Add block_scrutinee lint

Author: shivendra02467

CHANGELOG.md

@@ -6449,6 +6449,7 @@ Released 2018-09-13
 [`blanket_clippy_restriction_lints`]: https://rust-lang.github.io/rust-clippy/master/index.html#blanket_clippy_restriction_lints
 [`block_in_if_condition_expr`]: https://rust-lang.github.io/rust-clippy/master/index.html#block_in_if_condition_expr
 [`block_in_if_condition_stmt`]: https://rust-lang.github.io/rust-clippy/master/index.html#block_in_if_condition_stmt
+[`block_scrutinee`]: https://rust-lang.github.io/rust-clippy/master/index.html#block_scrutinee
 [`blocks_in_conditions`]: https://rust-lang.github.io/rust-clippy/master/index.html#blocks_in_conditions
 [`blocks_in_if_conditions`]: https://rust-lang.github.io/rust-clippy/master/index.html#blocks_in_if_conditions
 [`bool_assert_comparison`]: https://rust-lang.github.io/rust-clippy/master/index.html#bool_assert_comparison

clippy_lints/src/block_scrutinee.rs

@@ -0,0 +1,58 @@
+use clippy_utils::diagnostics::span_lint_and_sugg;
+use clippy_utils::source::snippet;
+use rustc_errors::Applicability;
+use rustc_hir::{Expr, ExprKind};
+use rustc_lint::{LateContext, LateLintPass};
+use rustc_session::declare_lint_pass;
+use rustc_span::edition::Edition;
+
+declare_clippy_lint! {
+    /// ### What it does
+    /// Warns when a match, if let, or while let scrutinee is wrapped in a block.
+    ///
+    /// ### Why is this bad?
+    /// Prior to the 2024 edition, wrapping the scrutinee in a block did not drop
+    /// temporaries before the body executes.
+    ///
+    /// ### Example
+    /// ```rust,ignore
+    /// if let Some(x) = { my_function() } { .. }
+    /// ```
+    #[clippy::version = "1.80.0"]
+    pub BLOCK_SCRUTINEE,
+    correctness,
+    "warns when the scrutinee is wrapped in a block in older editions"
+}
+
+declare_lint_pass!(BlockScrutinee => [BLOCK_SCRUTINEE]);
+
+impl<'tcx> LateLintPass<'tcx> for BlockScrutinee {
+    fn check_expr(&mut self, cx: &LateContext<'tcx>, expr: &'tcx Expr<'tcx>) {
+        if cx.tcx.sess.edition() >= Edition::Edition2024 {
+            return;
+        }
+
+        let scrutinee = match expr.kind {
+            ExprKind::Match(scrutinee, _, _) => scrutinee,
+            ExprKind::Let(let_expr) => let_expr.init,
+            _ => return,
+        };
+
+        if let ExprKind::Block(block, _) = scrutinee.kind
+            && block.stmts.is_empty()
+            && let Some(inner_expr) = block.expr
+        {
+            let inner_snippet = snippet(cx, inner_expr.span, "..");
+
+            span_lint_and_sugg(
+                cx,
+                BLOCK_SCRUTINEE,
+                scrutinee.span,
+                "scrutinee is wrapped in a block which will not drop temporaries until the end of the statement in this edition",
+                "remove the block",
+                inner_snippet.to_string(),
+                Applicability::MachineApplicable,
+            );
+        }
+    }
+}

clippy_lints/src/declared_lints.rs

@@ -33,6 +33,7 @@ pub static LINTS: &[&::declare_clippy_lint::LintInfo] = &[
     crate::await_holding_invalid::AWAIT_HOLDING_INVALID_TYPE_INFO,
     crate::await_holding_invalid::AWAIT_HOLDING_LOCK_INFO,
     crate::await_holding_invalid::AWAIT_HOLDING_REFCELL_REF_INFO,
+    crate::block_scrutinee::BLOCK_SCRUTINEE_INFO,
     crate::blocks_in_conditions::BLOCKS_IN_CONDITIONS_INFO,
     crate::bool_assert_comparison::BOOL_ASSERT_COMPARISON_INFO,
     crate::bool_comparison::BOOL_COMPARISON_INFO,

