(zilch lang rust): recursively resolve pkg-config paths

lang/rust/helpers/src/bin/buildscript-runner.rs

@@ -1,7 +1,7 @@
 use std::{
     collections::HashMap,
     env::{var, var_os},
-    fs::{File, create_dir},
+    fs::{File, create_dir, exists, read_to_string},
     io::{Read, Write},
     os::unix::process::CommandExt,
     process::Command,
@@ -73,5 +73,49 @@ fn main() {
         }
     }
 
-    panic!("failed to exec: {:?}", cmd.exec())
+    let mut pkg_config_path = Vec::new();
+    if let Ok(paths) = var("_zilch_pkgconfig") {
+        if !paths.is_empty() {
+            let mut paths_to_check = Vec::new();
+            for path in paths.split(':') {
+                paths_to_check.push(path.to_string());
+            }
+
+            while let Some(path) = paths_to_check.pop() {
+                let pkgconfig_path = format!("{}/lib/pkgconfig", path);
+                if exists(&pkgconfig_path).ok() == Some(true)
+                    && !pkg_config_path.contains(&pkgconfig_path)
+                {
+                    pkg_config_path.push(pkgconfig_path);
+                }
+
+                let pkgconfig_path = format!("{}/share/pkgconfig", path);
+                if exists(&pkgconfig_path).ok() == Some(true)
+                    && !pkg_config_path.contains(&pkgconfig_path)
+                {
+                    pkg_config_path.push(pkgconfig_path);
+                }
+
+                if let Ok(file) =
+                    read_to_string(format!("{}/nix-support/propagated-build-inputs", path))
+                {
+                    for path in file.split(' ') {
+                        if !path.is_empty() {
+                            paths_to_check.push(path.to_string());
+                        }
+                    }
+                }
+            }
+        }
+    }
+
+    cmd.env("PKG_CONFIG_PATH", pkg_config_path.join(":"));
+    let status = cmd.status().unwrap();
+
+    if !status.success() {
+        panic!(
+            "Failed to execute.\nstdout: {}",
+            std::fs::read_to_string(var_os("out").unwrap()).unwrap()
+        );
+    }
 }