Add empty_test lint

Empty test functions execute no test code and cannot verify behaviour.
Provide an opt-in restriction lint so projects can detect these no-op
tests while allowing temporary placeholders where appropriate.

Author: willwang-io

CHANGELOG.md

@@ -7023,6 +7023,7 @@ Released 2018-09-13
 [`empty_line_after_outer_attr`]: https://rust-lang.github.io/rust-clippy/main/index.html#empty_line_after_outer_attr
 [`empty_loop`]: https://rust-lang.github.io/rust-clippy/main/index.html#empty_loop
 [`empty_structs_with_brackets`]: https://rust-lang.github.io/rust-clippy/main/index.html#empty_structs_with_brackets
+[`empty_test`]: https://rust-lang.github.io/rust-clippy/main/index.html#empty_test
 [`enum_clike_unportable_variant`]: https://rust-lang.github.io/rust-clippy/main/index.html#enum_clike_unportable_variant
 [`enum_glob_use`]: https://rust-lang.github.io/rust-clippy/main/index.html#enum_glob_use
 [`enum_variant_names`]: https://rust-lang.github.io/rust-clippy/main/index.html#enum_variant_names

clippy_lints/src/declared_lints.rs

@@ -147,6 +147,7 @@ pub static LINTS: &[&::declare_clippy_lint::LintInfo] = &[
     crate::empty_enums::EMPTY_ENUMS_INFO,
     crate::empty_line_after::EMPTY_LINE_AFTER_DOC_COMMENTS_INFO,
     crate::empty_line_after::EMPTY_LINE_AFTER_OUTER_ATTR_INFO,
+    crate::empty_test::EMPTY_TEST_INFO,
     crate::empty_with_brackets::EMPTY_ENUM_VARIANTS_WITH_BRACKETS_INFO,
     crate::empty_with_brackets::EMPTY_STRUCTS_WITH_BRACKETS_INFO,
     crate::endian_bytes::BIG_ENDIAN_BYTES_INFO,

clippy_lints/src/empty_test.rs

@@ -0,0 +1,51 @@
+use clippy_utils::diagnostics::span_lint;
+use clippy_utils::is_test_function;
+use rustc_hir::intravisit::FnKind;
+use rustc_hir::{Body, ExprKind, FnDecl};
+use rustc_lint::{LateContext, LateLintPass};
+use rustc_session::declare_lint_pass;
+use rustc_span::Span;
+use rustc_span::def_id::LocalDefId;
+
+declare_clippy_lint! {
+    /// ### What it does
+    /// Checks for test functions with an empty body.
+    ///
+    /// ### Why restrict this?
+    /// Empty tests do not verify behavior. However, they can serve as temporary placeholders while
+    /// a test is being developed.
+    ///
+    /// ### Example
+    /// ```no_run
+    /// #[test]
+    /// fn empty_test() {}
+    /// ```
+    ///
+    /// Remove the test if it is no longer needed, or add assertions that exercise the behavior it
+    /// is intended to test.
+    #[clippy::version = "1.100.0"]
+    pub EMPTY_TEST,
+    restriction,
+    "test function with an empty body"
+}
+
+declare_lint_pass!(EmptyTest => [EMPTY_TEST]);
+
+impl LateLintPass<'_> for EmptyTest {
+    fn check_fn(
+        &mut self,
+        cx: &LateContext<'_>,
+        kind: FnKind<'_>,
+        _: &FnDecl<'_>,
+        body: &Body<'_>,
+        span: Span,
+        fn_def_id: LocalDefId,
+    ) {
+        if matches!(kind, FnKind::ItemFn(..))
+            && is_test_function(cx.tcx, fn_def_id)
+            && matches!(body.value.kind, ExprKind::Block(block, _) if block.stmts.is_empty() && block.expr.is_none())
+        {
+            span_lint(cx, EMPTY_TEST, span, "empty test function");
+        }
+    }
+}

clippy_lints/src/lib.rs

@@ -121,6 +121,7 @@ mod else_if_without_else;
 mod empty_drop;
 mod empty_enums;
 mod empty_line_after;
+mod empty_test;
 mod empty_with_brackets;
 mod endian_bytes;
 mod entry;
@@ -870,6 +871,7 @@ rustc_lint::late_lint_methods!(
         BlockScrutinee: block_scrutinee::BlockScrutinee = block_scrutinee::BlockScrutinee,
         NonnullUncheckedOnBoxPtr: nonnull_unchecked_on_box_ptr::NonnullUncheckedOnBoxPtr = nonnull_unchecked_on_box_ptr::NonnullUncheckedOnBoxPtr::new(conf),
         NeedlessNonzeroGet: needless_nonzero_get::NeedlessNonzeroGet = needless_nonzero_get::NeedlessNonzeroGet::new(conf),
+        EmptyTest: empty_test::EmptyTest = empty_test::EmptyTest,
         // add late passes here, used by `cargo dev new_lint`
     ]]
 );

tests/ui/empty_test.rs

@@ -0,0 +1,21 @@
+#![warn(clippy::empty_test)]
+
+#[test]
+fn empty() {}
+//~^ empty_test
+
+#[test]
+fn empty_with_comment() {
+    // This is still an empty body.
+}
+//~^^^ empty_test
+
+fn ordinary_empty_function() {}
+
+#[test]
+fn meaningful_test() {
+    let value = String::from("Clippy");
+    assert_eq!(value.len(), 6);
+}
+
+fn main() {}

tests/ui/empty_test.stderr

@@ -0,0 +1,19 @@
+error: empty test function
+  --> tests/ui/empty_test.rs:4:1
+   |
+LL | fn empty() {}
+   | ^^^^^^^^^^^^^
+   |
+   = note: `-D clippy::empty-test` implied by `-D warnings`
+   = help: to override `-D warnings` add `#[allow(clippy::empty_test)]`
+
+error: empty test function
+  --> tests/ui/empty_test.rs:8:1
+   |
+LL | / fn empty_with_comment() {
+LL | |     // This is still an empty body.
+LL | | }
+   | |_^
+
+error: aborting due to 2 previous errors
+