Lint ifs with the same then and else blocks

README.md

@@ -47,6 +47,7 @@ name
 [for_loop_over_option](https://github.com/Manishearth/rust-clippy/wiki#for_loop_over_option)                   | warn    | for-looping over an `Option`, which is more clearly expressed as an `if let`
 [for_loop_over_result](https://github.com/Manishearth/rust-clippy/wiki#for_loop_over_result)                   | warn    | for-looping over a `Result`, which is more clearly expressed as an `if let`
 [identity_op](https://github.com/Manishearth/rust-clippy/wiki#identity_op)                                     | warn    | using identity operations, e.g. `x + 0` or `y / 1`
+[if_same_then_else](https://github.com/Manishearth/rust-clippy/wiki#if_same_then_else)                         | warn    | if with the same *then* and *else* blocks
 [ifs_same_cond](https://github.com/Manishearth/rust-clippy/wiki#ifs_same_cond)                                 | warn    | consecutive `ifs` with the same condition
 [ineffective_bit_mask](https://github.com/Manishearth/rust-clippy/wiki#ineffective_bit_mask)                   | warn    | expressions where a bit mask will be rendered useless by a comparison, e.g. `(x | 1) > 2`
 [inline_always](https://github.com/Manishearth/rust-clippy/wiki#inline_always)                                 | warn    | `#[inline(always)]` is a bad idea in most cases

src/copies.rs

@@ -1,6 +1,6 @@
 use rustc::lint::*;
 use rustc_front::hir::*;
-use utils::{get_parent_expr, is_exp_equal, span_lint};
+use utils::{get_parent_expr, in_macro, is_exp_equal, is_stmt_equal, over, span_lint, span_note_and_lint};
 
 /// **What it does:** This lint checks for consecutive `ifs` with the same condition. This lint is
 /// `Warn` by default.
@@ -16,19 +16,66 @@ declare_lint! {
     "consecutive `ifs` with the same condition"
 }
 
+/// **What it does:** This lint checks for `if/else` with the same body as the *then* part and the
+/// *else* part. This lint is `Warn` by default.
+///
+/// **Why is this bad?** This is probably a copy & paste error.
+///
+/// **Known problems:** Hopefully none.
+///
+/// **Example:** `if .. { 42 } else { 42 }`
+declare_lint! {
+    pub IF_SAME_THEN_ELSE,
+    Warn,
+    "if with the same *then* and *else* blocks"
+}
+
 #[derive(Copy, Clone, Debug)]
 pub struct CopyAndPaste;
 
 impl LintPass for CopyAndPaste {
     fn get_lints(&self) -> LintArray {
         lint_array![
-            IFS_SAME_COND
+            IFS_SAME_COND,
+            IF_SAME_THEN_ELSE
         ]
     }
 }
 
 impl LateLintPass for CopyAndPaste {
     fn check_expr(&mut self, cx: &LateContext, expr: &Expr) {
+        if !in_macro(cx, expr.span) {
+            lint_same_then_else(cx, expr);
+            lint_same_cond(cx, expr);
+        }
+    }
+}
+
+/// Implementation of `IF_SAME_THEN_ELSE`.
+fn lint_same_then_else(cx: &LateContext, expr: &Expr) {
+    if let ExprIf(_, ref then_block, Some(ref else_expr)) = expr.node {
+        let must_lint = if let ExprBlock(ref else_block) = else_expr.node {
+            over(&then_block.stmts, &else_block.stmts, |l, r| is_stmt_equal(cx, l, r)) &&
+                match (&then_block.expr, &else_block.expr) {
+                    (&Some(ref then_expr), &Some(ref else_expr)) => {
+                        is_exp_equal(cx, &then_expr, &else_expr)
+                    }
+                    (&None, &None) => true,
+                    _ => false,
+                }
+        }
+        else {
+            false
+        };
+
+        if must_lint {
+            span_lint(cx, IF_SAME_THEN_ELSE, expr.span, "this if has the same then and else blocks");
+        }
+    }
+}
+
+/// Implementation of `IFS_SAME_COND`.
+fn lint_same_cond(cx: &LateContext, expr: &Expr) {
     // skip ifs directly in else, it will be checked in the parent if
     if let Some(&Expr{node: ExprIf(_, _, Some(ref else_expr)), ..}) = get_parent_expr(cx, expr) {
         if else_expr.id == expr.id {
@@ -41,8 +88,7 @@ impl LateLintPass for CopyAndPaste {
     for (n, i) in conds.iter().enumerate() {
         for j in conds.iter().skip(n+1) {
             if is_exp_equal(cx, i, j) {
-                    span_lint(cx, IFS_SAME_COND, j.span, "this if as the same condition as a previous if");
-                }
+                span_note_and_lint(cx, IFS_SAME_COND, j.span, "this if has the same condition as a previous if", i.span, "same as this");
             }
         }
     }

