Include trailing comma in single param vertical fn

When formatting a function with a single parameter with
`fn_params_layout` set to `Vertical`, the previous tactic would remove
any trailing commas before spilling to multiple lines for other reasons,
such as long function names or return types. Instead, use the same
tactic as `Tall` does, so that the trailing comma is kept if the end
result spans multiple lines.

`fn_params_layout = "Vertical"` is stable, so this is a breaking change.
Guard on the style edition so that it only takes effect starting in the
next edition.

Issue #6889 `fn_params_layout = "Vertical"` removes single argument's
            trailing comma

Author: jasonaowen

src/config/options.rs

@@ -96,11 +96,16 @@ pub enum Heuristics {
 }
 
 impl Density {
-    pub fn to_list_tactic(self, len: usize) -> ListTactic {
+    pub fn to_list_tactic(self, style_edition: StyleEdition, len: usize) -> ListTactic {
         match self {
             Density::Compressed => ListTactic::Mixed,
             Density::Tall => ListTactic::HorizontalVertical,
-            Density::Vertical if len == 1 => ListTactic::Horizontal,
+            Density::Vertical if style_edition < StyleEdition::Edition2027 && len == 1 => {
+                ListTactic::Horizontal
+            }
+            Density::Vertical if style_edition >= StyleEdition::Edition2027 && len == 1 => {
+                ListTactic::HorizontalVertical
+            }
             Density::Vertical => ListTactic::Vertical,
         }
     }

src/items.rs

@@ -2877,7 +2877,7 @@ fn rewrite_params(
         context
             .config
             .fn_params_layout()
-            .to_list_tactic(param_items.len()),
+            .to_list_tactic(context.config.style_edition(), param_items.len()),
         Separator::Comma,
         one_line_budget,
     );

tests/source/configs/fn_params_layout/vertical.rs

@@ -2,6 +2,10 @@
 // Function arguments density
 
 trait Lorem {
+    fn lorem(ipsum: Ipsum);
+
+    fn lorem(ipsum: Ipsum) -> Dolor;
+
     fn lorem(ipsum: Ipsum, dolor: Dolor, sit: Sit, amet: Amet);
 
     fn lorem(ipsum: Ipsum, dolor: Dolor, sit: Sit, amet: Amet) {
@@ -13,4 +17,22 @@ trait Lorem {
     fn lorem(ipsum: Ipsum, dolor: Dolor, sit: Sit, amet: Amet, consectetur: onsectetur, adipiscing: Adipiscing, elit: Elit) {
         // body
     }
+
+    fn long_param_name(lorem_ipsum_dolor_sit_amet_consectetur_adipiscing_elit_sed_do_eiusmod: Tempor);
+
+    fn long_param_type(lorem: IpsumDolorSitAmetConsecteturAdipiscingElitSedDoEiusmodTemporIncididuntUtLabore,
+    );
+
+    fn long_return_type(lorem: Lorem)
+        -> IpsumDolorSitAmetConsecteturAdipiscingElitSedDoEiusmodTemporIncididuntUtLabore;
+
+    fn lorem_ipsum_dolor_sit_amet_consectetur_adipiscing_elit_sed_do_eiusmod_tempor_incididunt
+        (lorem: Lorem);
+
+    fn lorem<T: IpsumDolorSitAmetConsecteturAdipiscingElitSedDoEiusmodTemporIncididuntUtLaboreEt>(t: T);
+
+    fn lorem<T: IpsumDolorSitAmetConsecteturAdipiscingElitSedDoEiusmodTemporIncididuntUtLaboreEtDolore>
+    (
+        t: T,
+    );
 }

tests/source/configs/fn_params_layout/vertical_style_2027.rs

@@ -0,0 +1,39 @@
+// rustfmt-style_edition: 2027
+// rustfmt-fn_params_layout: Vertical
+// Function arguments density
+
+trait Lorem {
+    fn lorem(ipsum: Ipsum);
+
+    fn lorem(ipsum: Ipsum) -> Dolor;
+
+    fn lorem(ipsum: Ipsum, dolor: Dolor, sit: Sit, amet: Amet);
+
+    fn lorem(ipsum: Ipsum, dolor: Dolor, sit: Sit, amet: Amet) {
+        // body
+    }
+
+    fn lorem(ipsum: Ipsum, dolor: Dolor, sit: Sit, amet: Amet, consectetur: onsectetur, adipiscing: Adipiscing, elit: Elit);
+
+    fn lorem(ipsum: Ipsum, dolor: Dolor, sit: Sit, amet: Amet, consectetur: onsectetur, adipiscing: Adipiscing, elit: Elit) {
+        // body
+    }
+
+    fn long_param_name(lorem_ipsum_dolor_sit_amet_consectetur_adipiscing_elit_sed_do_eiusmod: Tempor);
+
+    fn long_param_type(lorem: IpsumDolorSitAmetConsecteturAdipiscingElitSedDoEiusmodTemporIncididuntUtLabore,
+    );
+
+    fn long_return_type(lorem: Lorem)
+        -> IpsumDolorSitAmetConsecteturAdipiscingElitSedDoEiusmodTemporIncididuntUtLabore;
+
+    fn lorem_ipsum_dolor_sit_amet_consectetur_adipiscing_elit_sed_do_eiusmod_tempor_incididunt
+        (lorem: Lorem);
+
+    fn lorem<T: IpsumDolorSitAmetConsecteturAdipiscingElitSedDoEiusmodTemporIncididuntUtLaboreEt>(t: T);
+
+    fn lorem<T: IpsumDolorSitAmetConsecteturAdipiscingElitSedDoEiusmodTemporIncididuntUtLaboreEtDolore>
+    (
+        t: T,
+    );
+}

