Fix two follow-up issues: reject nonexistent roots, single-source extensions

- pipeline: fail index create/update with a clear error when the project root
  is not a directory instead of building a misleading zero-file index
- config: single LANGUAGE_BY_EXT table as source of truth; language_for and
  resolver code_extensions both derive from it (no more duplicated ext list)
- imports: resolver probes code_extensions() instead of a local RESOLVE_EXTS
- tests: regression for nonexistent-root rejection

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

src/config.rs

@@ -28,21 +28,52 @@ pub const SKIP_DIRS: &[&str] = &[
     "out",
 ];
 
+/// Extension → language for every supported source extension. Single source of
+/// truth: discovery (`language_for`) and import resolution (`code_extensions`)
+/// both derive from this table, so a new extension can never drift between the
+/// two. Code extensions come first (resolver probe order), markup/assets last.
+const LANGUAGE_BY_EXT: &[(&str, &str)] = &[
+    ("rs", "rust"),
+    ("py", "python"),
+    ("ts", "typescript"),
+    ("tsx", "typescript"),
+    ("mts", "typescript"),
+    ("cts", "typescript"),
+    ("js", "javascript"),
+    ("jsx", "javascript"),
+    ("mjs", "javascript"),
+    ("cjs", "javascript"),
+    ("c", "c"),
+    ("h", "c"),
+    ("cc", "cpp"),
+    ("cpp", "cpp"),
+    ("cxx", "cpp"),
+    ("hpp", "cpp"),
+    ("hh", "cpp"),
+    ("hxx", "cpp"),
+    ("java", "java"),
+    ("html", "html"),
+    ("htm", "html"),
+    ("css", "css"),
+];
+
 /// Map a file extension to a language label, or `None` if unsupported.
 pub fn language_for(path: &Path) -> Option<&'static str> {
     let ext = path.extension()?.to_str()?;
-    Some(match ext {
-        "rs" => "rust",
-        "java" => "java",
-        "py" => "python",
-        "c" | "h" => "c",
-        "cc" | "cpp" | "cxx" | "hpp" | "hh" | "hxx" => "cpp",
-        "ts" | "tsx" | "mts" | "cts" => "typescript",
-        "js" | "jsx" | "mjs" | "cjs" => "javascript",
-        "html" | "htm" => "html",
-        "css" => "css",
-        _ => return None,
-    })
+    LANGUAGE_BY_EXT
+        .iter()
+        .find(|(e, _)| *e == ext)
+        .map(|(_, lang)| *lang)
+}
+
+/// Code extensions probed when resolving extensionless import targets.
+/// Markup/asset languages (html/css) are always imported with an explicit
+/// extension, so they are excluded from probing.
+pub fn code_extensions() -> impl Iterator<Item = &'static str> {
+    LANGUAGE_BY_EXT
+        .iter()
+        .filter(|(_, lang)| *lang != "html" && *lang != "css")
+        .map(|(ext, _)| *ext)
 }
 
 /// Base cache directory for index databases.

src/index/imports.rs

@@ -5,6 +5,7 @@ use std::sync::OnceLock;
 
 use regex::Regex;
 
+use crate::config;
 use crate::discovery::SourceFile;
 
 #[derive(Debug, Clone)]
@@ -306,14 +307,7 @@ fn clean(target: &str) -> Option<String> {
 }
 
 /// Code file extensions probed when resolving extensionless import targets.
-/// Mirrors `config::language_for` for code languages (html/css assets are
-/// always imported with an explicit extension, so they need no probing).
-const RESOLVE_EXTS: &[&str] = &[
-    "rs", "py", "ts", "tsx", "mts", "cts", "js", "jsx", "mjs", "cjs", "c", "h", "cc", "cpp", "cxx",
-    "hpp", "hh", "hxx", "java",
-];
-
-/// Module-directory entry files tried for `dir/mod` style resolution.
+/// Derived from `config::language_for` so the two lists can't drift.
 const MODULE_ENTRIES: &[&str] = &["mod.rs", "__init__.py", "index.ts", "index.js", "index.tsx"];
 
 impl Resolver {
@@ -403,7 +397,7 @@ impl Resolver {
         if let Some(id) = self.by_path.get(&format!("{base}{t}")) {
             return Some(*id);
         }
-        for ext in RESOLVE_EXTS {
+        for ext in config::code_extensions() {
             if let Some(id) = self.by_path.get(&format!("{base}{t}.{ext}")) {
                 return Some(*id);
             }
@@ -417,7 +411,7 @@ impl Resolver {
     }
 
     fn try_module(&self, base: &str) -> Option<i64> {
-        for ext in RESOLVE_EXTS {
+        for ext in config::code_extensions() {
             if let Some(id) = self.by_path.get(&format!("{base}.{ext}")) {
                 return Some(*id);
             }

src/index/pipeline.rs

@@ -44,6 +44,10 @@ fn read_crate_name(project: &Path) -> Option<String> {
 
 /// (Re)index a project. `recreate` drops any existing index first.
 pub fn run(project: &Path, db_path: &Path, recreate: bool) -> Result<IndexStats> {
+    // A typo'd path must fail loudly, not create a misleading empty index.
+    if !project.is_dir() {
+        anyhow::bail!("project root is not a directory: {}", project.display());
+    }
     if recreate && db_path.exists() {
         std::fs::remove_file(db_path).context("remove old index")?;
     }