Improve mod resolution error for mods with multiple candidate files

src/modules.rs

@@ -81,6 +81,7 @@ pub struct ModuleResolutionError {
     pub(crate) kind: ModuleResolutionErrorKind,
 }
 
+/// Defines variants similar to those of [rustc_expand::module::ModError]
 #[derive(Debug, Error)]
 pub(crate) enum ModuleResolutionErrorKind {
     /// Find a file that cannot be parsed.
@@ -89,6 +90,12 @@ pub(crate) enum ModuleResolutionErrorKind {
     /// File cannot be found.
     #[error("{file} does not exist")]
     NotFound { file: PathBuf },
+    /// File a.rs and a/mod.rs both exist
+    #[error("file for module found at both {default_path:?} and {secondary_path:?}")]
+    MultipleCandidates {
+        default_path: PathBuf,
+        secondary_path: PathBuf,
+    },
 }
 
 #[derive(Clone)]
@@ -444,12 +451,31 @@ impl<'ast, 'sess, 'c> ModResolver<'ast, 'sess> {
                 }
                 Ok(Some(SubModKind::MultiExternal(mods_outside_ast)))
             }
-            Err(_) => Err(ModuleResolutionError {
-                module: mod_name.to_string(),
-                kind: ModuleResolutionErrorKind::NotFound {
-                    file: self.directory.path.clone(),
-                },
-            }),
+            Err(e) => match e {
+                ModError::FileNotFound(_, default_path, _secondary_path) => {
+                    Err(ModuleResolutionError {
+                        module: mod_name.to_string(),
+                        kind: ModuleResolutionErrorKind::NotFound { file: default_path },
+                    })
+                }
+                ModError::MultipleCandidates(_, default_path, secondary_path) => {
+                    Err(ModuleResolutionError {
+                        module: mod_name.to_string(),
+                        kind: ModuleResolutionErrorKind::MultipleCandidates {
+                            default_path,
+                            secondary_path,
+                        },
+                    })
+                }
+                ModError::ParserError(_)
+                | ModError::CircularInclusion(_)
+                | ModError::ModInBlock(_) => Err(ModuleResolutionError {
+                    module: mod_name.to_string(),
+                    kind: ModuleResolutionErrorKind::ParseError {
+                        file: self.directory.path.clone(),
+                    },
+                }),
+            },
         }
     }
 

src/parse/session.rs

@@ -163,8 +163,11 @@ impl ParseSess {
             |e| {
                 // If resloving a module relative to {dir_path}/{symbol} fails because a file
                 // could not be found, then try to resolve the module relative to {dir_path}.
+                // If we still can't find the module after searching for it in {dir_path},
+                // surface the original error.
                 if matches!(e, ModError::FileNotFound(..)) && relative.is_some() {
                     rustc_expand::module::default_submod_path(&self.parse_sess, id, None, dir_path)
+                        .map_err(|_| e)
                 } else {
                     Err(e)
                 }

tests/mod-resolver/issue-5167/src/lib.rs

@@ -0,0 +1 @@
+mod a;

tests/mod-resolver/module-not-found/bad_path_attribute/lib.rs

@@ -0,0 +1,3 @@
+// module resolution fails because the path does not exist.
+#[path = "path/to/does_not_exist.rs"]
+mod a;

tests/mod-resolver/module-not-found/relative_module/a.rs

@@ -0,0 +1,2 @@
+// module resolution fails because `./a/b.rs` does not exist
+mod b;

tests/mod-resolver/module-not-found/relative_module/lib.rs

@@ -0,0 +1 @@
+mod a;

tests/mod-resolver/module-not-found/sibling_module/lib.rs

@@ -0,0 +1,2 @@
+// module resolution fails because `./a.rs` does not exist
+mod a;

tests/rustfmt/main.rs

@@ -113,3 +113,47 @@ fn rustfmt_usage_text() {
     let (stdout, _) = rustfmt(&args);
     assert!(stdout.contains("Format Rust code\n\nusage: rustfmt [options] <file>..."));
 }
+
+#[test]
+fn mod_resolution_error_multiple_candidate_files() {
+    // See also https://github.com/rust-lang/rustfmt/issues/5167
+    let default_path = Path::new("tests/mod-resolver/issue-5167/src/a.rs");
+    let secondary_path = Path::new("tests/mod-resolver/issue-5167/src/a/mod.rs");
+    let error_message = format!(
+        "file for module found at both {:?} and {:?}",
+        default_path.canonicalize().unwrap(),
+        secondary_path.canonicalize().unwrap(),
+    );
+
+    let args = ["tests/mod-resolver/issue-5167/src/lib.rs"];
+    let (_stdout, stderr) = rustfmt(&args);
+    assert!(stderr.contains(&error_message))
+}
+
+#[test]
+fn mod_resolution_error_sibling_module_not_found() {
+    let args = ["tests/mod-resolver/module-not-found/sibling_module/lib.rs"];
+    let (_stdout, stderr) = rustfmt(&args);
+    // Module resolution fails because we're unable to find `a.rs` in the same directory as lib.rs
+    assert!(stderr.contains("a.rs does not exist"))
+}
+
+#[test]
+fn mod_resolution_error_relative_module_not_found() {
+    let args = ["tests/mod-resolver/module-not-found/relative_module/lib.rs"];
+    let (_stdout, stderr) = rustfmt(&args);
+    // The file `./a.rs` and directory `./a` both exist.
+    // Module resolution fails becuase we're unable to find `./a/b.rs`
+    #[cfg(not(windows))]
+    assert!(stderr.contains("a/b.rs does not exist"));
+    #[cfg(windows)]
+    assert!(stderr.contains("a\\b.rs does not exist"));
+}
+
+#[test]
+fn mod_resolution_error_path_attribute_does_not_exist() {
+    let args = ["tests/mod-resolver/module-not-found/bad_path_attribute/lib.rs"];
+    let (_stdout, stderr) = rustfmt(&args);
+    // The path attribute points to a file that does not exist
+    assert!(stderr.contains("does_not_exist.rs does not exist"));
+}