fix: redundant_closure_call lint for closures with block (#15144)

Fixes rust-lang/rust-clippy#9583

When a closure with no parameters is called immediately after being
defined, clippy can now suggest replacing the entire expression with
just the closure's body.

----

changelog: [`redundant_closure_call`]: add fixes for closures with block

clippy_lints/src/redundant_closure_call.rs

@@ -5,9 +5,7 @@ use hir::Param;
 use rustc_errors::Applicability;
 use rustc_hir as hir;
 use rustc_hir::intravisit::{Visitor as HirVisitor, Visitor};
-use rustc_hir::{
-    ClosureKind, CoroutineDesugaring, CoroutineKind, CoroutineSource, ExprKind, Node, intravisit as hir_visit,
-};
+use rustc_hir::{ClosureKind, CoroutineDesugaring, CoroutineKind, CoroutineSource, ExprKind, intravisit as hir_visit};
 use rustc_lint::{LateContext, LateLintPass, LintContext};
 use rustc_middle::hir::nested_filter;
 use rustc_middle::ty;
@@ -198,15 +196,15 @@ impl<'tcx> LateLintPass<'tcx> for RedundantClosureCall {
                             hint = hint.asyncify();
                         }
 
-                        let is_in_fn_call_arg = if let Node::Expr(expr) = cx.tcx.parent_hir_node(expr.hir_id) {
-                            matches!(expr.kind, ExprKind::Call(_, _))
-                        } else {
-                            false
-                        };
-
-                        // avoid clippy::double_parens
-                        if !is_in_fn_call_arg {
-                            hint = hint.maybe_paren();
+                        // If the closure body is a block with a single expression, suggest just the inner expression,
+                        // not the block. Example: `(|| { Some(true) })()` should suggest
+                        // `Some(true)`
+                        if let ExprKind::Block(block, _) = body.kind
+                            && block.stmts.is_empty()
+                            && let Some(expr) = block.expr
+                        {
+                            hint = Sugg::hir_with_context(cx, expr, full_expr.span.ctxt(), "..", &mut applicability)
+                                .maybe_paren();
                         }
 
                         diag.span_suggestion(full_expr.span, "try doing something like", hint, applicability);

tests/ui/redundant_closure_call_fixable.fixed

@@ -60,7 +60,7 @@ fn issue9956() {
     //~^ redundant_closure_call
 
     // immediately calling only one closure, so we can't remove the other ones
-    let a = (|| || 123);
+    let a = || || 123;
     //~^ redundant_closure_call
     dbg!(a()());
 
@@ -144,3 +144,15 @@ fn issue_12358() {
     // different.
     make_closure!(x)();
 }
+
+#[rustfmt::skip]
+fn issue_9583() {
+    Some(true) == Some(true);
+     //~^ redundant_closure_call
+    Some(true) == Some(true);
+    //~^ redundant_closure_call
+    Some(if 1 > 2 {1} else {2}) == Some(2);
+    //~^ redundant_closure_call
+    Some( 1 > 2 ) == Some(true);
+    //~^ redundant_closure_call
+}

tests/ui/redundant_closure_call_fixable.rs

@@ -144,3 +144,15 @@ fn issue_12358() {
     // different.
     make_closure!(x)();
 }
+
+#[rustfmt::skip]
+fn issue_9583() {
+    (|| { Some(true) })() == Some(true);
+     //~^ redundant_closure_call
+    (|| Some(true))() == Some(true);
+    //~^ redundant_closure_call
+    (|| { Some(if 1 > 2 {1} else {2}) })() == Some(2);
+    //~^ redundant_closure_call
+    (|| { Some( 1 > 2 ) })() == Some(true);
+    //~^ redundant_closure_call
+}

tests/ui/redundant_closure_call_fixable.stderr

@@ -95,7 +95,7 @@ error: try not to call a closure in the expression where it is declared
   --> tests/ui/redundant_closure_call_fixable.rs:63:13
    |
 LL |     let a = (|| || || 123)();
-   |             ^^^^^^^^^^^^^^^^ help: try doing something like: `(|| || 123)`
+   |             ^^^^^^^^^^^^^^^^ help: try doing something like: `|| || 123`
 
 error: try not to call a closure in the expression where it is declared
   --> tests/ui/redundant_closure_call_fixable.rs:68:13
@@ -145,5 +145,29 @@ error: try not to call a closure in the expression where it is declared
 LL |     std::convert::identity((|| 13_i32 + 36_i32)()).leading_zeros();
    |                            ^^^^^^^^^^^^^^^^^^^^^^ help: try doing something like: `13_i32 + 36_i32`
 
-error: aborting due to 17 previous errors
+error: try not to call a closure in the expression where it is declared
+  --> tests/ui/redundant_closure_call_fixable.rs:150:5
+   |
+LL |     (|| { Some(true) })() == Some(true);
+   |     ^^^^^^^^^^^^^^^^^^^^^ help: try doing something like: `Some(true)`
+
+error: try not to call a closure in the expression where it is declared
+  --> tests/ui/redundant_closure_call_fixable.rs:152:5
+   |
+LL |     (|| Some(true))() == Some(true);
+   |     ^^^^^^^^^^^^^^^^^ help: try doing something like: `Some(true)`
+
+error: try not to call a closure in the expression where it is declared
+  --> tests/ui/redundant_closure_call_fixable.rs:154:5
+   |
+LL |     (|| { Some(if 1 > 2 {1} else {2}) })() == Some(2);
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: try doing something like: `Some(if 1 > 2 {1} else {2})`
+
+error: try not to call a closure in the expression where it is declared
+  --> tests/ui/redundant_closure_call_fixable.rs:156:5
+   |
+LL |     (|| { Some( 1 > 2 ) })() == Some(true);
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^ help: try doing something like: `Some( 1 > 2 )`
+
+error: aborting due to 21 previous errors