add flag to force newlines in let else statements

Configurations.md

@@ -1731,6 +1731,58 @@ use core::slice;
 #[cfg(feature = "alloc")] use core::slice;
 ```
 
+## `let_else_force_newline`
+
+Force `else` branch of a `let_else` expression to always come after a newline.
+
+- **Default value**: `false`
+- **Possible values**: `true`, `false`
+- **Stable**: No (Tracking issue: [TODO](todo))
+
+#### `false` (default):
+
+```rust
+fn main() {
+    let Some(w) = opt else { return Ok(()) };
+
+    let Some(x) = opt else { return };
+
+    let Some(y) = opt else {
+        return;
+    };
+
+    let Some(z) = some_very_very_very_very_long_name else {
+        return;
+    };
+}
+```
+
+#### `true`:
+
+```rust
+fn main() {
+    let Some(w) = opt
+    else {
+        return Ok(());
+    };
+
+    let Some(x) = opt
+    else {
+        return;
+    };
+
+    let Some(y) = opt
+    else {
+        return;
+    };
+
+    let Some(z) = some_very_very_very_very_long_name
+    else {
+        return;
+    };
+}
+```
+
 ## `match_arm_blocks`
 
 Controls whether arm bodies are wrapped in cases where the first line of the body cannot fit on the same line as the `=>` operator.

src/config/mod.rs

@@ -161,6 +161,8 @@ create_config! {
     format_generated_files: FormatGeneratedFiles, false, "Format generated files";
     generated_marker_line_search_limit: GeneratedMarkerLineSearchLimit, false, "Number of lines to \
         check for a `@generated` marker when `format_generated_files` is enabled";
+    let_else_force_newline: LetElseForceNewline, true, "Always put the else branch of let_else \
+        statements on a new line.";
 
     // Options that can change the source code beyond whitespace/blocks (somewhat linty things)
     merge_derives: MergeDerives, true, "Merge multiple `#[derive(...)]` into a single one";
@@ -820,6 +822,7 @@ version = "One"
 inline_attribute_width = 0
 format_generated_files = true
 generated_marker_line_search_limit = 5
+let_else_force_newline = false
 merge_derives = true
 use_try_shorthand = false
 use_field_init_shorthand = false
@@ -912,6 +915,7 @@ version = "Two"
 inline_attribute_width = 0
 format_generated_files = true
 generated_marker_line_search_limit = 5
+let_else_force_newline = false
 merge_derives = true
 use_try_shorthand = false
 use_field_init_shorthand = false

src/config/options.rs

@@ -683,6 +683,7 @@ config_option_with_style_edition_default!(
     InlineAttributeWidth, usize, _ => 0;
     FormatGeneratedFiles, bool, _ => true;
     GeneratedMarkerLineSearchLimit, usize, _ => 5;
+    LetElseForceNewline, bool, _ => false;
 
     // Options that can change the source code beyond whitespace/blocks (somewhat linty things)
     MergeDerives, bool, _ => true;

src/items.rs

@@ -144,7 +144,8 @@ impl Rewrite for ast::Local {
                     result.as_str()
                 };
                 let force_newline_else = pat_str.contains('\n')
-                    || !same_line_else_kw_and_brace(init_str, context, else_kw_span, nested_shape);
+                    || !same_line_else_kw_and_brace(init_str, context, else_kw_span, nested_shape)
+                    || context.config.let_else_force_newline();
                 let else_kw = rewrite_else_kw_with_comments(
                     force_newline_else,
                     true,

tests/source/let_else_force_newline.rs

@@ -0,0 +1,182 @@
+// rustfmt-let_else_force_newline: true
+
+fn main() {
+    // Although this won't compile it still parses so make sure we can format empty else blocks
+    let Some(x) = opt else {};
+
+    // let-else may be formatted on a single line if they are "short"
+    // and only contain a single expression
+    let Some(x) = opt else { return };
+
+    let Some(x) = opt else {
+        return
+    };
+
+    let Some(x) = opt else { return; };
+
+    let Some(x) = opt else {
+        // nope
+        return;
+    };
+
+    let Some(x) = opt else { let y = 1; return y };
+
+    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() };
+}
+
+fn with_comments_around_else_keyword() {
+    let Some(x) = opt /* pre else keyword block-comment */ else { return };
+
+    let Some(x) = opt else /* post else keyword block-comment */ { return };
+
+    let Some(x) = opt /* pre else keyword block-comment */ else /* post else keyword block-comment */ { return };
+
+    let Some(x) = opt // pre else keyword line-comment
+    else { return };
+
+    let Some(x) = opt else
+     // post else keyword line-comment
+    { return };
+
+    let Some(x) = opt // pre else keyword line-comment
+    else
+    // post else keyword line-comment
+    { return };
+
+}
+
+fn unbreakable_initializer_expr_pre_formatting_let_else_length_near_max_width() {
+    // Pre Formatting:
+    // The length of `(indent)let pat = init else block;` is 100 (max_width)
+    // Post Formatting:
+    // The formatting is left unchanged!
