feat: add match_arm_indent option

Allows to disable the indentation of match arms.

Related issue: #2937

Author: KAAtheWiseGit

Configurations.md

@@ -1862,6 +1862,42 @@ fn foo() {
 }
 ```
 
+## `match_arm_indent`
+
+Controls whether match arms are indented.  If disabled, the match patterns will be located on the indentation level as the match statement itself.  This means 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)
@@ -2475,7 +2511,7 @@ required_version=">=1.0.0, <2.0.0"
 
 The width threshold for an array element to be considered "short".
 
-The layout of an array is dependent on the length of each of its elements. 
+The layout of an array is dependent on the length of each of its elements.
 If the length of every element in an array is below this threshold (all elements are "short") then the array can be formatted in the mixed/compressed style, but if any one element has a length that exceeds this threshold then the array elements will have to be formatted vertically.
 
 - **Default value**: `10`

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"
+    }
+    }
+}