feat: add match_arm_indent option

Allows to disable the indentation of match arms.

Related issue: #2937

Co-authored-by: Yacin Tmimi <[email protected]>

Author: KAAtheWiseGit

Configurations.md

@@ -1862,6 +1862,42 @@ fn foo() {
 }
 ```
 
+## `match_arm_indent`
+
+Controls whether match arms are indented.  If disabled, match arms will be formatted at the same indentation level as the outer `match` statement.  Meaning that match blocks will only be indented once, not twice.
+
+- **Default value**: `true`
+- **Possible values**: `true`, `false`
+- **Stable**: No (TODO tracking issue)
+
+#### `true` (default):
+
+```rust
+fn main() {
+    match value {
+        Enum::A => {
+            let mut work = first();
+            work += second();
+        }
+        Enum::B => short_work(),
+    }
+}
+```
+
+#### `false`:
+
+```rust
+fn main() {
+    match value {
+    Enum::A => {
+        let mut work = first();
+        work += second();
+    }
+    Enum::B => short_work(),
+    }
+}
+```
+
 ## `match_block_trailing_comma`
 
 Put a trailing comma after a block based match arm (non-block arms are not affected)

src/config/mod.rs

@@ -131,6 +131,8 @@ create_config! {
         on the same line with the pattern of arms";
     match_arm_leading_pipes: MatchArmLeadingPipeConfig, true,
         "Determines whether leading pipes are emitted on match arms";
+    match_arm_indent: MatchArmIndent, false,
+        "Determines whether match arms are indented";
     force_multiline_blocks: ForceMultilineBlocks, false,
         "Force multiline closure bodies and match arms to be wrapped in a block";
     fn_args_layout: FnArgsLayout, true,
@@ -803,6 +805,7 @@ struct_field_align_threshold = 0
 enum_discrim_align_threshold = 0
 match_arm_blocks = true
 match_arm_leading_pipes = "Never"
+match_arm_indent = true
 force_multiline_blocks = false
 fn_params_layout = "Tall"
 brace_style = "SameLineWhere"
@@ -894,6 +897,7 @@ struct_field_align_threshold = 0
 enum_discrim_align_threshold = 0
 match_arm_blocks = true
 match_arm_leading_pipes = "Never"
+match_arm_indent = true
 force_multiline_blocks = false
 fn_params_layout = "Tall"
 brace_style = "SameLineWhere"

src/config/options.rs

@@ -665,6 +665,7 @@ config_option_with_style_edition_default!(
     EnumDiscrimAlignThreshold, usize, _ => 0;
     MatchArmBlocks, bool, _ => true;
     MatchArmLeadingPipeConfig, MatchArmLeadingPipe, _ => MatchArmLeadingPipe::Never;
+    MatchArmIndent, bool, _ => true;
     ForceMultilineBlocks, bool, _ => false;
     FnArgsLayout, Density, _ => Density::Tall;
     FnParamsLayout, Density, _ => Density::Tall;

src/matches.rs

@@ -100,17 +100,22 @@ pub(crate) fn rewrite_match(
         _ => " ",
     };
 
-    let nested_indent_str = shape
-        .indent
-        .block_indent(context.config)
-        .to_string(context.config);
+    let nested_indent = if context.config.match_arm_indent() {
+        shape.indent.block_indent(context.config)
+    } else {
+        shape.indent
+    };
+    let nested_indent_str = nested_indent.to_string(context.config);
+
     // Inner attributes.
     let inner_attrs = &inner_attributes(attrs);
     let inner_attrs_str = if inner_attrs.is_empty() {
         String::new()
     } else {
         let shape = if context.config.style_edition() <= StyleEdition::Edition2021 {
             shape
+        } else if !context.config.match_arm_indent() {
+            shape
         } else {
             shape.block_indent(context.config.tab_spaces())
         };
@@ -204,9 +209,12 @@ fn rewrite_match_arms(
     span: Span,
     open_brace_pos: BytePos,
 ) -> RewriteResult {
-    let arm_shape = shape
-        .block_indent(context.config.tab_spaces())
-        .with_max_width(context.config);
+    let arm_shape = if context.config.match_arm_indent() {
+        shape.block_indent(context.config.tab_spaces())
+    } else {
+        shape
+    }
+    .with_max_width(context.config);
 
     let arm_len = arms.len();
     let is_last_iter = repeat(false)

tests/source/match-unindent.rs

@@ -0,0 +1,19 @@
+// rustfmt-match_arm_indent: false
+// Unindent the match arms
+
+fn foo() {
+    match x {
+        a => {
+            "line1";
+            "line2"
+        }
+        ThisIsA::Guard if true => {
+            "line1";
+            "line2"
+        }
+        ThisIsA::ReallyLongPattern(ThatWillForce::TheGuard, ToWrapOnto::TheFollowingLine) if true => {
+            "line1";
+            "line2"
+        }
+    }
+}

tests/target/match-noindent-tab.rs

@@ -0,0 +1,23 @@
+// rustfmt-match_arm_indent: false
+// rustfmt-hard_tabs: true
+// rustfmt-tab_spaces: 8
+
+// Large-indentation style, brought to you by the Linux kernel
+fn foo() {
+	match value {
+	0 => {
+		"one";
+		"two";
+	}
+	1 | 2 | 3 => {
+		"line1";
+		"line2";
+	}
+	100..1000 => oneline(),
+
+	_ => {
+		// catch-all
+		todo!();
+	}
+	}
+}

tests/target/match-noindent.rs

@@ -0,0 +1,21 @@
+// rustfmt-match_arm_indent: false
+// Don't indent the match arms
+
+fn foo() {
+    match value {
+    0 => {
+        "one";
+        "two";
+    }
+    1 | 2 | 3 => {
+        "line1";
+        "line2";
+    }
+    100..1000 => oneline(),
+
+    _ => {
+        // catch-all
+        todo!();
+    }
+    }
+}

tests/target/match-unindent.rs

@@ -0,0 +1,19 @@
+// rustfmt-match_arm_indent: false
+// Unindent the match arms
+
+fn foo() {
+    match x {
+    a => {
+        "line1";
+        "line2"
+    }
+    ThisIsA::Guard if true => {
+        "line1";
+        "line2"
+    }
+    ThisIsA::ReallyLongPattern(ThatWillForce::TheGuard, ToWrapOnto::TheFollowingLine) if true => {
+        "line1";
+        "line2"
+    }
+    }
+}