Disable "if_not_else" lints firing on else-ifs

1. Convert IfNotElse to LateLintPass and use clippy_utils::is_else_clause for checking.
2. Handle the case where the span comes from desugaring.
3. Update tests.

clippy_lints/src/if_not_else.rs

@@ -2,9 +2,9 @@
 //! on the condition
 
 use clippy_utils::diagnostics::span_lint_and_help;
-use rustc_ast::ast::{BinOpKind, Expr, ExprKind, UnOp};
-use rustc_lint::{EarlyContext, EarlyLintPass};
-use rustc_middle::lint::in_external_macro;
+use clippy_utils::is_else_clause;
+use rustc_hir::{BinOpKind, Expr, ExprKind, UnOp};
+use rustc_lint::{LateContext, LateLintPass};
 use rustc_session::{declare_lint_pass, declare_tool_lint};
 
 declare_clippy_lint! {
@@ -46,14 +46,21 @@
 
 declare_lint_pass!(IfNotElse => [IF_NOT_ELSE]);
 
-impl EarlyLintPass for IfNotElse {
-    fn check_expr(&mut self, cx: &EarlyContext<'_>, item: &Expr) {
-        if in_external_macro(cx.sess, item.span) {
+impl LateLintPass<'_> for IfNotElse {
+    fn check_expr(&mut self, cx: &LateContext<'_>, item: &Expr<'_>) {
+        // While loops will be desugared to ExprKind::If. This will cause the lint to fire.
+        // To fix this, return early if this span comes from a macro or desugaring.
+        if item.span.from_expansion() {
             return;
         }
-        if let ExprKind::If(ref cond, _, Some(ref els)) = item.kind {
+        if let ExprKind::If(cond, _, Some(els)) = item.kind {
             if let ExprKind::Block(..) = els.kind {
-                match cond.kind {
+                // Disable firing the lint in "else if" expressions.
+                if is_else_clause(cx.tcx, item) {
+                    return;
+                }
+
+                match cond.peel_drop_temps().kind {
                     ExprKind::Unary(UnOp::Not, _) => {
                         span_lint_and_help(
                             cx,

clippy_lints/src/lib.rs

@@ -668,7 +668,6 @@ pub fn register_plugins(store: &mut rustc_lint::LintStore, sess: &Session, conf:
     store.register_early_pass(|| Box::new(double_parens::DoubleParens));
     store.register_late_pass(|| Box::new(to_string_in_display::ToStringInDisplay::new()));
     store.register_early_pass(|| Box::new(unsafe_removed_from_name::UnsafeNameRemoval));
-    store.register_early_pass(|| Box::new(if_not_else::IfNotElse));
     store.register_early_pass(|| Box::new(else_if_without_else::ElseIfWithoutElse));
     store.register_early_pass(|| Box::new(int_plus_one::IntPlusOne));
     store.register_early_pass(|| Box::new(formatting::Formatting));
@@ -722,6 +721,7 @@ pub fn register_plugins(store: &mut rustc_lint::LintStore, sess: &Session, conf:
     store.register_late_pass(|| Box::new(option_if_let_else::OptionIfLetElse));
     store.register_late_pass(|| Box::new(future_not_send::FutureNotSend));
     store.register_late_pass(|| Box::new(if_let_mutex::IfLetMutex));
+    store.register_late_pass(|| Box::new(if_not_else::IfNotElse));
     store.register_late_pass(|| Box::new(equatable_if_let::PatternEquality));
     store.register_late_pass(|| Box::new(mut_mutex_lock::MutMutexLock));
     store.register_late_pass(|| Box::new(match_on_vec_items::MatchOnVecItems));

tests/ui/if_not_else.rs

@@ -1,6 +1,9 @@
 #![warn(clippy::all)]
 #![warn(clippy::if_not_else)]
 
+fn foo() -> bool {
+    unimplemented!()
+}
 fn bla() -> bool {
     unimplemented!()
 }
@@ -16,4 +19,11 @@ fn main() {
     } else {
         println!("Bunny");
     }
+    if !foo() {
+        println!("Foo");
+    } else if !bla() {
+        println!("Bugs");
+    } else {
+        println!("Bunny");
+    }
 }

tests/ui/if_not_else.stderr

@@ -1,5 +1,5 @@
 error: unnecessary boolean `not` operation
-  --> $DIR/if_not_else.rs:9:5
+  --> $DIR/if_not_else.rs:12:5
    |
 LL | /     if !bla() {
 LL | |         println!("Bugs");
@@ -12,7 +12,7 @@ LL | |     }
    = help: remove the `!` and swap the blocks of the `if`/`else`
 
 error: unnecessary `!=` operation
-  --> $DIR/if_not_else.rs:14:5
+  --> $DIR/if_not_else.rs:17:5
    |
 LL | /     if 4 != 5 {
 LL | |         println!("Bugs");