Show correct location in error messages by tracking caller of utility assert_tokens functions

serde_test/build.rs

@@ -0,0 +1,33 @@
+use std::env;
+use std::process::Command;
+use std::str;
+
+// The rustc-cfg strings below are *not* public API. Please let us know by
+// opening a GitHub issue if your build environment requires some way to enable
+// these cfgs other than by executing our build script.
+fn main() {
+    let minor = match rustc_minor_version() {
+        Some(minor) => minor,
+        None => return,
+    };
+
+    // Check ability to use #[track_caller]:
+    // https://doc.rust-lang.org/reference/attributes/codegen.html#the-track_caller-attribute
+    //
+    // Perhaps sometime it will be possible to replace by built-in `version` attribute:
+    // https://github.com/rust-lang/rust/issues/64796
+    if minor >= 46 {
+        println!("cargo:rustc-cfg=has_track_caller");
+    }
+}
+
+fn rustc_minor_version() -> Option<u32> {
+    let rustc = env::var_os("RUSTC")?;
+    let output = Command::new(rustc).arg("--version").output().ok()?;
+    let version = str::from_utf8(&output.stdout).ok()?;
+    let mut pieces = version.split('.');
+    if pieces.next() != Some("rustc 1") {
+        return None;
+    }
+    pieces.next()?.parse().ok()
+}