Changes for #4394 - Support preserving block wrapping of closure bodies (#4519)

* Changes for #4394 - Support preserving block wrapping of closure bodies

* Add preserve_closure_block_wrapping config option support

* Code purifying based on the discussion in PR #4519

* Further purifying code

* Cosmetic comment change

Author: davidBar-On

Diff for: Configurations.md

@@ -1836,6 +1836,33 @@ fn say_hi() {
 }
 ```
 
+## `preserve_closure_block_wrapping`
+
+Preserves block wraping arround closures.  For example, useful when the closure `||` can be
+confused with OR.
+
+- **Default value**: `false`
+- **Possible values**: `true`, `false`
+- **Stable**: No
+
+#### `true`:
+Original block wrapping is preserved:
+```rust
+fn main() {
+    let explicit_conversion_preserves_semantics =
+        || { !is_mod || (is_mod && attrs.map_or(true, |a| a.is_empty())) };
+}
+```
+
+#### `false` (default):
+Block is not preserved:
+```rust
+fn main() {
+    let explicit_conversion_preserves_semantics =
+        || !is_mod || (is_mod && attrs.map_or(true, |a| a.is_empty()));
+}
+```
+
 ## `overflow_delimited_expr`
 
 When structs, slices, arrays, and block/array-like macros are used as the last

Diff for: src/config.rs

@@ -136,6 +136,7 @@ create_config! {
     format_generated_files: bool, false, false, "Format generated files";
     preserve_block_start_blank_lines: bool, false, false, "Preserve blank lines at the start of \
         blocks.";
+    preserve_closure_block_wrapping: bool, false , false, "Preserve block wrapping around closures";
 
     // Options that can change the source code beyond whitespace/blocks (somewhat linty things)
     merge_derives: bool, true, true, "Merge multiple `#[derive(...)]` into a single one";
@@ -626,6 +627,7 @@ edition = "2018"
 inline_attribute_width = 0
 format_generated_files = false
 preserve_block_start_blank_lines = false
+preserve_closure_block_wrapping = false
 merge_derives = true
 use_try_shorthand = false
 use_field_init_shorthand = false

Diff for: src/formatting/closures.rs

@@ -55,8 +55,19 @@ pub(crate) fn rewrite_closure(
                 .map(|s| format!("{} {}", prefix, s));
         }
 
+        // Whether a closure block wrapping may not be preserved (#4394).
+        let can_try_rewrite_without_block = if context.inside_macro() {
+            false
+        } else if context.config.preserve_closure_block_wrapping()
+            && context.snippet(body.span).trim_start().starts_with('{')
+        {
+            false
+        } else {
+            true
+        };
+
         let result = match fn_decl.output {
-            ast::FnRetTy::Default(_) if !context.inside_macro() => {
+            ast::FnRetTy::Default(_) if can_try_rewrite_without_block => {
                 try_rewrite_without_block(body, &prefix, capture, context, shape, body_shape)
             }
             _ => None,

Diff for: tests/source/preserve_closure_block_wrapping.rs

@@ -0,0 +1,32 @@
+// rustfmt-preserve_closure_block_wrapping: true
+
+fn main() {
+let explicit_conversion_preserves_semantics =
+    || { !is_mod || (is_mod && attrs.map_or(true, |a| a.is_empty())) };
+}
+
+fn main() {
+|| {{}};
+}
+
+fn issue1524() {
+let f = |x| {{{{x}}}};
+let f = |x| {{{x}}};
+let f = |x| {{x}};
+let f = |x| {x};
+let f = |x| x;
+}
+
+fn main() {
+let arg_test2 = |big_argument_name, test123| {looooooooooooooooooong_function_naaaaaaaaaaaaaaaaame()};
+}
+
+impl Foo {
+pub fn bar(&self) {
+    Some(SomeType {
+        push_closure_out_to_100_chars: iter(otherwise_it_works_ok.into_iter().map(|f| {
+            Ok(f)
+        })),
+    })
+}
+}

Diff for: tests/target/preserve_closure_block_wrapping.rs

@@ -0,0 +1,45 @@
+// rustfmt-preserve_closure_block_wrapping: true
+
+fn main() {
+    let explicit_conversion_preserves_semantics =
+        || { !is_mod || (is_mod && attrs.map_or(true, |a| a.is_empty())) };
+}
+
+fn main() {
+    || { {} };
+}
+
+fn issue1524() {
+    let f = |x| {
+        {
+            {
+                { x }
+            }
+        }
+    };
+    let f = |x| {
+        {
+            { x }
+        }
+    };
+    let f = |x| {
+        { x }
+    };
+    let f = |x| { x };
+    let f = |x| x;
+}
+
+fn main() {
+    let arg_test2 =
+        |big_argument_name, test123| { looooooooooooooooooong_function_naaaaaaaaaaaaaaaaame() };
+}
+
+impl Foo {
+    pub fn bar(&self) {
+        Some(SomeType {
+            push_closure_out_to_100_chars: iter(
+                otherwise_it_works_ok.into_iter().map(|f| { Ok(f) }),
+            ),
+        })
+    }
+}