suggest a op= b over a = a op b

CHANGELOG.md

@@ -76,6 +76,8 @@ All notable changes to this project will be documented in this file.
 [`absurd_extreme_comparisons`]: https://github.com/Manishearth/rust-clippy/wiki#absurd_extreme_comparisons
 [`almost_swapped`]: https://github.com/Manishearth/rust-clippy/wiki#almost_swapped
 [`approx_constant`]: https://github.com/Manishearth/rust-clippy/wiki#approx_constant
+[`assign_op_pattern`]: https://github.com/Manishearth/rust-clippy/wiki#assign_op_pattern
+[`assign_ops`]: https://github.com/Manishearth/rust-clippy/wiki#assign_ops
 [`bad_bit_mask`]: https://github.com/Manishearth/rust-clippy/wiki#bad_bit_mask
 [`blacklisted_name`]: https://github.com/Manishearth/rust-clippy/wiki#blacklisted_name
 [`block_in_if_condition_expr`]: https://github.com/Manishearth/rust-clippy/wiki#block_in_if_condition_expr

README.md

@@ -17,13 +17,15 @@ Table of contents:
 
 ## Lints
 
-There are 147 lints included in this crate:
+There are 149 lints included in this crate:
 
 name                                                                                                                 | default | meaning
 ---------------------------------------------------------------------------------------------------------------------|---------|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
 [absurd_extreme_comparisons](https://github.com/Manishearth/rust-clippy/wiki#absurd_extreme_comparisons)             | warn    | a comparison involving a maximum or minimum value involves a case that is always true or always false
 [almost_swapped](https://github.com/Manishearth/rust-clippy/wiki#almost_swapped)                                     | warn    | `foo = bar; bar = foo` sequence
 [approx_constant](https://github.com/Manishearth/rust-clippy/wiki#approx_constant)                                   | warn    | the approximate of a known float constant (in `std::f64::consts` or `std::f32::consts`) is found; suggests to use the constant
+[assign_op_pattern](https://github.com/Manishearth/rust-clippy/wiki#assign_op_pattern)                               | warn    | assigning the result of an operation on a variable to that same variable
+[assign_ops](https://github.com/Manishearth/rust-clippy/wiki#assign_ops)                                             | allow   | Any assignment operation
 [bad_bit_mask](https://github.com/Manishearth/rust-clippy/wiki#bad_bit_mask)                                         | warn    | expressions of the form `_ & mask == select` that will only ever return `true` or `false` (because in the example `select` containing bits that `mask` doesn't have)
 [blacklisted_name](https://github.com/Manishearth/rust-clippy/wiki#blacklisted_name)                                 | warn    | usage of a blacklisted/placeholder name
 [block_in_if_condition_expr](https://github.com/Manishearth/rust-clippy/wiki#block_in_if_condition_expr)             | warn    | braces can be eliminated in conditions that are expressions, e.g `if { true } ...`

