single_line_if config option

Configurations.md

@@ -2481,6 +2481,34 @@ fn main() {
 
 See also [`max_width`](#max_width) and [`use_small_heuristics`](#use_small_heuristics)
 
+## `single_line_simple_if`
+
+Allows simple single expression if blocks to format on one line. Useful in the case of keeping `let-else` guards format consistent with `if` guards.
+
+Note that line will still break if:
+1. The condition length is over the `max_width`/`single_line_simple_if_max_width length`
+2. The block contains a trailing semicolon
+3. The block contains a single or multi-lined comment
+
+- **Default value**: `false`
+- **Possible values**: `true`, `false`
+- **Stable**: No
+
+#### `false` (default):
+
+## `single_line_simple_if_max_width`
+
+Maximum line length for single line if with a simple inner expression. Useful in the case of keeping `let-else` guards format consistent with `if` guards.
+
+A value of `0` (zero) results in if-else expressions always being broken into multiple lines. Note this occurs when `use_small_heuristics` is set to `Off`.
+
+- **Default value**: `50`
+- **Possible values**: any positive integer that is less than or equal to the value specified for [`max_width`](#max_width)
+- **Stable**: Yes
+
+By default this option is set as a percentage of [`max_width`](#max_width) provided by [`use_small_heuristics`](#use_small_heuristics), but a value set directly for `single_line_if_else_max_width` will take precedence.
+
+See also [`max_width`](#max_width) and [`use_small_heuristics`](#use_small_heuristics)
 
 ## `space_after_colon`
 

src/config/config_type.rs

@@ -122,6 +122,7 @@ macro_rules! create_config {
                     | "fn_call_width"
                     | "single_line_if_else_max_width"
                     | "single_line_let_else_max_width"
+                    | "single_line_simple_if_max_width"
                     | "attr_fn_like_width"
                     | "struct_lit_width"
                     | "struct_variant_width"
@@ -273,6 +274,7 @@ macro_rules! create_config {
                     | "fn_call_width"
                     | "single_line_if_else_max_width"
                     | "single_line_let_else_max_width"
+                    | "single_line_simple_if_max_width"
                     | "attr_fn_like_width"
                     | "struct_lit_width"
                     | "struct_variant_width"
@@ -421,6 +423,14 @@ macro_rules! create_config {
                     "single_line_let_else_max_width",
                 );
                 self.single_line_let_else_max_width.2 = single_line_let_else_max_width;
+
+                let single_line_simple_if_max_width = get_width_value(
+                    self.was_set().single_line_simple_if_max_width(),
+                    self.single_line_simple_if_max_width.2,
+                    heuristics.single_line_simple_if_max_width,
+                    "single_line_simple_if_max_width",
+                );
+                self.single_line_simple_if_max_width.2 = single_line_simple_if_max_width;
             }
 
             fn set_heuristics(&mut self) {

src/config/mod.rs

@@ -61,6 +61,8 @@ create_config! {
     single_line_let_else_max_width: usize, 50, true, "Maximum line length for single line \
         let-else statements. A value of zero means always format the divergent `else` block \
         over multiple lines.";
+    single_line_simple_if_max_width: usize, 50, false, "Maximum line length for single line \
+        if statement. A value of zero means always format to multiple lines.";
 
     // Comments. macros, and strings
     wrap_comments: bool, false, false, "Break comments to fit on the line";
@@ -81,6 +83,7 @@ create_config! {
         "Format hexadecimal integer literals";
 
     // Single line expressions and items
+    single_line_simple_if: bool, false, false, "Simple if statements can format to a single line";
     empty_item_single_line: bool, true, false,
         "Put empty-body functions and impls on a single line";
     struct_lit_single_line: bool, true, false,
@@ -490,6 +493,8 @@ mod test {
             single_line_let_else_max_width: usize, 50, false, "Maximum line length for single \
                 line let-else statements. A value of zero means always format the divergent \
                 `else` block over multiple lines.";
+            single_line_simple_if_max_width: usize, 50, false, "Maximum line length for \
+                single line if statement. A value of zero means always format to multiple lines.";
 
