Better support for nightly

By treating nightly as the then-beta, time no longer relies on a feature
that may still be unstable.

build.rs

@@ -1,8 +1,8 @@
 use std::env;
 use version_check as rustc;
 
-const MSRV: &str = "1.32.0";
-const NO_STD_MSRV: &str = "1.36.0";
+const MSRV: u16 = 32;
+const NO_STD_MSRV: u16 = 36;
 
 macro_rules! cfg_emit {
     ($s:ident) => {
@@ -24,16 +24,34 @@ fn main() {
         cfg_emit!(__time_02_cargo_web);
     }
 
+    // Treat nightly & dev compilers as the equivalent of the then-beta.
+    // As features are never stabilized in patch versions, we can safely ignore it.
+    let (effective_compiler_version, channel) = match rustc::triple() {
+        Some((version, channel, _)) if channel.is_dev() => (version.to_mmp().1 - 2, channel),
+        Some((version, channel, _)) if channel.is_nightly() => (version.to_mmp().1 - 1, channel),
+        Some((version, channel, _)) => (version.to_mmp().1, channel),
+        None => {
+            warning!(
+                "Unable to determine rustc version. Assuming rustc 1.{}.0.",
+                MSRV
+            );
+            (
+                MSRV,
+                rustc::Channel::parse("1.0.0").expect("This is a generic stable version."),
+            )
+        }
+    };
+
     // Warn if the version is below MSRV.
-    if !rustc::is_min_version(MSRV).unwrap_or(false) {
+    if effective_compiler_version < MSRV {
         warning!(
             "The time crate has a minimum supported rust version of {}.",
             MSRV
         );
     }
 
     // Warn if the version is below `#![no_std]` MSRV.
-    if !rustc::is_min_version(NO_STD_MSRV).unwrap_or(false) {
+    if effective_compiler_version < NO_STD_MSRV {
         #[cfg(not(feature = "std"))]
         warning!(
             "Using the time crate without the standard library enabled requires a global \
@@ -44,7 +62,7 @@ fn main() {
     }
 
     // Warn if the `__doc` feature is used on stable or beta.
-    if !rustc::Channel::read().map_or(false, |channel| channel.supports_features()) {
+    if !channel.supports_features() {
         #[cfg(__time_02_docs)]
         warning!(
             "`--cfg __time_02_docs` requires a nightly compiler, and is intended for internal \
@@ -55,19 +73,19 @@ fn main() {
     // ==== features that affect runtime directly ====
 
     // `#[non_exhaustive]` was stabilized in 1.40.0.
-    if rustc::is_min_version("1.40.0").unwrap_or(false) {
+    if effective_compiler_version >= 40 {
         cfg_emit!(__time_02_supports_non_exhaustive);
     }
 
     // `Instant::checked_add` and `Instant::checked_sub` were added in 1.34.0.
     // `NonZeroI*` was stabilized in 1.34.0.
-    if rustc::is_min_version("1.34.0").unwrap_or(false) {
+    if effective_compiler_version >= 34 {
         cfg_emit!(__time_02_instant_checked_ops);
         cfg_emit!(__time_02_nonzero_signed);
     }
 
     // `use <trait> as _;` was stabilized in 1.33.0.
-    if rustc::is_min_version("1.33.0").unwrap_or(false) {
+    if effective_compiler_version >= 33 {
         cfg_emit!(__time_02_use_trait_as_underscore);
     }
 }