feat: support configurability of leading pipe

Author: calebcartwright

rustfmt-core/rustfmt-config/src/lib.rs

@@ -105,6 +105,7 @@ create_config! {
         "Align enum variants discrims, if their diffs fit within threshold";
     match_arm_blocks: bool, true, false, "Wrap the body of arms in blocks when it does not fit on \
         the same line with the pattern of arms";
+    strip_leading_match_arm_pipes: bool, true, true, "";
     force_multiline_blocks: bool, false, false,
         "Force multiline closure bodies and match arms to be wrapped in a block";
     fn_args_layout: Density, Density::Tall, true,
@@ -555,6 +556,7 @@ overflow_delimited_expr = false
 struct_field_align_threshold = 0
 enum_discrim_align_threshold = 0
 match_arm_blocks = true
+strip_leading_match_arm_pipes = true
 force_multiline_blocks = false
 fn_args_layout = "Tall"
 brace_style = "SameLineWhere"

rustfmt-core/rustfmt-lib/src/matches.rs

@@ -55,7 +55,13 @@ impl<'a> Spanned for ArmWrapper<'a> {
 
 impl<'a> Rewrite for ArmWrapper<'a> {
     fn rewrite(&self, context: &RewriteContext<'_>, shape: Shape) -> Option<String> {
-        rewrite_match_arm(context, self.arm, shape, self.is_last)
+        rewrite_match_arm(
+            context,
+            self.arm,
+            shape,
+            self.is_last,
+            self.beginning_vert.is_some(),
+        )
     }
 }
 
@@ -215,6 +221,7 @@ fn rewrite_match_arm(
     arm: &ast::Arm,
     shape: Shape,
     is_last: bool,
+    has_leading_pipe: bool,
 ) -> Option<String> {
     let (missing_span, attrs_str) = if !arm.attrs.is_empty() {
         if contains_skip(&arm.attrs) {
@@ -232,9 +239,17 @@ fn rewrite_match_arm(
         (mk_sp(arm.span().lo(), arm.span().lo()), String::new())
     };
 
+    let needs_leading_pipe = has_leading_pipe && !context.config.strip_leading_match_arm_pipes();
+    let (pipe_offset, pipe_str) = if needs_leading_pipe {
+        (2, "| ")
+    } else {
+        (0, "")
+    };
     // Patterns
     // 5 = ` => {`
-    let pat_shape = shape.sub_width(5)?;
+    // 2 = `| `
+    let pat_shape = shape.sub_width(5)?.offset_left(pipe_offset)?;
+
     let pats_str = arm.pat.rewrite(context, pat_shape)?;
 
     // Guard
@@ -251,7 +266,7 @@ fn rewrite_match_arm(
     let lhs_str = combine_strs_with_missing_comments(
         context,
         &attrs_str,
-        &format!("{}{}", pats_str, guard_str),
+        &format!("{}{}{}", pipe_str, pats_str, guard_str),
         missing_span,
         shape,
         false,