Add empty closure lint

CHANGELOG.md

@@ -1242,6 +1242,7 @@ Released 2018-09-13
 [`duplicate_underscore_argument`]: https://rust-lang.github.io/rust-clippy/master/index.html#duplicate_underscore_argument
 [`duration_subsec`]: https://rust-lang.github.io/rust-clippy/master/index.html#duration_subsec
 [`else_if_without_else`]: https://rust-lang.github.io/rust-clippy/master/index.html#else_if_without_else
+[`empty_closure`]: https://rust-lang.github.io/rust-clippy/master/index.html#empty_closure
 [`empty_enum`]: https://rust-lang.github.io/rust-clippy/master/index.html#empty_enum
 [`empty_line_after_outer_attr`]: https://rust-lang.github.io/rust-clippy/master/index.html#empty_line_after_outer_attr
 [`empty_loop`]: https://rust-lang.github.io/rust-clippy/master/index.html#empty_loop

README.md

@@ -5,7 +5,7 @@
 
 A collection of lints to catch common mistakes and improve your [Rust](https://github.com/rust-lang/rust) code.
 
-[There are 361 lints included in this crate!](https://rust-lang.github.io/rust-clippy/master/index.html)
+[There are 362 lints included in this crate!](https://rust-lang.github.io/rust-clippy/master/index.html)
 
 We have a bunch of lint categories to allow you to choose how much Clippy is supposed to ~~annoy~~ help you:
 

clippy_lints/src/empty_closure.rs

@@ -0,0 +1,48 @@
+use crate::utils::{match_def_path, paths, span_lint};
+use if_chain::if_chain;
+use rustc_hir::{Expr, ExprKind};
+use rustc_lint::{LateContext, LateLintPass};
+use rustc_session::{declare_lint_pass, declare_tool_lint};
+
+declare_clippy_lint! {
+    /// **What it does:** Checks for empty closure.
+    ///
+    /// **Why is this bad?** Empty closure does nothing, it can be removed.
+    ///
+    /// **Known problems:** None.
+    ///
+    /// **Example:**
+    ///
+    /// ```rust
+    /// std::thread::spawn(|| {});
+    /// ```
+    pub EMPTY_CLOSURE,
+    style,
+    "closure with empty body"
+}
+
+declare_lint_pass!(EmptyClosure => [EMPTY_CLOSURE]);
+
+impl LateLintPass<'_, '_> for EmptyClosure {
+    fn check_expr(&mut self, cx: &LateContext<'_, '_>, expr: &'_ Expr<'_>) {
+        if_chain! {
+            if let ExprKind::Call(ref path_expr, ref args) = expr.kind;
+            if args.len() == 1;
+            if let ExprKind::Path(ref qpath) = path_expr.kind;
+            if let Some(def_id) = cx.tables.qpath_res(qpath, path_expr.hir_id).opt_def_id();
+            if match_def_path(cx, def_id, &paths::THREAD_SPAWN);
+            if let ExprKind::Closure(_, _, body_id, ..) = args[0].kind;
+            let body = cx.tcx.hir().body(body_id);
+            if let ExprKind::Block(ref b, _) = body.value.kind;
+            if b.stmts.is_empty() && b.expr.is_none();
+            then {
+                span_lint(
+                    cx,
+                    EMPTY_CLOSURE,
+                    args[0].span,
+                    "Empty closure may be removed",
+                );
+            }
+        }
+    }
+}

clippy_lints/src/lib.rs

@@ -199,6 +199,7 @@ pub mod drop_bounds;
 pub mod drop_forget_ref;
 pub mod duration_subsec;
 pub mod else_if_without_else;
+pub mod empty_closure;
 pub mod empty_enum;
 pub mod entry;
 pub mod enum_clike;
@@ -527,6 +528,7 @@ pub fn register_plugins(store: &mut rustc_lint::LintStore, sess: &Session, conf:
         &drop_forget_ref::FORGET_REF,
         &duration_subsec::DURATION_SUBSEC,
         &else_if_without_else::ELSE_IF_WITHOUT_ELSE,
+        &empty_closure::EMPTY_CLOSURE,
         &empty_enum::EMPTY_ENUM,
         &entry::MAP_ENTRY,
         &enum_clike::ENUM_CLIKE_UNPORTABLE_VARIANT,
@@ -1027,6 +1029,7 @@ pub fn register_plugins(store: &mut rustc_lint::LintStore, sess: &Session, conf:
     store.register_early_pass(|| box macro_use::MacroUseImports);
     store.register_late_pass(|| box verbose_file_reads::VerboseFileReads);
     store.register_late_pass(|| box redundant_pub_crate::RedundantPubCrate::default());
+    store.register_late_pass(|| box empty_closure::EmptyClosure);
 
