new: let an explicit --lang win over build-system detection

The wizard overwrote cli.lang with the detected ecosystem even when the
user passed --lang, and re-asked the language question in the ambiguous
and skeleton cases despite the documented 'flag > detected > default'
merge order. The flag now short-circuits the language step entirely
(detection stays informational); behavior without the flag is
unchanged.

src/new/questions.rs

@@ -122,10 +122,30 @@ async fn run_wizard(mut cli: NewCli) -> Result<NewOptions, Box<dyn Error>> {
         cli.name = Some(answer);
     }
 
-    // 2. Language / build system.
-    let mut preselected: Option<TemplateId> = None;
-    match &detection {
-        Detection::Single(id) if detection_decides => {
+    // 2. Language / build system. An explicit `--lang` wins over any
+    //    detection (flag > detected/probe > default): it is never
+    //    overwritten and the question is never re-asked — the detection is
+    //    only logged for information.
+    match language_choice(cli.lang.as_deref(), &detection, detection_decides) {
+        LanguageChoice::Flag => match &detection {
+            Detection::Single(id) => log::info!(
+                "Detected: {} project in {}; --lang takes precedence",
+                id.display_name(),
+                detect_dir.display()
+            ),
+            Detection::Ambiguous(candidates) => log::info!(
+                "Several build systems found in {} ({}); --lang takes \
+                 precedence",
+                detect_dir.display(),
+                candidates
+                    .iter()
+                    .map(|id| id.as_str())
+                    .collect::<Vec<_>>()
+                    .join(", ")
+            ),
+            Detection::Empty => {}
+        },
+        LanguageChoice::Detected(id) => {
             log::info!(
                 "Detected: {} project in {}",
                 id.display_name(),
@@ -133,12 +153,16 @@ async fn run_wizard(mut cli: NewCli) -> Result<NewOptions, Box<dyn Error>> {
             );
             cli.lang = Some(id.as_str().to_string());
         }
-        Detection::Single(id) => {
-            // A skeleton was asked for: still ask, preselecting the
-            // detected ecosystem.
-            preselected = Some(*id);
+        LanguageChoice::Ask(preselected) => {
+            let menu = language_menu(&[]);
+            let default = preselected
+                .unwrap_or(TemplateId::Empty)
+                .display_name()
+                .to_string();
+            let id = select_template(&menu, &default)?;
+            cli.lang = Some(id.as_str().to_string());
         }
-        Detection::Ambiguous(candidates) => {
+        LanguageChoice::Ambiguous(candidates) => {
             log::info!(
                 "Several build systems found in {} ({}): candidates listed \
                  first, the highest-precedence one preselected",
@@ -149,20 +173,10 @@ async fn run_wizard(mut cli: NewCli) -> Result<NewOptions, Box<dyn Error>> {
                     .collect::<Vec<_>>()
                     .join(", ")
             );
-            let menu = language_menu(candidates);
+            let menu = language_menu(&candidates);
             let id = select_template(&menu, &menu[0])?;
             cli.lang = Some(id.as_str().to_string());
         }
-        Detection::Empty => {}
-    }
-    if cli.lang.is_none() {
-        let menu = language_menu(&[]);
-        let default = preselected
-            .unwrap_or(TemplateId::Empty)
-            .display_name()
-            .to_string();
-        let id = select_template(&menu, &default)?;
-        cli.lang = Some(id.as_str().to_string());
     }
     let template = TemplateId::parse(cli.lang.as_deref().unwrap_or_default())?;
 
@@ -690,6 +704,42 @@ fn language_menu(candidates: &[TemplateId]) -> Vec<String> {
         .collect()
 }
 
+/// What the language step does with the detection result: how an explicit
+/// `--lang` flag combines with it (flag > detected/probe > default).
+#[derive(Debug, PartialEq, Eq)]
+enum LanguageChoice {
+    /// An explicit `--lang` wins: it is kept as-is, the question is never
+    /// re-asked and the detection stays informational.
+    Flag,
+    /// No flag, confident detection, packaging the detected directory: the
+    /// detection decides.
+    Detected(TemplateId),
+    /// Ask the question, preselecting the detected ecosystem (a skeleton
+    /// was asked for); `None` when nothing was detected.
+    Ask(Option<TemplateId>),
+    /// Ask the question with the ambiguous candidates listed first (the
+    /// highest-precedence one preselected).
+    Ambiguous(Vec<TemplateId>),
+}
+
+/// The language step's decision for one wizard run. With the flag absent
+/// this mirrors the historical behavior exactly; the flag always wins.
+fn language_choice(
+    flag: Option<&str>,
+    detection: &Detection,
+    detection_decides: bool,
+) -> LanguageChoice {
+    if flag.is_some() {
+        return LanguageChoice::Flag;
+    }
+    match detection {
+        Detection::Single(id) if detection_decides => LanguageChoice::Detected(*id),
+        Detection::Single(id) => LanguageChoice::Ask(Some(*id)),
+        Detection::Ambiguous(candidates) => LanguageChoice::Ambiguous(candidates.clone()),
+        Detection::Empty => LanguageChoice::Ask(None),
+    }
+}
+
 /// The license menu: the curated SPDX list plus the free-text entry.
 fn license_menu() -> Vec<String> {
     KNOWN_LICENSES
@@ -1043,6 +1093,59 @@ mod tests {
         assert_eq!(unique.len(), menu.len());
     }
 
+    /// An explicit `--lang` wins over any detection (the flag outranks the
+    /// detection and the defaults): the flag is kept and the question is
+    /// never re-asked, whatever the detection found (regression: a
+    /// confident detection used to overwrite the flag and an ambiguous one
+    /// re-asked the question).
+    #[test]
+    fn language_choice_flag_wins_over_detection() {
+        let detections = [
+            Detection::Single(Tid::Rust),
+            Detection::Ambiguous(vec![Tid::Rust, Tid::Python]),
+            Detection::Empty,
+        ];
+        for detection in &detections {
+            for decides in [false, true] {
+                assert_eq!(
+                    language_choice(Some("python"), detection, decides),
+                    LanguageChoice::Flag,
+                    "detection {detection:?}, decides {decides}"
+                );
+            }
+        }
+    }
+
+    /// Without the flag the detection behaves exactly as before: it decides
+    /// for a confident detection (packaging the detected directory), asks
+    /// with a preselection for a skeleton run, lists the candidates first
+    /// when ambiguous, and falls back to the plain menu otherwise.
+    #[test]
+    fn language_choice_without_flag_follows_detection() {
+        assert_eq!(
+            language_choice(None, &Detection::Single(Tid::Rust), true),
+            LanguageChoice::Detected(Tid::Rust)
+        );
+        // Skeleton run: ask, preselecting the detected ecosystem.
+        assert_eq!(
+            language_choice(None, &Detection::Single(Tid::Rust), false),
+            LanguageChoice::Ask(Some(Tid::Rust))
+        );
+        assert_eq!(
+            language_choice(
+                None,
+                &Detection::Ambiguous(vec![Tid::Go, Tid::Python]),
+                true
+            ),
+            LanguageChoice::Ambiguous(vec![Tid::Go, Tid::Python])
+        );
+        // Nothing detected: plain menu, the empty template preselected.
+        assert_eq!(
+            language_choice(None, &Detection::Empty, false),
+            LanguageChoice::Ask(None)
+        );
+    }
+
     #[test]
     fn license_menu_and_defaults() {
         let menu = license_menu();