Warn about explicit self-assignment

Warn about assignments where left-hand side place expression is the same
as right-hand side value expression. For example, warn about assignment in:

```rust
pub struct Event {
    id: usize,
    x: i32,
    y: i32,
}

pub fn copy_position(a: &mut Event, b: &Event) {
    a.x = b.x;
    a.y = a.y;
}
```

CHANGELOG.md

@@ -1690,6 +1690,7 @@ Released 2018-09-13
 [`same_functions_in_if_condition`]: https://rust-lang.github.io/rust-clippy/master/index.html#same_functions_in_if_condition
 [`same_item_push`]: https://rust-lang.github.io/rust-clippy/master/index.html#same_item_push
 [`search_is_some`]: https://rust-lang.github.io/rust-clippy/master/index.html#search_is_some
+[`self_assignment`]: https://rust-lang.github.io/rust-clippy/master/index.html#self_assignment
 [`serde_api_misuse`]: https://rust-lang.github.io/rust-clippy/master/index.html#serde_api_misuse
 [`shadow_reuse`]: https://rust-lang.github.io/rust-clippy/master/index.html#shadow_reuse
 [`shadow_same`]: https://rust-lang.github.io/rust-clippy/master/index.html#shadow_same

clippy_lints/src/lib.rs

@@ -284,6 +284,7 @@ mod reference;
 mod regex;
 mod repeat_once;
 mod returns;
+mod self_assignment;
 mod serde_api;
 mod shadow;
 mod single_component_path_imports;
@@ -773,6 +774,7 @@ pub fn register_plugins(store: &mut rustc_lint::LintStore, sess: &Session, conf:
         &repeat_once::REPEAT_ONCE,
         &returns::LET_AND_RETURN,
         &returns::NEEDLESS_RETURN,
+        &self_assignment::SELF_ASSIGNMENT,
         &serde_api::SERDE_API_MISUSE,
         &shadow::SHADOW_REUSE,
         &shadow::SHADOW_SAME,
@@ -1090,6 +1092,7 @@ pub fn register_plugins(store: &mut rustc_lint::LintStore, sess: &Session, conf:
     store.register_late_pass(|| box pattern_type_mismatch::PatternTypeMismatch);
     store.register_late_pass(|| box stable_sort_primitive::StableSortPrimitive);
     store.register_late_pass(|| box repeat_once::RepeatOnce);
+    store.register_late_pass(|| box self_assignment::SelfAssignment);
 
     store.register_group(true, "clippy::restriction", Some("clippy_restriction"), vec![
         LintId::of(&arithmetic::FLOAT_ARITHMETIC),
@@ -1421,6 +1424,7 @@ pub fn register_plugins(store: &mut rustc_lint::LintStore, sess: &Session, conf:
         LintId::of(&repeat_once::REPEAT_ONCE),
         LintId::of(&returns::LET_AND_RETURN),
         LintId::of(&returns::NEEDLESS_RETURN),
+        LintId::of(&self_assignment::SELF_ASSIGNMENT),
         LintId::of(&serde_api::SERDE_API_MISUSE),
         LintId::of(&single_component_path_imports::SINGLE_COMPONENT_PATH_IMPORTS),
         LintId::of(&slow_vector_initialization::SLOW_VECTOR_INITIALIZATION),
@@ -1714,6 +1718,7 @@ pub fn register_plugins(store: &mut rustc_lint::LintStore, sess: &Session, conf:
         LintId::of(&ptr::MUT_FROM_REF),
         LintId::of(&ranges::REVERSED_EMPTY_RANGES),
         LintId::of(&regex::INVALID_REGEX),
+        LintId::of(&self_assignment::SELF_ASSIGNMENT),
         LintId::of(&serde_api::SERDE_API_MISUSE),
         LintId::of(&suspicious_trait_impl::SUSPICIOUS_ARITHMETIC_IMPL),
         LintId::of(&suspicious_trait_impl::SUSPICIOUS_OP_ASSIGN_IMPL),

clippy_lints/src/self_assignment.rs

@@ -0,0 +1,51 @@
+use crate::utils::{eq_expr_value, snippet, span_lint};
+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 explicit self-assignments.
+    ///
+    /// **Why is this bad?** Self-assignments are redundant and unlikely to be
+    /// intentional.
+    ///
+    /// **Known problems:** If expression contains any deref coercions or
+    /// indexing operations they are assumed not to have any side effects.
+    ///
+    /// **Example:**
+    ///
+    /// ```rust
+    /// struct Event {
+    ///     id: usize,
+    ///     x: i32,
+    ///     y: i32,
+    /// }
+    ///
+    /// fn copy_position(a: &mut Event, b: &Event) {
+    ///     a.x = b.x;
+    ///     a.y = a.y;
+    /// }
+    /// ```
+    pub SELF_ASSIGNMENT,
+    correctness,
+    "explicit self-assignment"
+}
+
+declare_lint_pass!(SelfAssignment => [SELF_ASSIGNMENT]);
+
+impl<'tcx> LateLintPass<'tcx> for SelfAssignment {
+    fn check_expr(&mut self, cx: &LateContext<'tcx>, expr: &'tcx Expr<'_>) {
+        if let ExprKind::Assign(lhs, rhs, _) = &expr.kind {
+            if eq_expr_value(cx, lhs, rhs) {
+                let lhs = snippet(cx, lhs.span, "<lhs>");
+                let rhs = snippet(cx, rhs.span, "<rhs>");
+                span_lint(
+                    cx,
+                    SELF_ASSIGNMENT,
+                    expr.span,
+                    &format!("self-assignment of `{}` to `{}`", rhs, lhs),
+                );
+            }
+        }
+    }
+}