src/assign_ops.rs

@@ -0,0 +1,158 @@
+use rustc::hir;
+use rustc::lint::*;
+use utils::{span_lint_and_then, span_lint, snippet_opt, SpanlessEq, get_trait_def_id, implements_trait};
+
+/// **What it does:** This lint checks for `+=` operations and similar
+///
+/// **Why is this bad?** Projects with many developers from languages without those operations
+///                      may find them unreadable and not worth their weight
+///
+/// **Known problems:** Types implementing `OpAssign` don't necessarily implement `Op`
+///
+/// **Example:**
+/// ```
+/// a += 1;
+/// ```
+declare_restriction_lint! {
+    pub ASSIGN_OPS,
+    "Any assignment operation"
+}
+
+/// **What it does:** Check for `a = a op b` or `a = b commutative_op a` patterns
+///
+/// **Why is this bad?** These can be written as the shorter `a op= b`
+///
+/// **Known problems:** While forbidden by the spec, `OpAssign` traits may have implementations that differ from the regular `Op` impl
+///
+/// **Example:**
+///
+/// ```
+/// let mut a = 5;
+/// ...
+/// a = a + b;
+/// ```
+declare_lint! {
+    pub ASSIGN_OP_PATTERN,
+    Warn,
+    "assigning the result of an operation on a variable to that same variable"
+}
+
+#[derive(Copy, Clone, Default)]
+pub struct AssignOps;
+
+impl LintPass for AssignOps {
+    fn get_lints(&self) -> LintArray {
+        lint_array!(ASSIGN_OPS, ASSIGN_OP_PATTERN)
+    }
+}
+
+impl LateLintPass for AssignOps {
+    fn check_expr(&mut self, cx: &LateContext, expr: &hir::Expr) {
+        match expr.node {
+            hir::ExprAssignOp(op, ref lhs, ref rhs) => {
+                if let (Some(l), Some(r)) = (snippet_opt(cx, lhs.span), snippet_opt(cx, rhs.span)) {
+                    span_lint_and_then(cx,
+                                       ASSIGN_OPS,
+                                       expr.span,
+                                       "assign operation detected",
+                                       |db| {
+                                           match rhs.node {
+                                               hir::ExprBinary(op2, _, _) if op2 != op => {
+                                                   db.span_suggestion(expr.span,
+                                                                       "replace it with",
+                                                                       format!("{} = {} {} ({})", l, l, op.node.as_str(), r));
+                                               },
+                                               _ => {
+                                                   db.span_suggestion(expr.span,
+                                                                       "replace it with",
+                                                                       format!("{} = {} {} {}", l, l, op.node.as_str(), r));
+                                               }
+                                           }
+                                       });
+                } else {
+                    span_lint(cx,
+                              ASSIGN_OPS,
+                              expr.span,
+                              "assign operation detected");
+                }
+            },
+            hir::ExprAssign(ref assignee, ref e) => {
+                if let hir::ExprBinary(op, ref l, ref r) = e.node {
+                    let lint = |assignee: &hir::Expr, rhs: &hir::Expr| {
+                        let ty = cx.tcx.expr_ty(assignee);
+                        if ty.walk_shallow().next().is_some() {
+                            return; // implements_trait does not work with generics
+                        }
+                        let rty = cx.tcx.expr_ty(rhs);
+                        if rty.walk_shallow().next().is_some() {
+                            return; // implements_trait does not work with generics
+                        }
+                        macro_rules! ops {
+                            ($op:expr, $cx:expr, $ty:expr, $rty:expr, $($trait_name:ident:$full_trait_name:ident),+) => {
+                                match $op {
+                                    $(hir::$full_trait_name => {
+                                        let [krate, module] = ::utils::paths::OPS_MODULE;
+                                        let path = [krate, module, concat!(stringify!($trait_name), "Assign")];
+                                        let trait_id = if let Some(trait_id) = get_trait_def_id($cx, &path) {
+                                            trait_id
+                                        } else {
+                                            return; // useless if the trait doesn't exist
+                                        };
+                                        implements_trait($cx, $ty, trait_id, vec![$rty])
+                                    },)*
+                                    _ => false,
+                                }
+                            }
+                        }
+                        if ops!(op.node, cx, ty, rty, Add:BiAdd,
+                                                      Sub:BiSub,
+                                                      Mul:BiMul,
+                                                      Div:BiDiv,
+                                                      Rem:BiRem,
+                                                      And:BiAnd,
+                                                      Or:BiOr,
+                                                      BitAnd:BiBitAnd,
+                                                      BitOr:BiBitOr,
+                                                      BitXor:BiBitXor,
+                                                      Shr:BiShr,
+                                                      Shl:BiShl
+                        ) {
+                            if let (Some(snip_a), Some(snip_r)) = (snippet_opt(cx, assignee.span), snippet_opt(cx, rhs.span)) {
+                                span_lint_and_then(cx,
+                                                   ASSIGN_OP_PATTERN,
+                                                   expr.span,
+                                                   "manual implementation of an assign operation",
+                                                   |db| {
+                                                       db.span_suggestion(expr.span,
+                                                                           "replace it with",
+                                                                           format!("{} {}= {}", snip_a, op.node.as_str(), snip_r));
+                                                   });
+                            } else {
+                                span_lint(cx,
+                                          ASSIGN_OP_PATTERN,
+                                          expr.span,
+                                          "manual implementation of an assign operation");
+                            }
+                        }
+                    };
+                    // a = a op b
+                    if SpanlessEq::new(cx).ignore_fn().eq_expr(assignee, l) {
+                        lint(assignee, r);
+                    }
+                    // a = b commutative_op a
+                    if SpanlessEq::new(cx).ignore_fn().eq_expr(assignee, r) {
+                        match op.node {
+                            hir::BiAdd | hir::BiMul |
+                            hir::BiAnd | hir::BiOr |
+                            hir::BiBitXor | hir::BiBitAnd | hir::BiBitOr => {
+                                lint(assignee, l);
+                            },
+                            _ => {},
+                        }
+                    }
+                }
+            },
+            _ => {},
+        }
+    }
+}

src/lib.rs

@@ -7,7 +7,6 @@
 #![feature(question_mark)]
 #![feature(stmt_expr_attributes)]
 #![allow(indexing_slicing, shadow_reuse, unknown_lints)]
-#![allow(float_arithmetic, integer_arithmetic)]
 
 extern crate rustc_driver;
 extern crate getopts;
@@ -192,6 +191,7 @@ pub mod utils;
 pub mod approx_const;
 pub mod arithmetic;
 pub mod array_indexing;
+pub mod assign_ops;
 pub mod attrs;
 pub mod bit_mask;
 pub mod blacklisted_name;
@@ -383,10 +383,12 @@ pub fn plugin_registrar(reg: &mut Registry) {
     reg.register_late_lint_pass(box unsafe_removed_from_name::UnsafeNameRemoval);
     reg.register_late_lint_pass(box mem_forget::MemForget);
     reg.register_late_lint_pass(box arithmetic::Arithmetic::default());
+    reg.register_late_lint_pass(box assign_ops::AssignOps);
 
