Vendor dependencies

Let's see how I like this workflow.

vendor/quote/build.rs

@@ -0,0 +1,38 @@
+use std::env;
+use std::process::{self, Command};
+use std::str;
+
+fn main() {
+    println!("cargo:rerun-if-changed=build.rs");
+
+    let version = match rustc_version() {
+        Some(version) => version,
+        None => return,
+    };
+
+    if version.minor < 31 {
+        eprintln!("Minimum supported rustc version is 1.31");
+        process::exit(1);
+    }
+
+    if version.minor < 53 {
+        // https://github.com/rust-lang/rust/issues/43081
+        println!("cargo:rustc-cfg=needs_invalid_span_workaround");
+    }
+}
+
+struct RustcVersion {
+    minor: u32,
+}
+
+fn rustc_version() -> Option<RustcVersion> {
+    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;
+    }
+    let minor = pieces.next()?.parse().ok()?;
+    Some(RustcVersion { minor })
+}