Auto merge of #10822 - Alexendoo:needless-else-cfg, r=llogiq

Ignore `#[cfg]`'d out code in `needless_else`

changelog: none (same release as #10810)

`#[cfg]` making things fun once more

This lead me to think about macro calls that expand to nothing as well, but apparently they produce an empty stmt in the AST so are already handled, added a test for that

r? `@llogiq`

clippy_lints/src/needless_else.rs

@@ -1,4 +1,5 @@
-use clippy_utils::{diagnostics::span_lint_and_sugg, source::trim_span, span_extract_comment};
+use clippy_utils::source::snippet_opt;
+use clippy_utils::{diagnostics::span_lint_and_sugg, source::trim_span};
 use rustc_ast::ast::{Expr, ExprKind};
 use rustc_errors::Applicability;
 use rustc_lint::{EarlyContext, EarlyLintPass, LintContext};
@@ -35,23 +36,26 @@ declare_lint_pass!(NeedlessElse => [NEEDLESS_ELSE]);
 
 impl EarlyLintPass for NeedlessElse {
     fn check_expr(&mut self, cx: &EarlyContext<'_>, expr: &Expr) {
-        if let ExprKind::If(_, then_block, Some(else_clause)) = &expr.kind &&
-            let ExprKind::Block(block, _) = &else_clause.kind &&
-            !expr.span.from_expansion() &&
-            !else_clause.span.from_expansion() &&
-            block.stmts.is_empty() {
-                let span = trim_span(cx.sess().source_map(), expr.span.trim_start(then_block.span).unwrap());
-                if span_extract_comment(cx.sess().source_map(), span).is_empty() {
-                    span_lint_and_sugg(
-                        cx,
-                        NEEDLESS_ELSE,
-                        span,
-                        "this else branch is empty",
-                        "you can remove it",
-                        String::new(),
-                        Applicability::MachineApplicable,
-                    );
-                }
-            }
+        if let ExprKind::If(_, then_block, Some(else_clause)) = &expr.kind
+            && let ExprKind::Block(block, _) = &else_clause.kind
+            && !expr.span.from_expansion()
+            && !else_clause.span.from_expansion()
+            && block.stmts.is_empty()
+            && let Some(trimmed) = expr.span.trim_start(then_block.span)
+            && let span = trim_span(cx.sess().source_map(), trimmed)
+            && let Some(else_snippet) = snippet_opt(cx, span)
+            // Ignore else blocks that contain comments or #[cfg]s
+            && !else_snippet.contains(['/', '#'])
+        {
+            span_lint_and_sugg(
+                cx,
+                NEEDLESS_ELSE,
+                span,
+                "this else branch is empty",
+                "you can remove it",
+                String::new(),
+                Applicability::MachineApplicable,
+            );
+        }
     }
 }

tests/ui/needless_else.fixed

@@ -12,6 +12,10 @@ macro_rules! mac {
     };
 }
 
+macro_rules! empty_expansion {
+    () => {};
+}
+
 fn main() {
     let b = std::hint::black_box(true);
 
@@ -37,4 +41,17 @@ fn main() {
 
     // Do not lint because inside a macro
     mac!(b);
+
+    if b {
+        println!("Foobar");
+    } else {
+        #[cfg(foo)]
+        "Do not lint cfg'd out code"
+    }
+
+    if b {
+        println!("Foobar");
+    } else {
+        empty_expansion!();
+    }
 }

tests/ui/needless_else.rs

@@ -12,6 +12,10 @@ macro_rules! mac {
     };
 }
 
+macro_rules! empty_expansion {
+    () => {};
+}
+
 fn main() {
     let b = std::hint::black_box(true);
 
@@ -38,4 +42,17 @@ fn main() {
 
     // Do not lint because inside a macro
     mac!(b);
+
+    if b {
+        println!("Foobar");
+    } else {
+        #[cfg(foo)]
+        "Do not lint cfg'd out code"
+    }
+
+    if b {
+        println!("Foobar");
+    } else {
+        empty_expansion!();
+    }
 }

tests/ui/needless_else.stderr

@@ -1,5 +1,5 @@
 error: this else branch is empty
-  --> $DIR/needless_else.rs:20:7
+  --> $DIR/needless_else.rs:24:7
    |
 LL |       } else {
    |  _______^