Auto merge of #10810 - samueltardieu:needless-else, r=llogiq

needless_else: new lint to check for empty `else` clauses

Empty `else` clauses are useless. They happen in the wild and are not linted yet: https://github.com/uutils/coreutils/pull/4880/files

`else` clauses containing or preceded by comments are not linted as the comments might be important.

changelog: [`needless_else`]: new lint

clippy_lints/src/declared_lints.rs

@@ -449,6 +449,7 @@ pub(crate) static LINTS: &[&crate::LintInfo] = &[
     crate::needless_bool::NEEDLESS_BOOL_ASSIGN_INFO,
     crate::needless_borrowed_ref::NEEDLESS_BORROWED_REFERENCE_INFO,
     crate::needless_continue::NEEDLESS_CONTINUE_INFO,
+    crate::needless_else::NEEDLESS_ELSE_INFO,
     crate::needless_for_each::NEEDLESS_FOR_EACH_INFO,
     crate::needless_late_init::NEEDLESS_LATE_INIT_INFO,
     crate::needless_parens_on_range_literals::NEEDLESS_PARENS_ON_RANGE_LITERALS_INFO,

clippy_lints/src/lib.rs

@@ -218,6 +218,7 @@ mod needless_arbitrary_self_type;
 mod needless_bool;
 mod needless_borrowed_ref;
 mod needless_continue;
+mod needless_else;
 mod needless_for_each;
 mod needless_late_init;
 mod needless_parens_on_range_literals;
@@ -992,6 +993,7 @@ pub fn register_plugins(store: &mut rustc_lint::LintStore, sess: &Session, conf:
     store.register_late_pass(|_| Box::new(items_after_test_module::ItemsAfterTestModule));
     store.register_early_pass(|| Box::new(ref_patterns::RefPatterns));
     store.register_late_pass(|_| Box::new(default_constructed_unit_structs::DefaultConstructedUnitStructs));
+    store.register_early_pass(|| Box::new(needless_else::NeedlessElse));
     // add lints here, do not remove this comment, it's used in `new_lint`
 }
 

clippy_lints/src/needless_else.rs

@@ -0,0 +1,57 @@
+use clippy_utils::{diagnostics::span_lint_and_sugg, source::trim_span, span_extract_comment};
+use rustc_ast::ast::{Expr, ExprKind};
+use rustc_errors::Applicability;
+use rustc_lint::{EarlyContext, EarlyLintPass, LintContext};
+use rustc_session::{declare_lint_pass, declare_tool_lint};
+
+declare_clippy_lint! {
+    /// ### What it does
+    /// Checks for empty `else` branches.
+    ///
+    /// ### Why is this bad?
+    /// An empty else branch does nothing and can be removed.
+    ///
+    /// ### Example
+    /// ```rust
+    ///# fn check() -> bool { true }
+    /// if check() {
+    ///     println!("Check successful!");
+    /// } else {
+    /// }
+    /// ```
+    /// Use instead:
+    /// ```rust
+    ///# fn check() -> bool { true }
+    /// if check() {
+    ///     println!("Check successful!");
+    /// }
+    /// ```
+    #[clippy::version = "1.71.0"]
+    pub NEEDLESS_ELSE,
+    style,
+    "empty else branch"
+}
+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,
+                    );
+                }
+            }
+    }
+}