fix: Do not warn tests outside a module for integration tests

clippy_lints/src/tests_outside_test_module.rs

@@ -5,7 +5,9 @@ use rustc_hir::{Body, FnDecl};
 use rustc_lint::{LateContext, LateLintPass};
 use rustc_session::declare_lint_pass;
 use rustc_span::def_id::LocalDefId;
-use rustc_span::Span;
+use rustc_span::source_map::SourceMap;
+use rustc_span::{FileName, Span};
+use std::path::PathBuf;
 
 declare_clippy_lint! {
     /// ### What it does
@@ -15,6 +17,7 @@ declare_clippy_lint! {
     /// ### Why restrict this?
     /// The idiomatic (and more performant) way of writing tests is inside a testing module (flagged with `#[cfg(test)]`),
     /// having test functions outside of this module is confusing and may lead to them being "hidden".
+    /// This does not apply to integration tests though, and this lint will ignore those.
     ///
     /// ### Example
     /// ```no_run
@@ -59,6 +62,7 @@ impl LateLintPass<'_> for TestsOutsideTestModule {
     ) {
         if !matches!(kind, FnKind::Closure)
             && is_in_test_function(cx.tcx, body.id().hir_id)
+            && !is_integration_test(cx.tcx.sess.source_map(), sp)
             && !is_in_cfg_test(cx.tcx, body.id().hir_id)
         {
             #[expect(clippy::collapsible_span_lint_calls, reason = "rust-clippy#7797")]
@@ -74,3 +78,24 @@ impl LateLintPass<'_> for TestsOutsideTestModule {
         }
     }
 }
+
+fn is_integration_test(sm: &SourceMap, sp: Span) -> bool {
+    // This part is from https://github.com/rust-lang/rust/blob/a91f7d72f12efcc00ecf71591f066c534d45ddf7/compiler/rustc_expand/src/expand.rs#L402-L409 (fn expand_crate)
+    // Extract crate base path from the filename path.
+    let file_path = match sm.span_to_filename(sp) {
+        FileName::Real(name) => name
+            .into_local_path()
+            .expect("attempting to resolve a file path in an external file"),
+        other => PathBuf::from(other.prefer_local().to_string()),
+    };
+
+    // Root path contains the topmost sources directory of the crate.
+    // I.e., for `project` with sources in `src` and tests in `tests` folders
+    // (no matter how many nested folders lie inside),
+    // there will be two different root paths: `/project/src` and `/project/tests`.
+    let root_path = file_path.parent().unwrap_or(&file_path).to_owned();
+
+    // The next part matches logic in https://github.com/rust-lang/rust/blob/a91f7d72f12efcc00ecf71591f066c534d45ddf7/compiler/rustc_builtin_macros/src/test.rs#L526 (fn test_type)
+    // Integration tests are under /tests directory in the crate root_path we determined above.
+    root_path.ends_with("tests")
+}

tests/ui/tests_outside_test_module.rs

@@ -5,11 +5,10 @@ fn main() {
     // test code goes here
 }
 
-// Should lint
+// Should not lint
+// Because we're inside an integration test
 #[test]
 fn my_test() {}
-//~^ ERROR: this function marked with #[test] is outside a #[cfg(test)] module
-//~| NOTE: move it to a testing module marked with #[cfg(test)]
 
 #[cfg(test)]
 mod tests {