     reg.register_lint_group("clippy_restrictions", vec![
         arithmetic::FLOAT_ARITHMETIC,
         arithmetic::INTEGER_ARITHMETIC,
+        assign_ops::ASSIGN_OPS,
     ]);
 
     reg.register_lint_group("clippy_pedantic", vec![
@@ -421,6 +423,7 @@ pub fn plugin_registrar(reg: &mut Registry) {
     reg.register_lint_group("clippy", vec![
         approx_const::APPROX_CONSTANT,
         array_indexing::OUT_OF_BOUNDS_INDEXING,
+        assign_ops::ASSIGN_OP_PATTERN,
         attrs::DEPRECATED_SEMVER,
         attrs::INLINE_ALWAYS,
         bit_mask::BAD_BIT_MASK,

src/utils/paths.rs

@@ -29,6 +29,7 @@ pub const LINKED_LIST: [&'static str; 3] = ["collections", "linked_list", "Linke
 pub const MEM_FORGET: [&'static str; 3] = ["core", "mem", "forget"];
 pub const MUTEX: [&'static str; 4] = ["std", "sync", "mutex", "Mutex"];
 pub const OPEN_OPTIONS: [&'static str; 3] = ["std", "fs", "OpenOptions"];
+pub const OPS_MODULE: [&'static str; 2] = ["core", "ops"];
 pub const OPTION: [&'static str; 3] = ["core", "option", "Option"];
 pub const RANGE: [&'static str; 3] = ["core", "ops", "Range"];
 pub const RANGE_FROM: [&'static str; 3] = ["core", "ops", "RangeFrom"];

tests/compile-fail/assign_ops.rs

@@ -0,0 +1,65 @@
+#![feature(plugin)]
+#![plugin(clippy)]
+
+#[deny(assign_ops)]
+#[allow(unused_assignments)]
+fn main() {
+    let mut i = 1i32;
+    i += 2; //~ ERROR assign operation detected
+    //~^ HELP replace it with
+    //~| SUGGESTION i = i + 2
+    i -= 6; //~ ERROR assign operation detected
+    //~^ HELP replace it with
+    //~| SUGGESTION i = i - 6
+    i *= 5; //~ ERROR assign operation detected
+    //~^ HELP replace it with
+    //~| SUGGESTION i = i * 5
+    i /= 32; //~ ERROR assign operation detected
+    //~^ HELP replace it with
+    //~| SUGGESTION i = i / 32
+    i %= 42; //~ ERROR assign operation detected
+    //~^ HELP replace it with
+    //~| SUGGESTION i = i % 42
+    i >>= i; //~ ERROR assign operation detected
+    //~^ HELP replace it with
+    //~| SUGGESTION i = i >> i
+    i <<= 9 + 6 - 7; //~ ERROR assign operation detected
+    //~^ HELP replace it with
+    //~| SUGGESTION i = i << (9 + 6 - 7)
+}
+
+#[allow(dead_code, unused_assignments)]
+#[deny(assign_op_pattern)]
+fn bla() {
+    let mut a = 5;
+    a = a + 1; //~ ERROR manual implementation of an assign operation
+    //~^ HELP replace it with
+    //~| SUGGESTION a += 1
+    a = 1 + a; //~ ERROR manual implementation of an assign operation
+    //~^ HELP replace it with
+    //~| SUGGESTION a += 1
+    a = a - 1; //~ ERROR manual implementation of an assign operation
+    //~^ HELP replace it with
+    //~| SUGGESTION a -= 1
+    a = a * 99; //~ ERROR manual implementation of an assign operation
+    //~^ HELP replace it with
+    //~| SUGGESTION a *= 99
+    a = 42 * a; //~ ERROR manual implementation of an assign operation
+    //~^ HELP replace it with
+    //~| SUGGESTION a *= 42
+    a = a / 2; //~ ERROR manual implementation of an assign operation
+    //~^ HELP replace it with
+    //~| SUGGESTION a /= 2
+    a = a % 5; //~ ERROR manual implementation of an assign operation
+    //~^ HELP replace it with
+    //~| SUGGESTION a %= 5
+    a = a & 1; //~ ERROR manual implementation of an assign operation
+    //~^ HELP replace it with
+    //~| SUGGESTION a &= 1
+    a = 1 - a;
+    a = 5 / a;
+    a = 42 % a;
+    a = 6 << a;
+    let mut s = String::new();
+    s = s + "bla";
+}

tests/compile-fail/strings.rs

@@ -65,6 +65,6 @@ fn main() {
 
     // the add is only caught for String
     let mut x = 1;
-    x = x + 1;
+    x = x + 1; //~ WARN assign_op_pattern
     assert_eq!(2, x);
 }