tests/target/configs/fn_params_layout/vertical.rs

@@ -2,6 +2,10 @@
 // Function arguments density
 
 trait Lorem {
+    fn lorem(ipsum: Ipsum);
+
+    fn lorem(ipsum: Ipsum) -> Dolor;
+
     fn lorem(
         ipsum: Ipsum,
         dolor: Dolor,
@@ -39,4 +43,30 @@ trait Lorem {
     ) {
         // body
     }
+
+    fn long_param_name(
+        lorem_ipsum_dolor_sit_amet_consectetur_adipiscing_elit_sed_do_eiusmod: Tempor
+    );
+
+    fn long_param_type(
+        lorem: IpsumDolorSitAmetConsecteturAdipiscingElitSedDoEiusmodTemporIncididuntUtLabore
+    );
+
+    fn long_return_type(
+        lorem: Lorem
+    ) -> IpsumDolorSitAmetConsecteturAdipiscingElitSedDoEiusmodTemporIncididuntUtLabore;
+
+    fn lorem_ipsum_dolor_sit_amet_consectetur_adipiscing_elit_sed_do_eiusmod_tempor_incididunt(
+        lorem: Lorem
+    );
+
+    fn lorem<T: IpsumDolorSitAmetConsecteturAdipiscingElitSedDoEiusmodTemporIncididuntUtLaboreEt>(
+        t: T
+    );
+
+    fn lorem<
+        T: IpsumDolorSitAmetConsecteturAdipiscingElitSedDoEiusmodTemporIncididuntUtLaboreEtDolore,
+    >(
+        t: T
+    );
 }

tests/target/configs/fn_params_layout/vertical_style_2027.rs

@@ -0,0 +1,73 @@
+// rustfmt-style_edition: 2027
+// rustfmt-fn_params_layout: Vertical
+// Function arguments density
+
+trait Lorem {
+    fn lorem(ipsum: Ipsum);
+
+    fn lorem(ipsum: Ipsum) -> Dolor;
+
+    fn lorem(
+        ipsum: Ipsum,
+        dolor: Dolor,
+        sit: Sit,
+        amet: Amet,
+    );
+
+    fn lorem(
+        ipsum: Ipsum,
+        dolor: Dolor,
+        sit: Sit,
+        amet: Amet,
+    ) {
+        // body
+    }
+
+    fn lorem(
+        ipsum: Ipsum,
+        dolor: Dolor,
+        sit: Sit,
+        amet: Amet,
+        consectetur: onsectetur,
+        adipiscing: Adipiscing,
+        elit: Elit,
+    );
+
+    fn lorem(
+        ipsum: Ipsum,
+        dolor: Dolor,
+        sit: Sit,
+        amet: Amet,
+        consectetur: onsectetur,
+        adipiscing: Adipiscing,
+        elit: Elit,
+    ) {
+        // body
+    }
+
+    fn long_param_name(
+        lorem_ipsum_dolor_sit_amet_consectetur_adipiscing_elit_sed_do_eiusmod: Tempor,
+    );
+
+    fn long_param_type(
+        lorem: IpsumDolorSitAmetConsecteturAdipiscingElitSedDoEiusmodTemporIncididuntUtLabore,
+    );
+
+    fn long_return_type(
+        lorem: Lorem,
+    ) -> IpsumDolorSitAmetConsecteturAdipiscingElitSedDoEiusmodTemporIncididuntUtLabore;
+
+    fn lorem_ipsum_dolor_sit_amet_consectetur_adipiscing_elit_sed_do_eiusmod_tempor_incididunt(
+        lorem: Lorem,
+    );
+
+    fn lorem<T: IpsumDolorSitAmetConsecteturAdipiscingElitSedDoEiusmodTemporIncididuntUtLaboreEt>(
+        t: T,
+    );
+
+    fn lorem<
+        T: IpsumDolorSitAmetConsecteturAdipiscingElitSedDoEiusmodTemporIncididuntUtLaboreEtDolore,
+    >(
+        t: T,
+    );
+}