refactor: rename KeepExisting->Preserve variant for match arm pipes (#4305)

Author: calebcartwright

Diff for: Configurations.md

@@ -1550,7 +1550,7 @@ See also: [`match_block_trailing_comma`](#match_block_trailing_comma).
 Controls whether to include a leading pipe on match arms
 
 - **Default value**: `Never`
-- **Possible values**: `Always`, `Never`, `KeepExisting`
+- **Possible values**: `Always`, `Never`, `Preserve`
 - **Stable**: Yes
 
 #### `Never` (default):
@@ -1607,7 +1607,7 @@ fn foo() {
 }
 ```
 
-#### `KeepExisting`:
+#### `Preserve`:
 ```rust
 fn foo() {
     match foo {

Diff for: src/config/options.rs

@@ -355,8 +355,8 @@ pub enum MatchArmLeadingPipe {
     Always,
     /// Never emit leading pipes on match arms
     Never,
-    /// Maintain any existing leading pipes
-    KeepExisting,
+    /// Preserve any existing leading pipes
+    Preserve,
 }
 
 #[cfg(test)]

Diff for: src/formatting/matches.rs

@@ -244,8 +244,8 @@ fn rewrite_match_arm(
     // 2 = `| `
     let (pipe_offset, pipe_str) = match context.config.match_arm_leading_pipes() {
         MatchArmLeadingPipe::Never => (0, ""),
-        MatchArmLeadingPipe::KeepExisting if !has_leading_pipe => (0, ""),
-        MatchArmLeadingPipe::KeepExisting | MatchArmLeadingPipe::Always => (2, "| "),
+        MatchArmLeadingPipe::Preserve if !has_leading_pipe => (0, ""),
+        MatchArmLeadingPipe::Preserve | MatchArmLeadingPipe::Always => (2, "| "),
     };
 
     // Patterns

Diff for: tests/source/configs/match_arm_leading_pipes/keep_existing.rs renamed to tests/source/configs/match_arm_leading_pipes/preserve.rs

@@ -1,4 +1,4 @@
-// rustfmt-match_arm_leading_pipes: KeepExisting
+// rustfmt-match_arm_leading_pipes: Preserve
 
 fn foo() {
     match foo {

Diff for: tests/target/configs/match_arm_leading_pipes/keep_existing.rs renamed to tests/target/configs/match_arm_leading_pipes/preserve.rs

@@ -1,4 +1,4 @@
-// rustfmt-match_arm_leading_pipes: KeepExisting
+// rustfmt-match_arm_leading_pipes: Preserve
 
 fn foo() {
     match foo {