diff --git a/src/rust/engine/process_execution/src/lib.rs b/src/rust/engine/process_execution/src/lib.rs
index a88b7e6cc70..e09c9ed93c2 100644
--- a/src/rust/engine/process_execution/src/lib.rs
+++ b/src/rust/engine/process_execution/src/lib.rs
@@ -21,7 +21,7 @@ extern crate tempdir;
 extern crate testutil;
 
 use bytes::Bytes;
-use std::collections::BTreeMap;
+use std::collections::{BTreeMap, BTreeSet};
 use std::path::PathBuf;
 
 pub mod local;
@@ -51,7 +51,7 @@ pub struct ExecuteProcessRequest {
 
   pub input_files: hashing::Digest,
 
-  pub output_files: Vec<PathBuf>,
+  pub output_files: BTreeSet<PathBuf>,
 }
 
 ///
diff --git a/src/rust/engine/process_execution/src/local.rs b/src/rust/engine/process_execution/src/local.rs
index b5549f3e6eb..086ae22e843 100644
--- a/src/rust/engine/process_execution/src/local.rs
+++ b/src/rust/engine/process_execution/src/local.rs
@@ -64,7 +64,7 @@ impl CommandRunner {
         .map(|posix_fs| Arc::new(posix_fs))
         .and_then(|posix_fs| {
           posix_fs
-            .path_stats(output_file_paths)
+            .path_stats(output_file_paths.into_iter().collect())
             .map_err(|e| format!("Error stating output files: {}", e))
             .and_then(move |paths| {
               fs::Snapshot::from_path_stats(
@@ -100,7 +100,7 @@ mod tests {
   use futures::Future;
   use super::{ExecuteProcessRequest, ExecuteProcessResult};
   use std;
-  use std::collections::BTreeMap;
+  use std::collections::{BTreeMap, BTreeSet};
   use std::env;
   use std::os::unix::fs::PermissionsExt;
   use std::path::{Path, PathBuf};
@@ -116,7 +116,7 @@ mod tests {
       argv: owned_string_vec(&["/bin/echo", "-n", "foo"]),
       env: BTreeMap::new(),
       input_files: fs::EMPTY_DIGEST,
-      output_files: vec![],
+      output_files: BTreeSet::new(),
     });
 
     assert_eq!(
@@ -137,7 +137,7 @@ mod tests {
       argv: owned_string_vec(&["/bin/bash", "-c", "echo -n foo ; echo >&2 -n bar ; exit 1"]),
       env: BTreeMap::new(),
       input_files: fs::EMPTY_DIGEST,
-      output_files: vec![],
+      output_files: BTreeSet::new(),
     });
 
     assert_eq!(
@@ -162,7 +162,7 @@ mod tests {
       argv: owned_string_vec(&["/usr/bin/env"]),
       env: env.clone(),
       input_files: fs::EMPTY_DIGEST,
-      output_files: vec![],
+      output_files: BTreeSet::new(),
     });
 
     let stdout = String::from_utf8(result.unwrap().stdout.to_vec()).unwrap();
@@ -194,7 +194,7 @@ mod tests {
         argv: owned_string_vec(&["/usr/bin/env"]),
         env: env,
         input_files: fs::EMPTY_DIGEST,
-        output_files: vec![],
+        output_files: BTreeSet::new(),
       }
     }
 
@@ -210,7 +210,7 @@ mod tests {
       argv: owned_string_vec(&["echo", "-n", "foo"]),
       env: BTreeMap::new(),
       input_files: fs::EMPTY_DIGEST,
-      output_files: vec![],
+      output_files: BTreeSet::new(),
     }).expect_err("Want Err");
   }
 
@@ -224,7 +224,7 @@ mod tests {
       ]),
       env: BTreeMap::new(),
       input_files: fs::EMPTY_DIGEST,
-      output_files: vec![],
+      output_files: BTreeSet::new(),
     });
     assert_eq!(
       result.unwrap(),
@@ -254,7 +254,7 @@ mod tests {
         ],
         env: BTreeMap::new(),
         input_files: fs::EMPTY_DIGEST,
-        output_files: vec![output_file_path.clone()],
+        output_files: vec![output_file_path.clone()].into_iter().collect(),
       },
       working_dir,
     );
diff --git a/src/rust/engine/process_execution/src/remote.rs b/src/rust/engine/process_execution/src/remote.rs
index 9ca7c3a20b6..e788e94157d 100644
--- a/src/rust/engine/process_execution/src/remote.rs
+++ b/src/rust/engine/process_execution/src/remote.rs
@@ -472,7 +472,7 @@ mod tests {
   use testutil::{as_bytes, owned_string_vec};
 
   use super::{CommandRunner, ExecuteProcessRequest, ExecuteProcessResult, ExecutionError};
-  use std::collections::BTreeMap;
+  use std::collections::{BTreeMap, BTreeSet};
   use std::iter::{self, FromIterator};
   use std::sync::Arc;
   use std::time::Duration;
@@ -501,7 +501,7 @@ mod tests {
           argv: owned_string_vec(&["/bin/echo", "-n", "bar"]),
           env: BTreeMap::new(),
           input_files: fs::EMPTY_DIGEST,
-          output_files: vec![],
+          output_files: BTreeSet::new(),
         }).unwrap()
           .1,
         vec![],
@@ -1119,7 +1119,7 @@ mod tests {
       argv: owned_string_vec(&["/bin/echo", "-n", "foo"]),
       env: BTreeMap::new(),
       input_files: fs::EMPTY_DIGEST,
-      output_files: vec![],
+      output_files: BTreeSet::new(),
     }
   }
 
@@ -1266,7 +1266,7 @@ mod tests {
       argv: owned_string_vec(&["/bin/cat", "roland"]),
       env: BTreeMap::new(),
       input_files: TestDirectory::containing_roland().digest(),
-      output_files: vec![],
+      output_files: BTreeSet::new(),
     }
   }
 }
diff --git a/src/rust/engine/process_executor/src/main.rs b/src/rust/engine/process_executor/src/main.rs
index f200083ff89..f99c4f41d91 100644
--- a/src/rust/engine/process_executor/src/main.rs
+++ b/src/rust/engine/process_executor/src/main.rs
@@ -11,7 +11,7 @@ use clap::{App, AppSettings, Arg};
 use futures::future::Future;
 use hashing::{Digest, Fingerprint};
 use tempdir::TempDir;
-use std::collections::BTreeMap;
+use std::collections::{BTreeMap, BTreeSet};
 use std::iter::Iterator;
 use std::process::exit;
 use std::sync::Arc;
@@ -128,7 +128,7 @@ fn main() {
     argv,
     env,
     input_files,
-    output_files: vec![],
+    output_files: BTreeSet::new(),
   };
 
   let result = match server_arg {