fix: Adjust matching code based on review feedback

Author: Berkus Karchebnyi

clippy_lints/src/tests_outside_test_module.rs

@@ -6,8 +6,8 @@ use rustc_lint::{LateContext, LateLintPass};
 use rustc_session::declare_lint_pass;
 use rustc_span::def_id::LocalDefId;
 use rustc_span::source_map::SourceMap;
-use rustc_span::{FileName, RealFileName, Span};
-use std::path::Component;
+use rustc_span::{FileName, Span};
+use std::path::PathBuf;
 
 declare_clippy_lint! {
     /// ### What it does
@@ -80,10 +80,22 @@ impl LateLintPass<'_> for TestsOutsideTestModule {
 }
 
 fn is_integration_test(sm: &SourceMap, sp: Span) -> bool {
-    match sm.span_to_filename(sp) {
-        FileName::Real(RealFileName::LocalPath(name)) => {
-            name.components().next() == Some(Component::Normal("tests".as_ref()))
-        },
-        _ => false,
-    }
+    // 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")
 }