src/lintlist/mod.rs

@@ -1956,6 +1956,13 @@ pub static ref ALL_LINTS: Vec<Lint> = vec![
         deprecation: None,
         module: "methods",
     },
+    Lint {
+        name: "self_assignment",
+        group: "correctness",
+        desc: "explicit self-assignment",
+        deprecation: None,
+        module: "self_assignment",
+    },
     Lint {
         name: "serde_api_misuse",
         group: "correctness",

tests/ui/self_assignment.rs

@@ -0,0 +1,67 @@
+#![warn(clippy::self_assignment)]
+
+pub struct S<'a> {
+    a: i32,
+    b: [i32; 10],
+    c: Vec<Vec<i32>>,
+    e: &'a mut i32,
+    f: &'a mut i32,
+}
+
+pub fn positives(mut a: usize, b: &mut u32, mut s: S) {
+    a = a;
+    *b = *b;
+    s = s;
+    s.a = s.a;
+    s.b[10] = s.b[5 + 5];
+    s.c[0][1] = s.c[0][1];
+    s.b[a] = s.b[a];
+    *s.e = *s.e;
+    s.b[a + 10] = s.b[10 + a];
+
+    let mut t = (0, 1);
+    t.1 = t.1;
+    t.0 = (t.0);
+}
+
+pub fn negatives_not_equal(mut a: usize, b: &mut usize, mut s: S) {
+    dbg!(&a);
+    a = *b;
+    dbg!(&a);
+    s.b[1] += s.b[1];
+    s.b[1] = s.b[2];
+    s.c[1][0] = s.c[0][1];
+    s.b[a] = s.b[*b];
+    s.b[a + 10] = s.b[a + 11];
+    *s.e = *s.f;
+
+    let mut t = (0, 1);
+    t.0 = t.1;
+}
+
+#[allow(clippy::eval_order_dependence)]
+pub fn negatives_side_effects() {
+    let mut v = vec![1, 2, 3, 4, 5];
+    let mut i = 0;
+    v[{
+        i += 1;
+        i
+    }] = v[{
+        i += 1;
+        i
+    }];
+
+    fn next(n: &mut usize) -> usize {
+        let v = *n;
+        *n += 1;
+        v
+    }
+
+    let mut w = vec![1, 2, 3, 4, 5];
+    let mut i = 0;
+    let i = &mut i;
+    w[next(i)] = w[next(i)];
+    w[next(i)] = w[next(i)];
+}
+
+fn main() {}

tests/ui/self_assignment.stderr

@@ -0,0 +1,70 @@
+error: self-assignment of `a` to `a`
+  --> $DIR/self_assignment.rs:12:5
+   |
+LL |     a = a;
+   |     ^^^^^
+   |
+   = note: `-D clippy::self-assignment` implied by `-D warnings`
+
+error: self-assignment of `*b` to `*b`
+  --> $DIR/self_assignment.rs:13:5
+   |
+LL |     *b = *b;
+   |     ^^^^^^^
+
+error: self-assignment of `s` to `s`
+  --> $DIR/self_assignment.rs:14:5
+   |
+LL |     s = s;
+   |     ^^^^^
+
+error: self-assignment of `s.a` to `s.a`
+  --> $DIR/self_assignment.rs:15:5
+   |
+LL |     s.a = s.a;
+   |     ^^^^^^^^^
+
+error: self-assignment of `s.b[5 + 5]` to `s.b[10]`
+  --> $DIR/self_assignment.rs:16:5
+   |
+LL |     s.b[10] = s.b[5 + 5];
+   |     ^^^^^^^^^^^^^^^^^^^^
+
+error: self-assignment of `s.c[0][1]` to `s.c[0][1]`
+  --> $DIR/self_assignment.rs:17:5
+   |
+LL |     s.c[0][1] = s.c[0][1];
+   |     ^^^^^^^^^^^^^^^^^^^^^
+
+error: self-assignment of `s.b[a]` to `s.b[a]`
+  --> $DIR/self_assignment.rs:18:5
+   |
+LL |     s.b[a] = s.b[a];
+   |     ^^^^^^^^^^^^^^^
+
+error: self-assignment of `*s.e` to `*s.e`
+  --> $DIR/self_assignment.rs:19:5
+   |
+LL |     *s.e = *s.e;
+   |     ^^^^^^^^^^^
+
+error: self-assignment of `s.b[10 + a]` to `s.b[a + 10]`
+  --> $DIR/self_assignment.rs:20:5
+   |
+LL |     s.b[a + 10] = s.b[10 + a];
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: self-assignment of `t.1` to `t.1`
+  --> $DIR/self_assignment.rs:23:5
+   |
+LL |     t.1 = t.1;
+   |     ^^^^^^^^^
+
+error: self-assignment of `(t.0)` to `t.0`
+  --> $DIR/self_assignment.rs:24:5
+   |
+LL |     t.0 = (t.0);
+   |     ^^^^^^^^^^^
+
+error: aborting due to 11 previous errors
+