implement single_line_let_else_max_width

This allows users to configure the maximum length of a single line
`let-else` statements. `let-else` statements that otherwise meet the
requirements to be formatted on a single line will have their divergent
`else` block formatted over multiple lines if they exceed this length.

**Note**: `single_line_let_else_max_widt` will be introduced as a stable
configuration option.

Author: ytmimi

Configurations.md

@@ -2392,6 +2392,78 @@ By default this option is set as a percentage of [`max_width`](#max_width) provi
 
 See also [`max_width`](#max_width) and [`use_small_heuristics`](#use_small_heuristics)
 
+## `single_line_let_else_max_width`
+
+Maximum line length for single line let-else statements.
+See the [let-else statement section of the Rust Style Guide](https://github.com/rust-lang/rust/blob/master/src/doc/style-guide/src/statements.md#else-blocks-let-else-statements) for more details on when a let-else statement may be written on a single line.
+A value of `0` (zero) means the divergent `else` block will always be formatted over multiple lines.
+Note this occurs when `use_small_heuristics` is set to `Off`.
+
+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_let_else_max_width` will take precedence.
+
+- **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
+
+#### `50` (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;
+    };
+}
+```
+
+#### `0`:
+
+```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;
+    };
+}
+```
+
+#### `100`:
+
+```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 };
+}
+```
+
+See also [`max_width`](#max_width) and [`use_small_heuristics`](#use_small_heuristics)
+
+
 ## `space_after_colon`
 
 Leave a space after the colon.
@@ -2804,6 +2876,7 @@ The ratios are:
 * [`array_width`](#array_width) - `60%`
 * [`chain_width`](#chain_width) - `60%`
 * [`single_line_if_else_max_width`](#single_line_if_else_max_width) - `50%`
+* [`single_line_let_else_max_width`](#single_line_let_else_max_width) - `50%`
 
 For example when `max_width` is set to `100`, the width settings are:
 * `fn_call_width=60`
@@ -2813,6 +2886,7 @@ For example when `max_width` is set to `100`, the width settings are:
 * `array_width=60`
 * `chain_width=60`
 * `single_line_if_else_max_width=50`
+* `single_line_let_else_max_width=50`
 
 and when `max_width` is set to `200`:
 * `fn_call_width=120`
@@ -2822,6 +2896,7 @@ and when `max_width` is set to `200`:
 * `array_width=120`
 * `chain_width=120`
 * `single_line_if_else_max_width=100`
+* `single_line_let_else_max_width=100`
 
 ```rust
 enum Lorem {
@@ -2891,6 +2966,7 @@ So if `max_width` is set to `200`, then all the width settings are also set to `
 * `array_width=200`
 * `chain_width=200`
 * `single_line_if_else_max_width=200`
+* `single_line_let_else_max_width=200`
 
 ```rust
 enum Lorem {
@@ -2918,6 +2994,7 @@ See also:
 * [`array_width`](#array_width)
 * [`chain_width`](#chain_width)
 * [`single_line_if_else_max_width`](#single_line_if_else_max_width)
+* [`single_line_let_else_max_width`](#single_line_let_else_max_width)
 
 ## `use_try_shorthand`
 

src/config/config_type.rs

@@ -121,6 +121,7 @@ macro_rules! create_config {
                     | "use_small_heuristics"
                     | "fn_call_width"
                     | "single_line_if_else_max_width"
+                    | "single_line_let_else_max_width"
                     | "attr_fn_like_width"
                     | "struct_lit_width"
                     | "struct_variant_width"
@@ -269,6 +270,7 @@ macro_rules! create_config {
                     | "use_small_heuristics"
                     | "fn_call_width"
                     | "single_line_if_else_max_width"
+                    | "single_line_let_else_max_width"
                     | "attr_fn_like_width"
                     | "struct_lit_width"
                     | "struct_variant_width"
@@ -407,6 +409,14 @@ macro_rules! create_config {
                     "single_line_if_else_max_width",
                 );
                 self.single_line_if_else_max_width.2 = single_line_if_else_max_width;
+
+                let single_line_let_else_max_width = get_width_value(
+                    self.was_set().single_line_let_else_max_width(),
+                    self.single_line_let_else_max_width.2,
+                    heuristics.single_line_let_else_max_width,
+                    "single_line_let_else_max_width",
+                );
+                self.single_line_let_else_max_width.2 = single_line_let_else_max_width;
             }
 
             fn set_heuristics(&mut self) {

src/config/mod.rs

@@ -58,6 +58,9 @@ create_config! {
     chain_width: usize, 60, true, "Maximum length of a chain to fit on a single line.";
     single_line_if_else_max_width: usize, 50, true, "Maximum line length for single line if-else \
         expressions. A value of zero means always break if-else expressions.";
+    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.";
 
