Add short_item_threshold config option

Allow custom short item threshold values via config

Configurations.md

@@ -2200,6 +2200,14 @@ specific version of rustfmt is used in your CI, use this option.
 - **Possible values**: any published version (e.g. `"0.3.8"`)
 - **Stable**: No (tracking issue: [#3386](https://github.com/rust-lang/rustfmt/issues/3386))
 
+## `short_item_threshold`
+
+Maximum width threshold for a short item.
+
+- **Default value**: `10`
+- **Possible values**: any positive integer that is less than or equal to the value specified for [`max_width`](#max_width)
+- **Stable**: Yes
+
 ## `skip_children`
 
 Don't reformat out of line modules

src/config/mod.rs

@@ -106,6 +106,7 @@ create_config! {
     // Misc.
     remove_nested_parens: bool, true, true, "Remove nested parens";
     combine_control_expr: bool, true, false, "Combine control expressions with function calls";
+    short_item_threshold: usize, 10, true, "Maximum width threshold for a short item";
     overflow_delimited_expr: bool, false, false,
         "Allow trailing bracket/brace delimited expressions to overflow";
     struct_field_align_threshold: usize, 0, false,
@@ -589,6 +590,7 @@ spaces_around_ranges = false
 binop_separator = "Front"
 remove_nested_parens = true
 combine_control_expr = true
+short_item_threshold = 10
 overflow_delimited_expr = false
 struct_field_align_threshold = 0
 enum_discrim_align_threshold = 0
@@ -902,6 +904,15 @@ make_backup = false
             assert_eq!(config.single_line_if_else_max_width(), 70);
         }
 
+        #[test]
+        fn test_short_item_threshold_config_exceeds_max_width() {
+            let toml = r#"
+                short_item_threshold = 10
+            "#;
+            let config = Config::from_toml(toml, Path::new("")).unwrap();
+            assert_eq!(config.short_item_threshold(), 10);
+        }
+
         #[test]
         fn test_override_fn_call_width_exceeds_max_width() {
             let mut config = Config::default();
@@ -950,5 +961,12 @@ make_backup = false
             config.override_value("single_line_if_else_max_width", "101");
             assert_eq!(config.single_line_if_else_max_width(), 100);
         }
+
+        #[test]
+        fn test_override_short_item_threshold_exceeds_max_width() {
+            let mut config = Config::default();
+            config.override_value("short_item_threshold", "20");
+            assert_eq!(config.short_item_threshold(), 20);
+        }
     }
 }

src/overflow.rs

@@ -26,8 +26,6 @@ use crate::spanned::Spanned;
 use crate::types::{can_be_overflowed_type, SegmentParam};
 use crate::utils::{count_newlines, extra_offset, first_line_width, last_line_width, mk_sp};
 
-const SHORT_ITEM_THRESHOLD: usize = 10;
-
 /// A list of `format!`-like macros, that take a long format string and a list of arguments to
 /// format.
 ///
@@ -572,7 +570,9 @@ impl<'a> Context<'a> {
                             if one_line {
                                 tactic = DefinitiveListTactic::SpecialMacro(num_args_before);
                             };
-                        } else if is_every_expr_simple(&self.items) && no_long_items(list_items) {
+                        } else if is_every_expr_simple(&self.items)
+                            && no_long_items(list_items, self.context.config.short_item_threshold())
+                        {
                             tactic = DefinitiveListTactic::Mixed;
                         }
                     }
@@ -755,9 +755,9 @@ fn shape_from_indent_style(
     }
 }
 
-fn no_long_items(list: &[ListItem]) -> bool {
+fn no_long_items(list: &[ListItem], short_item_threshold: usize) -> bool {
     list.iter()
-        .all(|item| item.inner_as_ref().len() <= SHORT_ITEM_THRESHOLD)
+        .all(|item| item.inner_as_ref().len() <= short_item_threshold)
 }
 
 /// In case special-case style is required, returns an offset from which we start horizontal layout.