util toml targets: infer file only if not a dir

Any directory entry ending with `.rs`, including directories, were
previously assumed to be files, and could end up as targets.

Now only regular files and symbolic links are inferred.

src/cargo/util/toml/targets.rs

@@ -624,10 +624,10 @@ fn infer_from_directory(directory: &Path) -> Vec<(String, PathBuf)> {
 }
 
 fn infer_any(entry: &DirEntry) -> Option<(String, PathBuf)> {
-    if entry.path().extension().and_then(|p| p.to_str()) == Some("rs") {
-        infer_file(entry)
-    } else if entry.file_type().map(|t| t.is_dir()).ok() == Some(true) {
+    if entry.file_type().map_or(false, |t| t.is_dir()) {
         infer_subdirectory(entry)
+    } else if entry.path().extension().and_then(|p| p.to_str()) == Some("rs") {
+        infer_file(entry)
     } else {
         None
     }

tests/testsuite/build.rs

@@ -5022,6 +5022,21 @@ fn inferred_benchmarks() {
     p.cargo("bench --bench=bar --bench=baz").run();
 }
 
+#[cargo_test]
+fn no_infer_dirs() {
+    let p = project()
+        .file("src/lib.rs", "fn main() {}")
+        .file("examples/file.rs", "fn main() {}")
+        .file("examples/dir.rs/dummy", "")
+        .file("benches/dir.rs/dummy", "")
+        .file("tests/dir.rs/dummy", "")
+        .build();
+
+    p.cargo("build --examples --benches --tests").run(); // should not fail with "is a directory"
+    assert!(p.bin("examples/file").is_file());
+    assert!(!p.bin("examples/dir").exists());
+}
+
 #[cargo_test]
 fn target_edition() {
     let p = project()