clippy_lints/src/lib.rs

@@ -72,6 +72,7 @@ mod assigning_clones;
 mod async_yields_async;
 mod attrs;
 mod await_holding_invalid;
+mod block_scrutinee;
 mod blocks_in_conditions;
 mod bool_assert_comparison;
 mod bool_comparison;
@@ -867,6 +868,7 @@ pub fn register_lint_passes(store: &mut rustc_lint::LintStore, conf: &'static Co
         Box::new(|_| Box::new(manual_checked_ops::ManualCheckedOps)),
         Box::new(move |tcx| Box::new(manual_pop_if::ManualPopIf::new(tcx, conf))),
         Box::new(move |_| Box::new(manual_noop_waker::ManualNoopWaker::new(conf))),
+        Box::new(|_| Box::new(block_scrutinee::BlockScrutinee)),
         // add late passes here, used by `cargo dev new_lint`
     ];
     store.late_passes.extend(late_lints);

tests/ui/block_scrutinee.fixed

@@ -0,0 +1,38 @@
+//@ edition: 2021
+#![warn(clippy::block_scrutinee)]
+#![allow(clippy::blocks_in_conditions)]
+
+fn my_function() -> Option<i32> {
+    Some(1)
+}
+
+fn main() {
+    if let Some(x) = my_function() {
+        //~^ ERROR: scrutinee is wrapped in a block
+        let _ = x;
+    }
+
+    match my_function() {
+        //~^ ERROR: scrutinee is wrapped in a block
+        Some(1) => println!("one"),
+        Some(_) => println!("other"),
+        None => println!("none"),
+    }
+
+    let mut v = vec![1, 2, 3];
+    while let Some(x) = v.pop() {
+        //~^ ERROR: scrutinee is wrapped in a block
+        let _ = x;
+    }
+
+    if let Some(x) = my_function() {
+        let _ = x;
+    }
+
+    if let Some(x) = {
+        let _y = 2;
+        my_function()
+    } {
+        let _ = x;
+    }
+}

tests/ui/block_scrutinee.rs

@@ -0,0 +1,38 @@
+//@ edition: 2021
+#![warn(clippy::block_scrutinee)]
+#![allow(clippy::blocks_in_conditions)]
+
+fn my_function() -> Option<i32> {
+    Some(1)
+}
+
+fn main() {
+    if let Some(x) = { my_function() } {
+        //~^ ERROR: scrutinee is wrapped in a block
+        let _ = x;
+    }
+
+    match { my_function() } {
+        //~^ ERROR: scrutinee is wrapped in a block
+        Some(1) => println!("one"),
+        Some(_) => println!("other"),
+        None => println!("none"),
+    }
+
+    let mut v = vec![1, 2, 3];
+    while let Some(x) = { v.pop() } {
+        //~^ ERROR: scrutinee is wrapped in a block
+        let _ = x;
+    }
+
+    if let Some(x) = my_function() {
+        let _ = x;
+    }
+
+    if let Some(x) = {
+        let _y = 2;
+        my_function()
+    } {
+        let _ = x;
+    }
+}

tests/ui/block_scrutinee.stderr

@@ -0,0 +1,23 @@
+error: scrutinee is wrapped in a block which will not drop temporaries until the end of the statement in this edition
+  --> tests/ui/block_scrutinee.rs:10:22
+   |
+LL |     if let Some(x) = { my_function() } {
+   |                      ^^^^^^^^^^^^^^^^^ help: remove the block: `my_function()`
+   |
+   = note: `-D clippy::block-scrutinee` implied by `-D warnings`
+   = help: to override `-D warnings` add `#[allow(clippy::block_scrutinee)]`
+
+error: scrutinee is wrapped in a block which will not drop temporaries until the end of the statement in this edition
+  --> tests/ui/block_scrutinee.rs:15:11
+   |
+LL |     match { my_function() } {
+   |           ^^^^^^^^^^^^^^^^^ help: remove the block: `my_function()`
+
+error: scrutinee is wrapped in a block which will not drop temporaries until the end of the statement in this edition
+  --> tests/ui/block_scrutinee.rs:23:25
+   |
+LL |     while let Some(x) = { v.pop() } {
+   |                         ^^^^^^^^^^^ help: remove the block: `v.pop()`
+
+error: aborting due to 3 previous errors
+