+    let Some(x) = some_really_really_really_really_really_really_really_long_name_A else { return };
+
+    // Pre Formatting:
+    // The length of `(indent)let pat = init else block;` is 100 (max_width)
+    // Post Formatting:
+    // The else keyword and opening brace remain on the same line as the initializer expr,
+    // and the else block is formatted over multiple lines because we can't fit the
+    // else block on the same line as the initializer expr.
+    let Some(x) = some_really_really_really_really_really_really_really_long_name___B else {return};
+
+    // Pre Formatting:
+    // The length of `(indent)let pat = init else block;` is 100 (max_width)
+    // Post Formatting:
+    // The else keyword and opening brace remain on the same line as the initializer expr,
+    // and the else block is formatted over multiple lines because we can't fit the
+    // else block on the same line as the initializer expr.
+    let Some(x) = some_really_really_really_really_long_name_____C else {some_divergent_function()};
+
+    // Pre Formatting:
+    // The length of `(indent)let pat = init else block;` is 101 (> max_width)
+    // Post Formatting:
+    // The else keyword and opening brace remain on the same line as the initializer expr,
+    // and the else block is formatted over multiple lines because we can't fit the
+    // else block on the same line as the initializer expr.
+    let Some(x) = some_really_really_really_really_really_really_really_long_name__D else { return };
+}
+
+fn unbreakable_initializer_expr_pre_formatting_length_up_to_opening_brace_near_max_width() {
+    // Pre Formatting:
+    // The length of `(indent)let pat = init else {` is 99 (< max_width)
+    // Post Formatting:
+    // The else keyword and opening brace remain on the same line as the initializer expr,
+    // and the else block is formatted over multiple lines because we can't fit the
+    // else block on the same line as the initializer expr.
+    let Some(x) = some_really_really_really_really_really_really_really_really_long_name___E else {return};
+
+    // Pre Formatting:
+    // The length of `(indent)let pat = init else {` is 101 (> max_width)
+    // Post Formatting:
+    // The else keyword and opening brace cannot fit on the same line as the initializer expr.
+    // They are formatted on the next line.
+    let Some(x) = some_really_really_really_really_really_really_really_really_long_name_____F else {return};
+}
+
+fn unbreakable_initializer_expr_pre_formatting_length_through_initializer_expr_near_max_width() {
+    // Pre Formatting:
+    // The length of `(indent)let pat = init` is 99 (< max_width)
+    // Post Formatting:
+    // The else keyword and opening brace cannot fit on the same line as the initializer expr.
+    // They are formatted on the next line.
+    let Some(x) = some_really_really_really_really_really_really_really_really_really_long_name___G else {return};
+
+    // Pre Formatting:
+    // The length of `(indent)let pat = init` is 100 (max_width)
+    // Post Formatting:
+    // Break after the `=` and put the initializer expr on it's own line.
+    // Because the initializer expr is multi-lined the else is placed on it's own line.
+    let Some(x) = some_really_really_really_really_really_really_really_really_really_long_name____H else {return};
+
+    // Pre Formatting:
+    // The length of `(indent)let pat = init` is 109 (> max_width)
+    // Post Formatting:
+    // Break after the `=` and put the initializer expr on it's own line.
+    // Because the initializer expr is multi-lined the else is placed on it's own line.
+    // The initializer expr has a length of 91, which when indented on the next line
+    // The `(indent)init` line has a length of 99. This is the max length that the `init` can be
+    // before we start running into max_width issues. I suspect this is because the shape is
+    // accounting for the `;` at the end of the `let-else` statement.
+    let Some(x) = some_really_really_really_really_really_really_really_really_really_really_long_name______I else {return};
+
+    // Pre Formatting:
+    // The length of `(indent)let pat = init` is 110 (> max_width)
+    // Post Formatting:
+    // Max length issues prevent us from formatting.
+    // The initializer expr has a length of 92, which if it would be indented on the next line
+    // the `(indent)init` line has a length of 100 which == max_width of 100.
