ui: control where prompt messages are written

cli/src/git_util.rs

@@ -70,7 +70,8 @@ pub fn is_colocated_git_workspace(workspace: &Workspace, repo: &ReadonlyRepo) ->
 }
 
 fn terminal_get_username(ui: &Ui, url: &str) -> Option<String> {
-    ui.prompt(&format!("Username for {url}")).ok()
+    ui.prompt(&mut ui.stdout(), &format!("Username for {url}"))
+        .ok()
 }
 
 fn terminal_get_pw(ui: &Ui, url: &str) -> Option<String> {

cli/src/movement_util.rs

@@ -214,6 +214,7 @@ fn choose_commit<'a>(
     drop(formatter);
 
     let choice = ui.prompt_choice(
+        &mut ui.stdout(),
         "enter the index of the commit you want to target",
         &choices,
         None,

cli/src/ui.rs

@@ -566,15 +566,15 @@ impl Ui {
                 .unwrap_or(false)
     }
 
-    pub fn prompt(&self, prompt: &str) -> io::Result<String> {
+    pub fn prompt(&self, writer: &mut dyn Write, prompt: &str) -> io::Result<String> {
         if !Self::can_prompt() {
             return Err(io::Error::new(
                 io::ErrorKind::Unsupported,
                 "Cannot prompt for input since the output is not connected to a terminal",
             ));
         }
-        write!(self.stdout(), "{prompt}: ")?;
-        self.stdout().flush()?;
+        write!(writer, "{prompt}: ")?;
+        writer.flush()?;
         let mut buf = String::new();
         io::stdin().read_line(&mut buf)?;
 
@@ -592,20 +592,21 @@ impl Ui {
     /// Repeat the given prompt until the input is one of the specified choices.
     pub fn prompt_choice(
         &self,
+        writer: &mut dyn Write,
         prompt: &str,
         choices: &[impl AsRef<str>],
         default: Option<&str>,
     ) -> io::Result<String> {
         if !Self::can_prompt() {
             if let Some(default) = default {
                 // Choose the default automatically without waiting.
-                writeln!(self.stdout(), "{prompt}: {default}")?;
+                writeln!(writer, "{prompt}: {default}")?;
                 return Ok(default.to_owned());
             }
         }
 
         loop {
-            let choice = self.prompt(prompt)?.trim().to_owned();
+            let choice = self.prompt(writer, prompt)?.trim().to_owned();
             if choice.is_empty() {
                 if let Some(default) = default {
                     return Ok(default.to_owned());
@@ -620,7 +621,12 @@ impl Ui {
     }
 
     /// Prompts for a yes-or-no response, with yes = true and no = false.
-    pub fn prompt_yes_no(&self, prompt: &str, default: Option<bool>) -> io::Result<bool> {
+    pub fn prompt_yes_no(
+        &self,
+        writer: &mut dyn Write,
+        prompt: &str,
+        default: Option<bool>,
+    ) -> io::Result<bool> {
         let default_str = match &default {
             Some(true) => "(Yn)",
             Some(false) => "(yN)",
@@ -629,6 +635,7 @@ impl Ui {
         let default_choice = default.map(|c| if c { "Y" } else { "N" });
 
         let choice = self.prompt_choice(
+            writer,
             &format!("{} {}", prompt, default_str),
             &["y", "n", "yes", "no", "Yes", "No", "YES", "NO"],
             default_choice,