Cleanup before upstream submission

Cargo.toml

@@ -5,6 +5,9 @@ edition = "2021"
 rust-version = "1.76"
 publish = false
 
+[dependencies]
+unicode-ident = "1.0.13"
+
 [features]
 
 ## Experimental API. This feature flag is **NOT** semver stable.

src/allow_use.rs

@@ -1,17 +1,46 @@
-use std::sync::OnceLock;
+use std::{process::Command, sync::OnceLock};
 
 fn rust_version_minor() -> u32 {
     static VERSION_MINOR: OnceLock<u32> = OnceLock::new();
     *VERSION_MINOR.get_or_init(|| {
         crate::input::cargo_pkg_rust_version()
             .split('.')
             .nth(1)
-            .unwrap_or("70")
+            // assume build-rs's MSRV if none specified for the current package
+            .unwrap_or(env!("CARGO_PKG_RUST_VERSION").split('.').nth(1).unwrap())
+            .parse()
+            .unwrap()
+    })
+}
+
+fn cargo_version_minor() -> u32 {
+    static VERSION_MINOR: OnceLock<u32> = OnceLock::new();
+    *VERSION_MINOR.get_or_init(|| {
+        let out = Command::new(crate::input::cargo())
+            .arg("-V")
+            .output()
+            .expect("running `cargo -V` should succeed");
+        assert!(out.status.success(), "running `cargo -V` should succeed");
+
+        // > cargo -V # example output
+        // cargo 1.82.0 (8f40fc59f 2024-08-21)
+
+        String::from_utf8(out.stdout).expect("`cargo -V` should output valid UTF-8")
+            ["cargo 1.".len()..]
+            .split('.')
+            .next()
+            .expect("`cargo -V` format should be stable")
             .parse()
             .unwrap()
     })
 }
 
 pub(crate) fn double_colon_directives() -> bool {
+    // cargo errors on `cargo::` directives with insufficient package.rust-version
     rust_version_minor() >= 77
 }
+
+pub(crate) fn check_cfg() -> bool {
+    // emit check-cfg if the toolchain being used supports it
+    cargo_version_minor() >= 80
+}

src/ident.rs

@@ -0,0 +1,27 @@
+use unicode_ident::{is_xid_continue, is_xid_start};
+
+pub(crate) fn is_feature_name(s: &str) -> bool {
+    s.chars()
+        .all(|ch| is_xid_continue(ch) || matches!(ch, '-' | '+' | '.'))
+}
+
+pub(crate) fn is_ident(s: &str) -> bool {
+    let mut cs = s.chars();
+    cs.next()
+        .is_some_and(|ch| is_xid_start(ch) || matches!(ch, '_'))
+        && cs.all(is_xid_continue)
+}
+
+pub(crate) fn is_ascii_ident(s: &str) -> bool {
+    let mut cs = s.chars();
+    cs.next()
+        .is_some_and(|ch| ch.is_ascii_alphabetic() || matches!(ch, '_'))
+        && cs.all(|ch| ch.is_ascii_alphanumeric() || matches!(ch, '_'))
+}
+
+pub(crate) fn is_crate_name(s: &str) -> bool {
+    let mut cs = s.chars();
+    cs.next()
+        .is_some_and(|ch| is_xid_start(ch) || matches!(ch, '-' | '_'))
+        && cs.all(|ch| is_xid_continue(ch) || matches!(ch, '-'))
+}