New lint for statement with no effect

README.md

@@ -6,7 +6,7 @@ A collection of lints to catch common mistakes and improve your Rust code.
 [Jump to usage instructions](#usage)
 
 ##Lints
-There are 69 lints included in this crate:
+There are 70 lints included in this crate:
 
 name                                                                                                   | default | meaning
 -------------------------------------------------------------------------------------------------------|---------|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
@@ -46,6 +46,7 @@ name
 [needless_range_loop](https://github.com/Manishearth/rust-clippy/wiki#needless_range_loop)             | warn    | for-looping over a range of indices where an iterator over items would do
 [needless_return](https://github.com/Manishearth/rust-clippy/wiki#needless_return)                     | warn    | using a return statement like `return expr;` where an expression would suffice
 [needless_update](https://github.com/Manishearth/rust-clippy/wiki#needless_update)                     | warn    | using `{ ..base }` when there are no missing fields
+[no_effect](https://github.com/Manishearth/rust-clippy/wiki#no_effect)                                 | warn    | statements with no effect
 [non_ascii_literal](https://github.com/Manishearth/rust-clippy/wiki#non_ascii_literal)                 | allow   | using any literal non-ASCII chars in a string literal; suggests using the \\u escape instead
 [nonsensical_open_options](https://github.com/Manishearth/rust-clippy/wiki#nonsensical_open_options)   | warn    | nonsensical combination of options for opening a file
 [option_unwrap_used](https://github.com/Manishearth/rust-clippy/wiki#option_unwrap_used)               | allow   | using `Option.unwrap()`, which should at least get a better message using `expect()`

src/lib.rs

@@ -52,6 +52,7 @@ pub mod zero_div_zero;
 pub mod open_options;
 pub mod needless_features;
 pub mod needless_update;
+pub mod no_effect;
 
 mod reexport {
     pub use syntax::ast::{Name, Ident, NodeId};
@@ -98,6 +99,7 @@ pub fn plugin_registrar(reg: &mut Registry) {
     reg.register_late_lint_pass(box mutex_atomic::MutexAtomic);
     reg.register_late_lint_pass(box needless_features::NeedlessFeaturesPass);
     reg.register_late_lint_pass(box needless_update::NeedlessUpdatePass);
+    reg.register_late_lint_pass(box no_effect::NoEffectPass);
 
     reg.register_lint_group("clippy_pedantic", vec![
         methods::OPTION_UNWRAP_USED,
@@ -159,6 +161,7 @@ pub fn plugin_registrar(reg: &mut Registry) {
         needless_features::UNSTABLE_AS_MUT_SLICE,
         needless_features::UNSTABLE_AS_SLICE,
         needless_update::NEEDLESS_UPDATE,
+        no_effect::NO_EFFECT,
         open_options::NONSENSICAL_OPEN_OPTIONS,
         precedence::PRECEDENCE,
         ptr_arg::PTR_ARG,

src/no_effect.rs

@@ -0,0 +1,61 @@
+use rustc::lint::{LateContext, LateLintPass, LintArray, LintPass};
+use rustc::middle::def::{DefStruct, DefVariant};
+use rustc_front::hir::{Expr, ExprCall, ExprLit, ExprPath, ExprStruct};
+use rustc_front::hir::{Stmt, StmtSemi};
+
+use utils::in_macro;
+use utils::span_lint;
+
+declare_lint! {
+    pub NO_EFFECT,
+    Warn,
+    "statements with no effect"
+}
+
+fn has_no_effect(cx: &LateContext, expr: &Expr) -> bool {
+    if in_macro(cx, expr.span) {
+        return false;
+    }
+    match expr.node {
+        ExprLit(..) |
+        ExprPath(..) => true,
+        ExprStruct(_, ref fields, ref base) => {
+            fields.iter().all(|field| has_no_effect(cx, &field.expr)) &&
+            match *base {
+                Some(ref base) => has_no_effect(cx, base),
+                None => true,
+            }
+        }
+        ExprCall(ref callee, ref args) => {
+            let def = cx.tcx.def_map.borrow().get(&callee.id).map(|d| d.full_def());
+            match def {
+                Some(DefStruct(..)) |
+                Some(DefVariant(..)) => {
+                    args.iter().all(|arg| has_no_effect(cx, arg))
+                }
+                _ => false,
+            }
+        }
+        _ => false,
+    }
+}
+
+#[derive(Copy, Clone)]
+pub struct NoEffectPass;
+
+impl LintPass for NoEffectPass {
+    fn get_lints(&self) -> LintArray {
+        lint_array!(NO_EFFECT)
+    }
+}
+
+impl LateLintPass for NoEffectPass {
+    fn check_stmt(&mut self, cx: &LateContext, stmt: &Stmt) {
+        if let StmtSemi(ref expr, _) = stmt.node {
+            if has_no_effect(cx, expr) {
+                span_lint(cx, NO_EFFECT, stmt.span,
+                          "statement with no effect");
+            }
+        }
+    }
+}

tests/compile-fail/needless_update.rs

@@ -2,6 +2,7 @@
 #![plugin(clippy)]
 
 #![deny(needless_update)]
+#![allow(no_effect)]
 
 struct S {
     pub a: i32,

tests/compile-fail/no_effect.rs

@@ -0,0 +1,39 @@
+#![feature(plugin)]
+#![plugin(clippy)]
+
+#![deny(no_effect)]
+#![allow(dead_code)]
+#![allow(path_statements)]
+
+struct Unit;
+struct Tuple(i32);
+struct Struct {
+    field: i32
+}
+enum Enum {
+    TupleVariant(i32),
+    StructVariant { field: i32 },
+}
+
+fn get_number() -> i32 { 0 }
+fn get_struct() -> Struct { Struct { field: 0 } }
+
+fn main() {
+    let s = get_struct();
+
+    0; //~ERROR statement with no effect
+    Unit; //~ERROR statement with no effect
+    Tuple(0); //~ERROR statement with no effect
+    Struct { field: 0 }; //~ERROR statement with no effect
+    Struct { ..s }; //~ERROR statement with no effect
+    Enum::TupleVariant(0); //~ERROR statement with no effect
+    Enum::StructVariant { field: 0 }; //~ERROR statement with no effect
+
+    // Do not warn
+    get_number();
+    Tuple(get_number());
+    Struct { field: get_number() };
+    Struct { ..get_struct() };
+    Enum::TupleVariant(get_number());
+    Enum::StructVariant { field: get_number() };
+}

tests/compile-fail/unit_cmp.rs

@@ -2,6 +2,7 @@
 #![plugin(clippy)]
 
 #![deny(unit_cmp)]
+#![allow(no_effect)]
 
 #[derive(PartialEq)]
 pub struct ContainsUnit(()); // should be fine