     store.register_group(true, "clippy::restriction", Some("clippy_restriction"), vec![
         LintId::of(&arithmetic::FLOAT_ARITHMETIC),
@@ -1172,6 +1175,7 @@ pub fn register_plugins(store: &mut rustc_lint::LintStore, sess: &Session, conf:
         LintId::of(&drop_forget_ref::FORGET_COPY),
         LintId::of(&drop_forget_ref::FORGET_REF),
         LintId::of(&duration_subsec::DURATION_SUBSEC),
+        LintId::of(&empty_closure::EMPTY_CLOSURE),
         LintId::of(&entry::MAP_ENTRY),
         LintId::of(&enum_clike::ENUM_CLIKE_UNPORTABLE_VARIANT),
         LintId::of(&enum_variants::ENUM_VARIANT_NAMES),
@@ -1404,6 +1408,7 @@ pub fn register_plugins(store: &mut rustc_lint::LintStore, sess: &Session, conf:
         LintId::of(&comparison_chain::COMPARISON_CHAIN),
         LintId::of(&doc::MISSING_SAFETY_DOC),
         LintId::of(&doc::NEEDLESS_DOCTEST_MAIN),
+        LintId::of(&empty_closure::EMPTY_CLOSURE),
         LintId::of(&enum_variants::ENUM_VARIANT_NAMES),
         LintId::of(&enum_variants::MODULE_INCEPTION),
         LintId::of(&eq_op::OP_REF),

clippy_lints/src/utils/paths.rs

@@ -116,6 +116,7 @@ pub const STRING: [&str; 3] = ["alloc", "string", "String"];
 pub const STRING_AS_MUT_STR: [&str; 4] = ["alloc", "string", "String", "as_mut_str"];
 pub const STRING_AS_STR: [&str; 4] = ["alloc", "string", "String", "as_str"];
 pub const SYNTAX_CONTEXT: [&str; 3] = ["rustc_span", "hygiene", "SyntaxContext"];
+pub const THREAD_SPAWN: [&str; 3] = ["std", "thread", "spawn"];
 pub const TO_OWNED: [&str; 3] = ["alloc", "borrow", "ToOwned"];
 pub const TO_OWNED_METHOD: [&str; 4] = ["alloc", "borrow", "ToOwned", "to_owned"];
 pub const TO_STRING: [&str; 3] = ["alloc", "string", "ToString"];

src/lintlist/mod.rs

@@ -6,7 +6,7 @@ pub use lint::Lint;
 pub use lint::LINT_LEVELS;
 
 // begin lint list, do not remove this comment, it’s used in `update_lints`
-pub const ALL_LINTS: [Lint; 361] = [
+pub const ALL_LINTS: [Lint; 362] = [
     Lint {
         name: "absurd_extreme_comparisons",
         group: "correctness",
@@ -427,6 +427,13 @@ pub const ALL_LINTS: [Lint; 361] = [
         deprecation: None,
         module: "else_if_without_else",
     },
+    Lint {
+        name: "empty_closure",
+        group: "style",
+        desc: "closure with empty body",
+        deprecation: None,
+        module: "empty_closure",
+    },
     Lint {
         name: "empty_enum",
         group: "pedantic",

tests/ui/empty_closure.rs

@@ -0,0 +1,8 @@
+#![warn(clippy::empty_closure)]
+
+fn main() {
+    // Lint
+    std::thread::spawn(|| {});
+    // No lint
+    vec![0, 1, 2].iter().map(|_| {}).collect::<Vec<()>>();
+}

tests/ui/empty_closure.stderr

@@ -0,0 +1,10 @@
+error: Empty closure may be removed
+  --> $DIR/empty_closure.rs:5:24
+   |
+LL |     std::thread::spawn(|| {});
+   |                        ^^^^^
+   |
+   = note: `-D clippy::empty-closure` implied by `-D warnings`
+
+error: aborting due to previous error
+

tests/ui/unused_unit.fixed

@@ -35,6 +35,7 @@ fn return_unit()  {  }
 #[allow(clippy::needless_return)]
 #[allow(clippy::never_loop)]
 #[allow(clippy::unit_cmp)]
+#[allow(clippy::empty_closure)]
 fn main() {
     let u = Unitter;
     assert_eq!(u.get_unit(|| {}, return_unit), u.into());

tests/ui/unused_unit.rs

@@ -36,6 +36,7 @@ fn return_unit() -> () { () }
 #[allow(clippy::needless_return)]
 #[allow(clippy::never_loop)]
 #[allow(clippy::unit_cmp)]
+#[allow(clippy::empty_closure)]
 fn main() {
     let u = Unitter;
     assert_eq!(u.get_unit(|| {}, return_unit), u.into());

tests/ui/unused_unit.stderr

@@ -37,13 +37,13 @@ LL | fn return_unit() -> () { () }
    |                          ^^ help: remove the final `()`
 
 error: unneeded `()`
-  --> $DIR/unused_unit.rs:44:14
+  --> $DIR/unused_unit.rs:45:14
    |
 LL |         break();
    |              ^^ help: remove the `()`
 
 error: unneeded `()`
-  --> $DIR/unused_unit.rs:46:11
+  --> $DIR/unused_unit.rs:47:11
    |
 LL |     return();
    |           ^^ help: remove the `()`