src/lib.rs

@@ -192,6 +192,7 @@ pub fn plugin_registrar(reg: &mut Registry) {
         block_in_if_condition::BLOCK_IN_IF_CONDITION_EXPR,
         block_in_if_condition::BLOCK_IN_IF_CONDITION_STMT,
         collapsible_if::COLLAPSIBLE_IF,
+        copies::IF_SAME_THEN_ELSE,
         copies::IFS_SAME_COND,
         cyclomatic_complexity::CYCLOMATIC_COMPLEXITY,
         derive::DERIVE_HASH_NOT_EQ,

src/utils.rs

@@ -589,6 +589,14 @@ fn parse_attrs<F: FnMut(u64)>(sess: &Session, attrs: &[ast::Attribute], name: &'
     }
 }
 
+pub fn is_stmt_equal(cx: &LateContext, left: &Stmt, right: &Stmt) -> bool {
+    match (&left.node, &right.node) {
+        (&StmtExpr(ref l, _), &StmtExpr(ref r, _)) => is_exp_equal(cx, l, r),
+        (&StmtSemi(ref l, _), &StmtSemi(ref r, _)) => is_exp_equal(cx, l, r),
+        _ => false,
+    }
+}
+
 pub fn is_exp_equal(cx: &LateContext, left: &Expr, right: &Expr) -> bool {
     if let (Some(l), Some(r)) = (constant(cx, left), constant(cx, right)) {
         if l == r {
@@ -649,7 +657,7 @@ fn is_qself_equal(left: &QSelf, right: &QSelf) -> bool {
     left.ty.node == right.ty.node && left.position == right.position
 }
 
-fn over<X, F>(left: &[X], right: &[X], mut eq_fn: F) -> bool
+pub fn over<X, F>(left: &[X], right: &[X], mut eq_fn: F) -> bool
     where F: FnMut(&X, &X) -> bool
 {
     left.len() == right.len() && left.iter().zip(right).all(|(x, y)| eq_fn(x, y))

tests/compile-fail/copies.rs

@@ -1,23 +1,61 @@
 #![feature(plugin)]
 #![plugin(clippy)]
 
+#![allow(dead_code)]
 #![deny(clippy)]
 
 fn foo() -> bool { unimplemented!() }
 
-fn main() {
+fn if_same_then_else() {
+    if true { //~ERROR this if has the same then and else blocks
+        foo();
+    }
+    else {
+        foo();
+    }
+
+    if true {
+        foo();
+        foo();
+    }
+    else {
+        foo();
+    }
+
+    let _ = if true { //~ERROR this if has the same then and else blocks
+        foo();
+        42
+    }
+    else {
+        foo();
+        42
+    };
+
+    if true {
+        foo();
+    }
+
+    let _ = if true { //~ERROR this if has the same then and else blocks
+        42
+    }
+    else {
+        42
+    };
+}
+
+fn ifs_same_cond() {
     let a = 0;
 
     if a == 1 {
     }
-    else if a == 1 { //~ERROR this if as the same condition as a previous if
+    else if a == 1 { //~ERROR this if has the same condition as a previous if
     }
 
     if 2*a == 1 {
     }
     else if 2*a == 2 {
     }
-    else if 2*a == 1 { //~ERROR this if as the same condition as a previous if
+    else if 2*a == 1 { //~ERROR this if has the same condition as a previous if
     }
     else if a == 1 {
     }
@@ -26,6 +64,8 @@ fn main() {
     // this to make the intention clearer anyway.
     if foo() {
     }
-    else if foo() { //~ERROR this if as the same condition as a previous if
+    else if foo() { //~ERROR this if has the same condition as a previous if
     }
 }
+
+fn main() {}

tests/compile-fail/needless_bool.rs

@@ -1,6 +1,7 @@
 #![feature(plugin)]
 #![plugin(clippy)]
 
+#[allow(if_same_then_else)]
 #[deny(needless_bool)]
 fn main() {
     let x = true;