diff --git a/build.rs b/build.rs
index f04125c9..5e4c14ee 100644
--- a/build.rs
+++ b/build.rs
@@ -3,6 +3,11 @@ use std::io::Write;
 use std::{env, fs, path::PathBuf};
 
 // Keep in sync with Cargo.toml.
+//
+// We don't populate this automatically from the Cargo.toml at build time
+// because doing so would require a heavy-weight deserialization lib dependency
+// (and it couldn't be a _dev_ dep for use in a build script) or doing brittle
+// by-hand parsing.
 const RUSTLS_CRATE_VERSION: &str = "0.23.4";
 
 fn main() {