+    // One might expect formatting to succeed, but I suspect the reason we hit max_width issues is
+    // because the Shape is accounting for the `;` at the end of the `let-else` statement.
+    let Some(x) = some_really_really_really_really_really_really_really_really_really_really_really_long_nameJ else {return};
+}
+
+fn long_patterns() {
+    let Foo {x: Bar(..), y: FooBar(..), z: Baz(..)} = opt else {
+        return;
+    };
+
+    // with version=One we don't wrap long array patterns
+    let [aaaaaaaaaaaaaaaa, bbbbbbbbbbbbbbb, cccccccccccccccccc, dddddddddddddddddd] = opt else {
+        return;
+    };
+
+    let ("aaaaaaaaaaaaaaaaaaa" | "bbbbbbbbbbbbbbbbb" | "cccccccccccccccccccccccc" | "dddddddddddddddd" | "eeeeeeeeeeeeeeee") = opt else {
+        return;
+    };
+
+    let Some(Ok((Message::ChangeColor(super::color::Color::Rgb(r, g, b)), Point { x, y, z }))) = opt else {
+        return;
+    };
+}
+
+fn with_trailing_try_operator() {
+    // Currently the trailing ? forces the else on the next line
+    // This may be revisited in style edition 2024
+    let Some(next_bucket) = ranking_rules[cur_ranking_rule_index].next_bucket(ctx, logger, &ranking_rule_universes[cur_ranking_rule_index])? else { return };
+
+    // Maybe this is a workaround?
+    let Ok(Some(next_bucket)) = ranking_rules[cur_ranking_rule_index].next_bucket(ctx, logger, &ranking_rule_universes[cur_ranking_rule_index]) else { return };
+}
+
+fn issue5901() {
+    #[cfg(target_os = "linux")]
+    let Some(x) = foo else { todo!() };
+
+    #[cfg(target_os = "linux")]
+    // Some comments between attributes and let-else statement
+    let Some(x) = foo else { todo!() };
+
+    #[cfg(target_os = "linux")]
+    #[cfg(target_arch = "x86_64")]
+    let Some(x) = foo else { todo!() };
+
+    // The else block will be multi-lined because attributes and comments before `let`
+    // are included when calculating max width
+    #[cfg(target_os = "linux")]
+    #[cfg(target_arch = "x86_64")]
+    // Some comments between attributes and let-else statement
+    let Some(x) = foo() else { todo!() };
+}

tests/source/let_else_force_newline_v2.rs

@@ -0,0 +1,56 @@
+// rustfmt-style_edition: 2024
+// rustfmt-let_else_force_newline: true
+
+fn issue5901() {
+    #[cfg(target_os = "linux")]
+    let Some(x) = foo else { todo!() };
+
+    #[cfg(target_os = "linux")]
+    // Some comments between attributes and let-else statement
+    let Some(x) = foo else { todo!() };
+
+    #[cfg(target_os = "linux")]
+    #[cfg(target_arch = "x86_64")]
+    let Some(x) = foo else { todo!() };
+
+    // The else block is multi-lined
+    #[cfg(target_os = "linux")]
+    let Some(x) = foo else { return; };
+
+    // The else block will be single-lined because attributes and comments before `let`
+    // are no longer included when calculating max width
+    #[cfg(target_os = "linux")]
+    #[cfg(target_arch = "x86_64")]
+    // Some comments between attributes and let-else statement
+    let Some(x) = foo else { todo!() };
+
+    // Some more test cases for v2 formatting with attributes
+
+    #[cfg(target_os = "linux")]
+    #[cfg(target_arch = "x86_64")]
+    let Some(x) = opt
+    // pre else keyword line-comment
+    else { return; };
+
+    #[cfg(target_os = "linux")]
+    #[cfg(target_arch = "x86_64")]
+    let Some(x) = opt else
+    // post else keyword line-comment
+    { return; };
+
+    #[cfg(target_os = "linux")]
+    #[cfg(target_arch = "x86_64")]
+    let Foo {x: Bar(..), y: FooBar(..), z: Baz(..)} = opt else {
+        return;
+    };
+
+    #[cfg(target_os = "linux")]
+    #[cfg(target_arch = "x86_64")]
+    let Some(Ok((Message::ChangeColor(super::color::Color::Rgb(r, g, b)), Point { x, y, z }))) = opt else {
+        return;
+    };
+
+    #[cfg(target_os = "linux")]
+    #[cfg(target_arch = "x86_64")]
+    let Some(x) = very_very_very_very_very_very_very_very_very_very_very_very_long_expression_in_assign_rhs() else { return; };
+}