feat(cheddar): Add a build script that injects BAT_SYNTAXES r/2557

The BAT_SYNTAXES environment variable is injected during Nix builds
and points to the build result of //third_party/bat_syntaxes, which is
included in the cheddar binary.

During development this is a bit annoying: Opening a Nix shell will
set the right variable, but it will also mess with the Cargo
environment in ways that make it annoying to use (*and* require a
pre-build of all dependencies as required by naersk).

For an easier development flow, this adds a build script that
automatically runs Nix, fetches the path to the syntax set and sets
the environment variable. It is a no-op if the variable is already
set.

This makes for a smoother development experience when working on
cheddar in a depot checkout.

Change-Id: Ib5435518897e3443247edd9f362821c0b9ff756b
Reviewed-on: https://cl.tvl.fyi/c/depot/+/3081
Tested-by: BuildkiteCI
Reviewed-by: flokli <flokli@flokli.de>

1 files changed, 50 insertions, 0 deletions

diff --git a/tools/cheddar/build.rs b/tools/cheddar/build.rs
new file mode 100644
index 000000000000..c04209d6a4c6
--- /dev/null
+++ b/tools/cheddar/build.rs
@@ -0,0 +1,50 @@
+//! Build script that can be used outside of Nix builds to inject the
+//! BAT_SYNTAXES variable when building in development mode.
+//!
+//! Note that this script assumes that cheddar is in a checkout of the
+//! TVL depot.
+
+use std::process::Command;
+
+static BAT_SYNTAXES: &str = "BAT_SYNTAXES";
+static ERROR_MESSAGE: &str = r#"Failed to build syntax set.
+
+When building during development, cheddar expects to be in a checkout
+of the TVL depot. This is required to automatically build the syntax
+highlighting files that are needed at compile time.
+
+As cheddar can not automatically detect the location of the syntax
+files, you must set the `BAT_SYNTAXES` environment variable to the
+right path.
+
+The expected syntax files are at //third_party/bat_syntaxes in the
+depot."#;
+
+fn main() {
+    // Do nothing if the variable is already set (e.g. via Nix)
+    if let Ok(_) = std::env::var(BAT_SYNTAXES) {
+        return;
+    }
+
+    // Otherwise ask Nix to build it and inject the result.
+    let output = Command::new("nix-build")
+        .arg("-A").arg("third_party.bat_syntaxes")
+        // ... assuming cheddar is at //tools/cheddar ...
+        .arg("../..")
+        .output()
+        .expect(ERROR_MESSAGE);
+
+    if !output.status.success() {
+        eprintln!("{}\nNix output: {}", ERROR_MESSAGE, String::from_utf8_lossy(&output.stderr));
+        return;
+    }
+
+    let out_path = String::from_utf8(output.stdout)
+        .expect("Nix returned invalid output after building syntax set");
+
+    // Return an instruction to Cargo that will set the environment
+    // variale during rustc calls.
+    //
+    // https://doc.rust-lang.org/cargo/reference/build-scripts.html#cargorustc-envvarvalue
+    println!("cargo:rustc-env={}={}", BAT_SYNTAXES, out_path.trim());
+}