Auto merge of #3590 - jorpic:i3559-if_same_then_else, r=phansch

Fix if_same_then_else false positive

This fixes false positive in #3559.
The problem was that `SpanlessEq` does not check patterns in declarations. So this two blocks considered equal.
```rust
if true {
    let (x, y) = foo();
} else {
   let (y, x) = foo();
}
```
Not sure if the proposed change is safe as `SpanlessEq` is used extensively in other lints, but I tried hard to come up with counterexample and failed.

clippy_lints/src/utils/hir_utils.rs

@@ -54,7 +54,9 @@ impl<'a, 'tcx: 'a> SpanlessEq<'a, 'tcx> {
         match (&left.node, &right.node) {
             (&StmtKind::Decl(ref l, _), &StmtKind::Decl(ref r, _)) => {
                 if let (&DeclKind::Local(ref l), &DeclKind::Local(ref r)) = (&l.node, &r.node) {
-                    both(&l.ty, &r.ty, |l, r| self.eq_ty(l, r)) && both(&l.init, &r.init, |l, r| self.eq_expr(l, r))
+                    self.eq_pat(&l.pat, &r.pat)
+                        && both(&l.ty, &r.ty, |l, r| self.eq_ty(l, r))
+                        && both(&l.init, &r.init, |l, r| self.eq_expr(l, r))
                 } else {
                     false
                 }

tests/ui/copies.rs

@@ -335,6 +335,17 @@ fn if_same_then_else() -> Result<&'static str, ()> {
         let foo = "";
         return Ok(&foo[0..]);
     }
+
+    // false positive if_same_then_else, let(x,y) vs let(y,x), see #3559
+    if true {
+        let foo = "";
+        let (x, y) = (1, 2);
+        return Ok(&foo[x..y]);
+    } else {
+        let foo = "";
+        let (y, x) = (1, 2);
+        return Ok(&foo[x..y]);
+    }
 }
 
 #[warn(clippy::ifs_same_cond)]

tests/ui/copies.stderr

@@ -352,38 +352,38 @@ LL | |     } else {
    | |_____^
 
 error: this `if` has the same condition as a previous if
-  --> $DIR/copies.rs:347:15
+  --> $DIR/copies.rs:358:15
    |
 LL |     } else if b {
    |               ^
    |
    = note: `-D clippy::ifs-same-cond` implied by `-D warnings`
 note: same as this
-  --> $DIR/copies.rs:346:8
+  --> $DIR/copies.rs:357:8
    |
 LL |     if b {
    |        ^
 
 error: this `if` has the same condition as a previous if
-  --> $DIR/copies.rs:352:15
+  --> $DIR/copies.rs:363:15
    |
 LL |     } else if a == 1 {
    |               ^^^^^^
    |
 note: same as this
-  --> $DIR/copies.rs:351:8
+  --> $DIR/copies.rs:362:8
    |
 LL |     if a == 1 {
    |        ^^^^^^
 
 error: this `if` has the same condition as a previous if
-  --> $DIR/copies.rs:358:15
+  --> $DIR/copies.rs:369:15
    |
 LL |     } else if 2 * a == 1 {
    |               ^^^^^^^^^^
    |
 note: same as this
-  --> $DIR/copies.rs:356:8
+  --> $DIR/copies.rs:367:8
    |
 LL |     if 2 * a == 1 {
    |        ^^^^^^^^^^