fix(linter): unicorn/switch-case-braces mangles code when applying fix (#8758)

Addresses #8491.

Essentially, the fix was ensuring that we track the indentation of a
`StatementExpression` or `BreakStatement` when we apply a fix for this
lint rule, and to make sure we add these statements on a new line with
the proper indentation. I also made sure we're indenting the closing
case brackets correctly.

To do this, I added a `get_preceding_indent_str` fn to the `ast_utils`
that handles getting the preceding indentation of a `Span` in a
`source_text` &str. It returns an Option in case no indentation is
found, or if the statement is not the only one on a given line.

This new fn was useful when addressing this particular bug, but I figure
it might be useful for other fixer use cases, too. I added some
documentation for this fn for clarity.

Author: taearls

Diff for: crates/oxc_linter/src/ast_util.rs

@@ -486,6 +486,41 @@ fn is_definitely_non_error_type(ty: &TSType) -> bool {
         _ => false,
     }
 }
+/// Get the preceding indentation string before the start of a Span in a given source_text string slice. Useful for maintaining the format of source code when applying a linting fix.
+///
+/// Slice into source_text until the start of given Span.
+/// Then, get the preceding spaces from the last line of the source_text.
+/// If there are any non-whitespace characters preceding the Span in the last line of source_text, return None.
+///
+/// Examples:
+///
+/// 1. Given the following source_text (with 2 preceding spaces):
+///
+/// ```ts
+///   break
+/// ```
+///
+/// and the Span encapsulating the break statement,
+///
+/// this function will return "  " (2 preceding spaces).
+///
+/// 2. Given the following source_text:
+///
+/// ```ts
+/// const foo = 'bar'; break;
+/// ```
+///
+/// and the Span encapsulating the break statement,
+///
+/// this function will return None because there is non-whitespace before the statement,
+/// meaning the line of source_text containing the Span is not indented on a new line.
+pub fn get_preceding_indent_str(source_text: &str, span: Span) -> Option<&str> {
+    let span_start = span.start as usize;
+    let preceding_source_text = &source_text[..span_start];
+
+    // only return last line if is whitespace
+    preceding_source_text.lines().last().filter(|&line| line.trim().is_empty())
+}
 
 pub fn could_be_error(ctx: &LintContext, expr: &Expression) -> bool {
     match expr.get_inner_expression() {

Diff for: crates/oxc_linter/src/rules/unicorn/switch_case_braces.rs

@@ -3,7 +3,7 @@ use oxc_diagnostics::OxcDiagnostic;
 use oxc_macros::declare_oxc_lint;
 use oxc_span::{GetSpan, Span};
 
-use crate::{context::LintContext, rule::Rule, AstNode};
+use crate::{ast_util::get_preceding_indent_str, context::LintContext, rule::Rule, AstNode};
 
 #[derive(Clone, Copy)]
 enum Diagnostic {
@@ -158,10 +158,33 @@ impl Rule for SwitchCaseBraces {
                                     formatter.print_ascii_byte(b'{');
 
                                     let source_text = ctx.source_text();
+
                                     for x in &case.consequent {
+                                        if matches!(
+                                            x,
+                                            Statement::ExpressionStatement(_)
+                                                | Statement::BreakStatement(_)
+                                        ) {
+                                            // indent the statement in the case consequent, if needed
+                                            if let Some(indent_str) =
+                                                get_preceding_indent_str(source_text, x.span())
+                                            {
+                                                formatter.print_ascii_byte(b'\n');
+                                                formatter.print_str(indent_str);
+                                            }
+                                        }
+
                                         formatter.print_str(x.span().source_text(source_text));
                                     }
 
+                                    // indent the closing case bracket, if needed
+                                    if let Some(case_indent_str) =
+                                        get_preceding_indent_str(source_text, case.span())
+                                    {
+                                        formatter.print_ascii_byte(b'\n');
+                                        formatter.print_str(case_indent_str);
+                                    }
+
                                     formatter.print_ascii_byte(b'}');
 
                                     formatter.into_source_text()
@@ -227,6 +250,35 @@ fn test() {
             "switch(foo) { default: doSomething(); }",
             Some(serde_json::json!(["avoid"])),
         ),
+        // Issue: https://github.com/oxc-project/oxc/issues/8491
+        (
+            "
+                const alpha = 7
+                let beta = ''
+                let gamma = 0
+
+                switch (alpha) {
+                    case 1: 
+                        beta = 'one'
+                        gamma = 1
+                        break
+                }
+            ",
+            "
+                const alpha = 7
+                let beta = ''
+                let gamma = 0
+
+                switch (alpha) {
+                    case 1: {
+                        beta = 'one'
+                        gamma = 1
+                        break
+                    }
+                }
+            ",
+            None,
+        ),
     ];
 
     Tester::new(SwitchCaseBraces::NAME, SwitchCaseBraces::PLUGIN, pass, fail)