     // Comments. macros, and strings
     wrap_comments: bool, false, false, "Break comments to fit on the line";
@@ -473,6 +476,9 @@ mod test {
             chain_width: usize, 60, true, "Maximum length of a chain to fit on a single line.";
             single_line_if_else_max_width: usize, 50, true, "Maximum line length for single \
                 line if-else expressions. A value of zero means always break if-else expressions.";
+            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.";
 
             // Options that are used by the tests
             stable_option: bool, false, true, "A stable option";
@@ -619,6 +625,7 @@ struct_variant_width = 35
 array_width = 60
 chain_width = 60
 single_line_if_else_max_width = 50
+single_line_let_else_max_width = 50
 wrap_comments = false
 format_code_in_doc_comments = false
 doc_comment_code_block_width = 100

src/config/options.rs

@@ -236,6 +236,9 @@ pub struct WidthHeuristics {
     // Maximum line length for single line if-else expressions. A value
     // of zero means always break if-else expressions.
     pub(crate) single_line_if_else_max_width: usize,
+    // 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,
 }
 
 impl fmt::Display for WidthHeuristics {
@@ -255,6 +258,7 @@ impl WidthHeuristics {
             array_width: usize::max_value(),
             chain_width: usize::max_value(),
             single_line_if_else_max_width: 0,
+            single_line_let_else_max_width: 0,
         }
     }
 
@@ -267,6 +271,7 @@ impl WidthHeuristics {
             array_width: max_width,
             chain_width: max_width,
             single_line_if_else_max_width: max_width,
+            single_line_let_else_max_width: max_width,
         }
     }
 
@@ -288,6 +293,7 @@ impl WidthHeuristics {
             array_width: (60.0 * max_width_ratio).round() as usize,
             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,
         }
     }
 }

src/items.rs

@@ -138,17 +138,30 @@ impl Rewrite for ast::Local {
                 );
                 result.push_str(&else_kw);
 
-                let allow_single_line = allow_single_line_let_else_block(&result, block);
+                // At this point we've written `let {pat} = {expr} else' into the buffer, and we
+                // want to calculate up front if there's room to write the divergent block on the
+                // same line. The available space varies based on indentation so we clamp the width
+                // on the smaller of `shape.width` and `single_line_let_else_max_width`.
+                let max_width =
+                    std::cmp::min(shape.width, context.config.single_line_let_else_max_width());
+
+                // If available_space hits zero we know for sure this will be a multi-lined block
+                let available_space = max_width.saturating_sub(result.len());
+
+                let allow_single_line = !force_newline_else
+                    && available_space > 0
+                    && allow_single_line_let_else_block(&result, block);
 
                 let mut rw_else_block =
                     rewrite_let_else_block(block, allow_single_line, context, shape)?;
 
-                if allow_single_line && !rw_else_block.contains('\n') {
-                    let available_space = shape.width.saturating_sub(result.len());
-                    if available_space <= rw_else_block.len() {
-                        // writing this on one line would exceed the available width
-                        rw_else_block = rewrite_let_else_block(block, false, context, shape)?;
-                    }
+                let single_line_else = !rw_else_block.contains('\n');
+                let else_block_exceeds_width = available_space <= rw_else_block.len();
+
+                if allow_single_line && single_line_else && else_block_exceeds_width {
+                    // writing this on one line would exceed the available width
+                    // so rewrite the else block over multiple lines.
+                    rw_else_block = rewrite_let_else_block(block, false, context, shape)?;
                 }
 
                 result.push_str(&rw_else_block);

