Activate `rust_v_1_46` when on high-enough rustc

This adds a build script that automatically activates the cfg if the
version is sufficient. We don't make API changes here, just improve
debugging by enabling `track_caller`.

internals/build.rs

@@ -0,0 +1,34 @@
+fn main() {
+    let rustc = std::env::var_os("RUSTC");
+    let rustc = rustc.as_ref().map(std::path::Path::new).unwrap_or_else(|| "rustc".as_ref());
+    let output = std::process::Command::new(rustc)
+        .arg("--version")
+        .output()
+        .unwrap_or_else(|error| panic!("Failed to run `{:?} --version`: {:?}", rustc, error));
+    assert!(output.status.success(), "{:?} -- version returned non-zero exit code", rustc);
+    let stdout = String::from_utf8(output.stdout).expect("rustc produced non-UTF-8 output");
+    let version_prefix = "rustc ";
+    if !stdout.starts_with(version_prefix) {
+        panic!("unexpected rustc output: {}", stdout);
+    }
+
+    let version = &stdout[version_prefix.len()..];
+    let end = version.find(&[' ', '-'] as &[_]).unwrap_or(version.len());
+    let version = &version[..end];
+    let mut version_components = version.split('.');
+    let major = version_components.next().unwrap();
+    assert_eq!(major, "1", "Unexpected Rust major version");
+    let minor = version_components
+        .next()
+        .unwrap_or("0")
+        .parse::<u64>()
+        .expect("invalid Rust minor version");
+
+    // print cfg for all interesting versions less than or equal to minor
+    // 46 adds `track_caller`
+    for version in &[46] {
+        if *version <= minor {
+            println!("cargo:rustc-cfg=rust_v_1_{}", version);
+        }
+    }
+}