             // Options that are used by the tests
             stable_option: bool, false, true, "A stable option";
@@ -634,6 +639,7 @@ array_width = 60
 chain_width = 60
 single_line_if_else_max_width = 50
 single_line_let_else_max_width = 50
+single_line_simple_if_max_width = 50
 wrap_comments = false
 format_code_in_doc_comments = false
 doc_comment_code_block_width = 100
@@ -645,6 +651,7 @@ format_macro_matchers = false
 format_macro_bodies = true
 skip_macro_invocations = []
 hex_literal_case = "Preserve"
+single_line_simple_if = false
 empty_item_single_line = true
 struct_lit_single_line = true
 fn_single_line = false

src/config/options.rs

@@ -241,6 +241,9 @@ pub struct WidthHeuristics {
     // Maximum line length for single line let-else statements. A value of zero means
     // always format the divergent `else` block over multiple lines.
     pub(crate) single_line_let_else_max_width: usize,
+    // Maximum line length for single line if statement.
+    // A value of zero means always format to multiple lines.
+    pub(crate) single_line_simple_if_max_width: usize,
 }
 
 impl fmt::Display for WidthHeuristics {
@@ -261,6 +264,7 @@ impl WidthHeuristics {
             chain_width: usize::max_value(),
             single_line_if_else_max_width: 0,
             single_line_let_else_max_width: 0,
+            single_line_simple_if_max_width: 0,
         }
     }
 
@@ -274,6 +278,7 @@ impl WidthHeuristics {
             chain_width: max_width,
             single_line_if_else_max_width: max_width,
             single_line_let_else_max_width: max_width,
+            single_line_simple_if_max_width: max_width,
         }
     }
 
@@ -296,6 +301,7 @@ impl WidthHeuristics {
             chain_width: (60.0 * max_width_ratio).round() as usize,
             single_line_if_else_max_width: (50.0 * max_width_ratio).round() as usize,
             single_line_let_else_max_width: (50.0 * max_width_ratio).round() as usize,
+            single_line_simple_if_max_width: (50.0 * max_width_ratio).round() as usize,
         }
     }
 }

src/expr.rs

@@ -1102,16 +1102,31 @@ impl<'a> Rewrite for ControlFlow<'a> {
         };
         let block_str = {
             let old_val = context.is_if_else_block.replace(self.else_block.is_some());
-            let allow_single_line =
-                allow_single_line_if(&cond_str, self.block) && self.keyword == "if";
+            let max_width = if context.config.single_line_simple_if() {
+                std::cmp::min(
+                    shape.width,
+                    context.config.single_line_simple_if_max_width(),
+                )
+            } else {
+                shape.width
+            };
+            let available_space = max_width.saturating_sub(used_width);
+            let allow_single_line = allow_single_line_if(&cond_str, self.block)
+                && self.keyword == "if"
+                && available_space > 0;
 
-            let result = if allow_single_line {
-                rewrite_block_inner(self.block, None, None, true, context, block_shape)
+            let mut result = if allow_single_line && context.config.single_line_simple_if() {
+                rewrite_block_inner(self.block, None, None, true, context, block_shape)?
             } else {
-                rewrite_block_with_visitor(context, "", self.block, None, None, block_shape, true)
+                rewrite_block_with_visitor(context, "", self.block, None, None, block_shape, true)?
             };
+
+            let block_exceeds_width = result.len() > available_space;
+            if allow_single_line && !result.contains('\n') && block_exceeds_width {
+                result = rewrite_block_inner(self.block, None, None, false, context, block_shape)?;
+            }
             context.is_if_else_block.replace(old_val);
-            result?
+            result
         };
 
         let mut result = format!("{cond_str}{block_str}");
@@ -1168,20 +1183,26 @@ fn allow_single_line_if(result: &str, block: &ast::Block) -> bool {
     if result.contains('\n') {
         return false;
     }
-
     if block.stmts.len() == 0 {
         return true;
     }
     if block.stmts.len() == 1 {
-        return is_simple_stmt(&block.stmts[0]);
+        return is_simple_control_flow_stmt(&block.stmts[0]);
     }
     false
 }
 
-fn is_simple_stmt(stmt: &ast::Stmt) -> bool {
+fn is_simple_control_flow_stmt(stmt: &ast::Stmt) -> bool {
     match stmt.kind {
         ast::StmtKind::Expr(ref expr) => match expr.kind {
-            ast::ExprKind::Ret(..) | ast::ExprKind::Continue(..) | ast::ExprKind::Break(..) => true,
+            ast::ExprKind::Continue(..) => true,
+            ast::ExprKind::Break(_, ref opt_expr) | ast::ExprKind::Ret(ref opt_expr) => {
+                if let Some(_) = *opt_expr {
+                    false
+                } else {
+                    true
+                }
+            }
             _ => false,
         },
         _ => false,