tests/source/configs/single_line_let_else_max_width/100.rs

@@ -0,0 +1,40 @@
+// rustfmt-single_line_let_else_max_width: 100
+
+fn main() {
+    let Some(a) = opt else {};
+
+    let Some(b) = opt else { return };
+
+    let Some(c) = opt else {
+        return
+    };
+
+    let Some(c) = opt else {
+        // a comment should always force the block to be multi-lined
+        return
+    };
+
+    let Some(c) = opt else { /* a comment should always force the block to be multi-lined */ return };
+
+    let Some(d) = some_very_very_very_very_long_name else { return };
+
+    let Expr::Slice(ast::ExprSlice { lower, upper, step, range: _ }) = slice.as_ref() else {
+        return
+    };
+
+    let Some((base_place, current)) = self.lower_expr_as_place(current, *base, true)? else {
+        return Ok(None)
+    };
+
+    let Some(doc_attr) = variant.attrs.iter().find(|attr| attr.path().is_ident("doc")) else {
+        return Err(Error::new(variant.span(), r#"expected a doc comment"#))
+    };
+
+    let Some((base_place, current)) = self.lower_expr_as_place(current, *base, true) else {
+        return Ok(None)
+    };
+
+    let Stmt::Expr(Expr::Call(ExprCall { args: some_args, .. }), _) = last_stmt else {
+        return Err(Error::new(last_stmt.span(), "expected last expression to be `Some(match (..) { .. })`"))
+    };
+}

tests/source/configs/single_line_let_else_max_width/50.rs

@@ -0,0 +1,40 @@
+// rustfmt-single_line_let_else_max_width: 50
+
+fn main() {
+    let Some(a) = opt else {};
+
+    let Some(b) = opt else { return };
+
+    let Some(c) = opt else {
+        return
+    };
+
+    let Some(c) = opt else {
+        // a comment should always force the block to be multi-lined
+        return
+    };
+
+    let Some(c) = opt else { /* a comment should always force the block to be multi-lined */ return };
+
+    let Some(d) = some_very_very_very_very_long_name else { return };
+
+    let Expr::Slice(ast::ExprSlice { lower, upper, step, range: _ }) = slice.as_ref() else {
+        return
+    };
+
+    let Some((base_place, current)) = self.lower_expr_as_place(current, *base, true)? else {
+        return Ok(None)
+    };
+
+    let Some(doc_attr) = variant.attrs.iter().find(|attr| attr.path().is_ident("doc")) else {
+        return Err(Error::new(variant.span(), r#"expected a doc comment"#))
+    };
+
+    let Some((base_place, current)) = self.lower_expr_as_place(current, *base, true) else {
+        return Ok(None)
+    };
+
+    let Stmt::Expr(Expr::Call(ExprCall { args: some_args, .. }), _) = last_stmt else {
+        return Err(Error::new(last_stmt.span(), "expected last expression to be `Some(match (..) { .. })`"))
+    };
+}

tests/source/configs/single_line_let_else_max_width/zero.rs

@@ -0,0 +1,40 @@
+// rustfmt-single_line_let_else_max_width: 0
+
+fn main() {
+    let Some(a) = opt else {};
+
+    let Some(b) = opt else { return };
+
+    let Some(c) = opt else {
+        return
+    };
+
+    let Some(c) = opt else {
+        // a comment should always force the block to be multi-lined
+        return
+    };
+
+    let Some(c) = opt else { /* a comment should always force the block to be multi-lined */ return };
+
+    let Some(d) = some_very_very_very_very_long_name else { return };
+
+    let Expr::Slice(ast::ExprSlice { lower, upper, step, range: _ }) = slice.as_ref() else {
+        return
+    };
+
+    let Some((base_place, current)) = self.lower_expr_as_place(current, *base, true)? else {
+        return Ok(None)
+    };
+
+    let Some(doc_attr) = variant.attrs.iter().find(|attr| attr.path().is_ident("doc")) else {
+        return Err(Error::new(variant.span(), r#"expected a doc comment"#))
+    };
+
+    let Some((base_place, current)) = self.lower_expr_as_place(current, *base, true) else {
+        return Ok(None)
+    };
+
+    let Stmt::Expr(Expr::Call(ExprCall { args: some_args, .. }), _) = last_stmt else {
+        return Err(Error::new(last_stmt.span(), "expected last expression to be `Some(match (..) { .. })`"))
+    };
+}

tests/source/configs/use_small_heuristics/default.rs

@@ -23,3 +23,13 @@ fn main() {
         sit
     };
 }
+
+fn format_let_else() {
+    let Some(a) = opt else {};
+
+    let Some(b) = opt else { return };
+
+    let Some(c) = opt else { return };
+
+    let Some(d) = some_very_very_very_very_long_name else { return };
+}