refactor(trace): dedupe downloads_dir secret resolution into one helper

crates/flowproof-agent/src/recorder.rs

@@ -1801,18 +1801,11 @@ fn mint_baseline<D: AppDriver>(
 ///
 /// `downloads_dir` is the one field here that resolves `${VAR}` (a runner's
 /// temp/download path is exactly the kind of thing that differs per
-/// environment) — every sibling field is a literal by design (geometry,
-/// a pinned clock/seed, raw Chrome flags), so this is the one seam that
-/// needs it.
+/// environment) — see `secret::resolve_downloads_dir`.
 fn web_browser_from_setup(
     setup: &flowproof_trace::format::BrowserSetup,
 ) -> Result<flowproof_driver::WebBrowserConfig, RecordError> {
-    let downloads_dir = setup
-        .downloads_dir
-        .as_deref()
-        .map(flowproof_trace::secret::resolve_refs)
-        .transpose()?
-        .map(std::path::PathBuf::from);
+    let downloads_dir = flowproof_trace::secret::resolve_downloads_dir(&setup.downloads_dir)?;
     Ok(flowproof_driver::WebBrowserConfig::from_setup_parts(
         setup
             .viewport

crates/flowproof-cli/src/lib.rs

@@ -705,16 +705,9 @@ fn stage_surface_browser(
         return Ok(());
     };
     // `downloads_dir` is the one field here resolved from `${VAR}` — see
-    // `web_browser_from_setup` in `flowproof-agent/src/recorder.rs`, the
-    // record-path sibling of this replay-path function; every other field
-    // is a literal by design.
-    let downloads_dir = browser
-        .downloads_dir
-        .as_deref()
-        .map(flowproof_trace::secret::resolve_refs)
-        .transpose()
-        .map_err(|e| flowproof_driver::DriverError::Uia(e.to_string()))?
-        .map(std::path::PathBuf::from);
+    // `secret::resolve_downloads_dir`.
+    let downloads_dir = flowproof_trace::secret::resolve_downloads_dir(&browser.downloads_dir)
+        .map_err(|e| flowproof_driver::DriverError::Uia(e.to_string()))?;
     driver.stage_browser(flowproof_driver::WebBrowserConfig::from_setup_parts(
         browser
             .viewport

crates/flowproof-replay/src/lib.rs

@@ -2457,15 +2457,10 @@ pub fn run_trace_with_exports<D: AppDriver>(
     // emulated phone viewport must not replay on a desktop one.
     if let Some(browser) = &header.browser {
         if !browser.is_empty() {
-            // `downloads_dir` is the one field here resolved from `${VAR}`;
-            // every sibling is a literal by design (geometry, a pinned
-            // clock/seed, raw Chrome flags).
-            let downloads_dir = browser
-                .downloads_dir
-                .as_deref()
-                .map(flowproof_trace::secret::resolve_refs)
-                .transpose()?
-                .map(std::path::PathBuf::from);
+            // `downloads_dir` is the one field here resolved from `${VAR}` —
+            // see `secret::resolve_downloads_dir`.
+            let downloads_dir =
+                flowproof_trace::secret::resolve_downloads_dir(&browser.downloads_dir)?;
             driver.stage_browser(flowproof_driver::WebBrowserConfig::from_setup_parts(
                 browser
                     .viewport

crates/flowproof-trace/src/secret.rs

@@ -92,10 +92,55 @@ pub fn resolve_refs_in_json(value: &serde_json::Value) -> Result<serde_json::Val
     })
 }
 
+/// Resolve a `downloads_dir` field: `${VAR}` references resolve from the
+/// environment (see [`resolve_refs`]), `None` stays `None`. This is the one
+/// browser-setup field that resolves refs — every sibling is a literal by
+/// design — so this composes [`resolve_refs`] into the shape each call site
+/// (record, replay, and the CLI's staging path) needs.
+pub fn resolve_downloads_dir(
+    raw: &Option<String>,
+) -> Result<Option<std::path::PathBuf>, MissingSecret> {
+    raw.as_deref()
+        .map(resolve_refs)
+        .transpose()
+        .map(|opt| opt.map(std::path::PathBuf::from))
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;
 
+    #[test]
+    fn downloads_dir_none_resolves_to_none() {
+        assert_eq!(resolve_downloads_dir(&None).expect("resolves"), None);
+    }
+
+    #[test]
+    fn downloads_dir_literal_path_resolves_unchanged() {
+        let raw = Some("/tmp/flowproof-downloads".to_string());
+        assert_eq!(
+            resolve_downloads_dir(&raw).expect("resolves"),
+            Some(std::path::PathBuf::from("/tmp/flowproof-downloads"))
+        );
+    }
+
+    #[test]
+    fn downloads_dir_ref_resolves_from_the_environment() {
+        std::env::set_var("FLOWPROOF_TEST_DOWNLOADS_DIR", "/runner/downloads");
+        let raw = Some("${FLOWPROOF_TEST_DOWNLOADS_DIR}".to_string());
+        assert_eq!(
+            resolve_downloads_dir(&raw).expect("resolves"),
+            Some(std::path::PathBuf::from("/runner/downloads"))
+        );
+    }
+
+    #[test]
+    fn downloads_dir_missing_ref_is_a_hard_error() {
+        let raw = Some("${FLOWPROOF_TEST_DOWNLOADS_DIR_UNSET_XYZ}".to_string());
+        let err = resolve_downloads_dir(&raw).expect_err("must fail");
+        assert_eq!(err.var, "FLOWPROOF_TEST_DOWNLOADS_DIR_UNSET_XYZ");
+    }
+
     #[test]
     fn plain_text_passes_through_untouched() {
         assert!(!has_refs("hello from flowproof"));