fix(build-rs)!: Updates from an audit (#14817)

### What does this PR try to resolve?

Did a quick audit for parts of the API and implementation that stood out
to me. So far, I've mostly focused on "input".

### How should we test and review this PR?

### Additional information

CC @CAD97

Author: weihanglo

crates/build-rs-test-lib/build.rs

@@ -47,6 +47,7 @@ fn smoke_test_inputs() {
     dbg!(cargo_encoded_rustflags());
     dbg!(cargo_feature("unstable"));
     dbg!(cargo_manifest_dir());
+    dbg!(cargo_manifest_path());
     dbg!(cargo_manifest_links());
     dbg!(cargo_pkg_authors());
     dbg!(cargo_pkg_description());

crates/build-rs/src/allow_use.rs

@@ -3,13 +3,9 @@ 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)
+        version_minor(&crate::input::cargo_pkg_rust_version().unwrap_or_default())
             // 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()
+            .unwrap_or_else(|| version_minor(env!("CARGO_PKG_RUST_VERSION")).unwrap())
     })
 }
 
@@ -25,16 +21,18 @@ fn cargo_version_minor() -> u32 {
         // > 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()
+        let out = std::str::from_utf8(&out.stdout).expect("`cargo -V` should output valid UTF-8");
+        let version = out.split(' ').nth(1).unwrap();
+        version_minor(version).unwrap()
     })
 }
 
+fn version_minor(version: &str) -> Option<u32> {
+    let minor = version.split('.').nth(1)?;
+    let minor = minor.parse().unwrap();
+    Some(minor)
+}
+
 pub(crate) fn double_colon_directives() -> bool {
     // cargo errors on `cargo::` directives with insufficient package.rust-version
     rust_version_minor() >= 77