get rid of ctx.prefix and add run cwd kwarg

src/builder.rs

@@ -1023,21 +1023,9 @@ fn plural<'a>(count: usize, singular: &'a str, plural: &'a str) -> &'a str {
     if count == 1 { singular } else { plural }
 }
 
-fn prefix_for(kind: RecipeKind) -> &'static str {
-    match kind {
-        RecipeKind::Package => "/usr",
-        RecipeKind::HostPackage => "/usr/local",
-    }
-}
-
 fn phase_context_for(recipe: &Recipe) -> PhaseContext {
     let files = recipe.files_dir().map(|_| "/files".to_owned());
-    PhaseContext::new(
-        source_dir_for(recipe),
-        prefix_for(recipe.kind()),
-        default_jobs(),
-        files,
-    )
+    PhaseContext::new(source_dir_for(recipe), default_jobs(), files)
 }
 
 fn source_dir_for(recipe: &Recipe) -> SourceDir {

src/phase.rs

@@ -211,18 +211,16 @@ impl<'v> StarlarkValue<'v> for SourceDir {
 pub struct PhaseContext {
     source_dir: SourceDir,
     build_dir: String,
-    prefix: String,
     sysroot: String,
     files: Option<String>,
     jobs: i32,
 }
 
 impl PhaseContext {
-    pub fn new(source_dir: SourceDir, prefix: &str, jobs: i32, files: Option<String>) -> Self {
+    pub fn new(source_dir: SourceDir, jobs: i32, files: Option<String>) -> Self {
         Self {
             source_dir,
             build_dir: "/build".to_owned(),
-            prefix: prefix.to_owned(),
             sysroot: "/sysroot".to_owned(),
             files,
             jobs,
@@ -285,6 +283,7 @@ fn phase_context_methods(builder: &mut MethodsBuilder) {
         #[starlark(this)] _this: Value<'v>,
         #[starlark(require = pos)] argv: UnpackList<Value<'v>>,
         #[starlark(require = named)] env: Option<SmallMap<String, Value<'v>>>,
+        #[starlark(require = named)] cwd: Option<String>,
     ) -> anyhow::Result<NoneType> {
         let argv: Vec<String> = argv
             .items
@@ -297,7 +296,7 @@ fn phase_context_methods(builder: &mut MethodsBuilder) {
                 env_strings.insert(k, coerce_path_string(v)?);
             }
         }
-        run_in_container(&argv, env_strings)?;
+        run_in_container(&argv, env_strings, cwd.as_deref().unwrap_or("/build"))?;
         Ok(NoneType)
     }
 }
@@ -305,6 +304,7 @@ fn phase_context_methods(builder: &mut MethodsBuilder) {
 fn run_in_container(
     argv: &[String],
     env_overrides: SmallMap<String, String>,
+    cwd: &str,
 ) -> anyhow::Result<()> {
     let (container, env) = with_current(|runtime| {
         let mut env: Vec<(String, String)> = runtime
@@ -328,7 +328,7 @@ fn run_in_container(
         }
         (runtime.container.clone(), env)
     })?;
-    container.borrow().exec(argv, &env, "/build")
+    container.borrow().exec(argv, &env, cwd)
 }
 
 #[derive(Debug, Clone, Allocative, ProvidesStaticType, NoSerialize)]