Warn when config file exists but fails to parse

crates/prek/src/cli/install.rs

@@ -18,7 +18,7 @@ use crate::fs::{CWD, Simplified};
 use crate::git::{GIT_ROOT, git_cmd};
 use crate::printer::Printer;
 use crate::store::Store;
-use crate::workspace::{Project, Workspace};
+use crate::workspace::{Error as WorkspaceError, Project, Workspace};
 use crate::{git, warn_user};
 
 #[allow(clippy::fn_params_excessive_bools)]
@@ -43,7 +43,15 @@ pub(crate) async fn install(
         );
     }
 
-    let project = Project::discover(config.as_deref(), &CWD).ok();
+    let project = match Project::discover(config.as_deref(), &CWD) {
+        Ok(project) => Some(project),
+        Err(err) => {
+            if let WorkspaceError::Config(err) = &err {
+                err.warn_parse_error();
+            }
+            None
+        }
+    };
     let hook_types = get_hook_types(hook_types, project.as_ref(), config.as_deref());
 
     let hooks_path = if let Some(dir) = git_dir {
@@ -133,13 +141,17 @@ fn get_hook_types(
             .iter()
             .map(Path::new)
             .filter(|p| p.exists());
-        config
-            .into_iter()
-            .chain(fallbacks)
-            .next()
-            .and_then(|p| load_config(p).ok())
-            .and_then(|cfg| cfg.default_install_hook_types.clone())
-            .unwrap_or_default()
+        if let Some(path) = config.into_iter().chain(fallbacks).next() {
+            match load_config(path) {
+                Ok(cfg) => cfg.default_install_hook_types.clone().unwrap_or_default(),
+                Err(err) => {
+                    err.warn_parse_error();
+                    vec![]
+                }
+            }
+        } else {
+            vec![]
+        }
     };
     if hook_types.is_empty() {
         hook_types = vec![HookType::PreCommit];

crates/prek/src/config.rs

@@ -1,6 +1,7 @@
 #[cfg(feature = "schemars")]
 use std::borrow::Cow;
 use std::collections::BTreeMap;
+use std::error::Error as _;
 use std::fmt::Display;
 use std::ops::RangeInclusive;
 use std::path::Path;
@@ -17,6 +18,7 @@ use serde::{Deserialize, Deserializer, Serialize};
 use crate::fs::Simplified;
 use crate::version;
 use crate::warn_user;
+use crate::warn_user_once;
 use crate::{identify, yaml};
 
 pub(crate) static CONFIG_FILE_REGEX: LazyLock<Regex> = LazyLock::new(|| {
@@ -1011,6 +1013,20 @@ pub(crate) enum Error {
     YamlMerge(String, #[source] yaml::MergeKeyError),
 }
 
+impl Error {
+    /// Warn the user if the config error is a parse error (not "file not found").
+    pub(crate) fn warn_parse_error(&self) {
+        if matches!(self, Self::NotFound(_)) {
+            return;
+        }
+        if let Some(cause) = self.source() {
+            warn_user_once!("{self}: {cause}");
+        } else {
+            warn_user_once!("{self}");
+        }
+    }
+}
+
 /// Keys that prek does not use.
 const EXPECTED_UNUSED: &[&str] = &["minimum_pre_commit_version", "ci"];
 

crates/prek/tests/install.rs

@@ -883,3 +883,29 @@ fn workspace_init_template_dir() -> anyhow::Result<()> {
 
     Ok(())
 }
+
+/// Test that a warning is shown when the config file exists but is invalid.
+#[test]
+fn install_invalid_config_warning() {
+    let context = TestContext::new();
+    context.init_project();
+
+    // Write an invalid config (missing required `rev` field).
+    context.write_pre_commit_config(indoc! {r"
+        repos:
+          - repo: https://github.com/pre-commit/pre-commit-hooks
+            hooks:
+              - id: trailing-whitespace
+    "});
+
+    // Install should succeed but show a warning about the invalid config.
+    cmd_snapshot!(context.filters(), context.install(), @r#"
+    success: true
+    exit_code: 0
+    ----- stdout -----
+    prek installed at `.git/hooks/pre-commit`
+
+    ----- stderr -----
+    warning: Failed to parse `.pre-commit-config.yaml`: Invalid remote repo: missing field `rev`
+    "#);
+}