CARGO: Support main.rs as crate root in various subfolder

This feature was added after cargo 0.21
(not released with any stable Rust version yet)
rust-lang/cargo#4496

src/main/kotlin/org/rust/cargo/project/model/impl/CargoTomlWatcher.kt

@@ -30,6 +30,8 @@ class CargoTomlWatcher(
         "/src/bin", "/examples", "/tests", "/benches"
     )
 
+    private val MAIN = "main.rs"
+
     override fun before(events: List<VFileEvent>) {
     }
 
@@ -38,9 +40,12 @@ class CargoTomlWatcher(
             if (isCargoTomlChange(event)) return true
             if (event.path.endsWith(RustToolchain.CARGO_LOCK)) return true
             if (event is VFileContentChangeEvent || PathUtil.getFileExtension(event.path) != "rs") return false
+            val parent = PathUtil.getParentPath(event.path)
+            val grandParent = PathUtil.getParentPath(parent)
+            val name = PathUtil.getFileName(event.path)
 
             if (IMPLICIT_TARGET_FILES.any { event.path.endsWith(it) }) return true
-            return IMPLICIT_TARGET_DIRS.any { PathUtil.getParentPath(event.path).endsWith(it) }
+            return IMPLICIT_TARGET_DIRS.any { parent.endsWith(it) || (name == MAIN && grandParent.endsWith(it)) }
         }
 
         if (events.any(::isInterestingEvent)) {

src/test/kotlin/org/rust/cargo/project/model/impl/CargoTomlWatcherTest.kt

@@ -44,6 +44,9 @@ class CargoTomlWatcherTest : RsTestBase() {
         watcher.checkTriggered(createEvent)
         watcher.checkNotTriggered(newChangeEvent(binFile))
 
+        // src/bin/*/main.rs
+        watcher.checkTriggered(newCreateEvent("src/bin/foo/main.rs").second)
+
         // src/main.rs
         watcher.checkTriggered(newCreateEvent("src/main.rs").second)
         watcher.checkNotTriggered(newCreateEvent("prefix_src/main.rs").second)
@@ -59,6 +62,11 @@ class CargoTomlWatcherTest : RsTestBase() {
         watcher.checkTriggered(newCreateEvent("tests/foo.rs").second)
         watcher.checkNotTriggered(newCreateEvent("prefix_tests/foo.rs").second)
 
+        // benches/*/main.rs, examples/*/main.rs, tests/*/main.rs
+        watcher.checkTriggered(newCreateEvent("benches/foo/main.rs").second)
+        watcher.checkTriggered(newCreateEvent("examples/foo/main.rs").second)
+        watcher.checkTriggered(newCreateEvent("tests/foo/main.rs").second)
+
         // build.rs
         watcher.checkTriggered(newCreateEvent("build.rs").second)
         watcher.checkNotTriggered(newCreateEvent("prefix_build.rs").second)