fix: embed C compiler path at build time for self-contained binary

build.rs captures the absolute path to cc/gcc/clang during compilation
and bakes it into the binary. On Nix systems this embeds the full
/nix/store path so `lux compile` works without cc on PATH.

Lookup order: $CC env var > embedded build-time path > PATH search.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

build.rs

@@ -0,0 +1,38 @@
+use std::path::PathBuf;
+
+fn main() {
+    // Capture the absolute C compiler path at build time so the binary is self-contained.
+    // This is critical for Nix builds where cc/gcc live in /nix/store paths.
+    let cc_path = std::env::var("CC").ok()
+        .filter(|s| !s.is_empty())
+        .and_then(|s| resolve_absolute(&s))
+        .or_else(|| find_in_path("cc"))
+        .or_else(|| find_in_path("gcc"))
+        .or_else(|| find_in_path("clang"))
+        .unwrap_or_default();
+
+    println!("cargo:rustc-env=LUX_CC_PATH={}", cc_path);
+    println!("cargo:rerun-if-env-changed=CC");
+    println!("cargo:rerun-if-env-changed=PATH");
+}
+
+/// Resolve a command name to its absolute path by searching PATH.
+fn find_in_path(cmd: &str) -> Option<String> {
+    let path_var = std::env::var("PATH").ok()?;
+    for dir in path_var.split(':') {
+        let candidate = PathBuf::from(dir).join(cmd);
+        if candidate.is_file() {
+            return Some(candidate.to_string_lossy().into_owned());
+        }
+    }
+    None
+}
+
+/// If the path is already absolute and exists, return it. Otherwise search PATH.
+fn resolve_absolute(cmd: &str) -> Option<String> {
+    let p = PathBuf::from(cmd);
+    if p.is_absolute() && p.is_file() {
+        return Some(cmd.to_string());
+    }
+    find_in_path(cmd)
+}

src/main.rs

@@ -726,6 +726,36 @@ fn collect_lux_files_nonrecursive(dir: &str, pattern: Option<&str>, files: &mut
     }
 }
 
+/// Find a C compiler. Priority: $CC env var, build-time embedded path, PATH search.
+fn find_c_compiler() -> String {
+    // 1. Explicit env var
+    if let Ok(cc) = std::env::var("CC") {
+        if !cc.is_empty() {
+            return cc;
+        }
+    }
+    // 2. Path captured at build time (e.g. absolute nix store path)
+    let built_in = env!("LUX_CC_PATH");
+    if !built_in.is_empty() && std::path::Path::new(built_in).exists() {
+        return built_in.to_string();
+    }
+    // 3. Search PATH
+    for name in &["cc", "gcc", "clang"] {
+        if let Ok(output) = std::process::Command::new("which").arg(name).output() {
+            if output.status.success() {
+                if let Ok(p) = String::from_utf8(output.stdout) {
+                    let p = p.trim();
+                    if !p.is_empty() {
+                        return p.to_string();
+                    }
+                }
+            }
+        }
+    }
+    // 4. Last resort
+    "cc".to_string()
+}
+
 fn compile_to_c(path: &str, output_path: Option<&str>, run_after: bool, emit_c: bool) {
     use codegen::c_backend::CBackend;
     use modules::ModuleLoader;
@@ -817,8 +847,8 @@ fn compile_to_c(path: &str, output_path: Option<&str>, run_after: bool, emit_c:
         std::process::exit(1);
     }
 
-    // Find C compiler
+    // Find C compiler: $CC env var > embedded build-time path > PATH search
-    let cc = std::env::var("CC").unwrap_or_else(|_| "cc".to_string());
+    let cc = find_c_compiler();
 
     let compile_result = Command::new(&cc)
         .args(["-O2", "-o"])