Implement wrapping rules to force else on a newline in let-else

Author: ytmimi

Diff for: src/expr.rs

@@ -1027,12 +1027,14 @@ impl<'a> ControlFlow<'a> {
 
 /// Rewrite the `else` keyword with surrounding comments.
 ///
+/// force_newline_else: whether or not to rewrite the `else` keyword on a newline.
 /// is_last: true if this is an `else` and `false` if this is an `else if` block.
 /// context: rewrite context
 /// span: Span between the end of the last expression and the start of the else block,
 ///       which contains the `else` keyword
 /// shape: Shape
 pub(crate) fn rewrite_else_kw_with_comments(
+    force_newline_else: bool,
     is_last: bool,
     context: &RewriteContext<'_>,
     span: Span,
@@ -1048,6 +1050,7 @@ pub(crate) fn rewrite_else_kw_with_comments(
 
     let newline_sep = &shape.indent.to_string_with_newline(context.config);
     let before_sep = match context.config.control_brace_style() {
+        _ if force_newline_else => newline_sep.as_ref(),
         ControlBraceStyle::AlwaysNextLine | ControlBraceStyle::ClosingNextLine => {
             newline_sep.as_ref()
         }
@@ -1132,6 +1135,7 @@ impl<'a> Rewrite for ControlFlow<'a> {
             };
 
             let else_kw = rewrite_else_kw_with_comments(
+                false,
                 last_in_chain,
                 context,
                 self.block.span.between(else_block.span),

Diff for: src/items.rs

@@ -126,10 +126,14 @@ impl Rewrite for ast::Local {
             )?;
 
             if let Some(block) = else_block {
+                let else_kw_span = init.span.between(block.span);
+                let force_newline_else =
+                    !same_line_else_kw_and_brace(&result, context, else_kw_span, nested_shape);
                 let else_kw = rewrite_else_kw_with_comments(
+                    force_newline_else,
                     true,
                     context,
-                    init.span.between(block.span),
+                    else_kw_span,
                     shape,
                 );
                 result.push_str(&else_kw);
@@ -148,6 +152,47 @@ impl Rewrite for ast::Local {
     }
 }
 
+/// When the initializer expression is multi-lined, then the else keyword and opening brace of the
+/// block ( i.e. "else {") should be put on the same line as the end of the initializer expression
+/// if all the following are true:
+///
+/// 1. The initializer expression ends with one or more closing parentheses, square brackets,
+///    or braces
+/// 2. There is nothing else on that line
+/// 3. That line is not indented beyond the indent on the first line of the let keyword
+fn same_line_else_kw_and_brace(
+    init_str: &str,
+    context: &RewriteContext<'_>,
+    else_kw_span: Span,
+    init_shape: Shape,
+) -> bool {
+    if !init_str.contains('\n') {
+        // initializer expression is single lined so the "else {" should be placed on the same line
+        return true;
+    }
+
+    // 1. The initializer expression ends with one or more `)`, `]`, `}`.
+    if !init_str.ends_with([')', ']', '}']) {
+        return false;
+    }
+
+    // 2. There is nothing else on that line
+    // For example, there are no comments
+    let else_kw_snippet = context.snippet(else_kw_span).trim();
+    if else_kw_snippet != "else" {
+        return false;
+    }
+
+    // 3. The last line of the initializer expression is not indented beyond the `let` keyword
+    let indent = init_shape.indent.to_string(context.config);
+    init_str
+        .lines()
+        .last()
+        .expect("initializer expression is multi-lined")
+        .strip_prefix(indent.as_ref())
+        .map_or(false, |l| !l.starts_with(char::is_whitespace))
+}
+
 // FIXME convert to using rewrite style rather than visitor
 // FIXME format modules in this style
 #[allow(dead_code)]

Diff for: tests/source/let_else.rs

@@ -7,4 +7,8 @@ fn main() {
         // nope
         return;
     };
+
+    let Some(x) = y.foo("abc", fairly_long_identifier, "def", "123456", "string", "cheese") else { bar() };
+
+    let Some(x) = abcdef().foo("abc", some_really_really_really_long_ident, "ident", "123456").bar().baz().qux("fffffffffffffffff") else { foo_bar() };
 }

Diff for: tests/target/let_else.rs

@@ -9,4 +9,29 @@ fn main() {
         // nope
         return;
     };
+
+    let Some(x) = y.foo(
+        "abc",
+        fairly_long_identifier,
+        "def",
+        "123456",
+        "string",
+        "cheese",
+    ) else {
+        bar()
+    };
+
+    let Some(x) = abcdef()
+        .foo(
+            "abc",
+            some_really_really_really_long_ident,
+            "ident",
+            "123456",
+        )
+        .bar()
+        .baz()
+        .qux("fffffffffffffffff")
+    else {
+        foo_bar()
+    };
 }