Compiler { minor, channel })
 }