Make a trailing comma always force tall

At first I thought, “but a single-element collection should always fit,
right?” And sure it fits, but then I reformatted a larger experimental
config I have, including some GitHub Actions, and it turns out that
sometimes I prefer even single-element litst to be tall. Black is right
about this. I should be less opinionated, leave it to the user.

docs/rcl_format.md

@@ -59,10 +59,10 @@ possible to stay within the limit.
 
 The formatter is not sensitive to initial formatting, with the exception of
 trailing commas, to give some control over how collections are formatted.[^1]
-Collections with at least two elements that have a trailing comma will be
-formatted tall, even when they fit on a line. To format the collection wide,
-remove the trailing comma. This applies to any place where trailing commas are
-allowed, not just collection literals.
+Collections that have a trailing comma will be formatted tall, even when they
+fit on a line. To format the collection wide, remove the trailing comma. This
+applies to any place where trailing commas are allowed, not just collection
+literals.
 
 ```rcl
 // This collection is formatted wide.

examples/github_actions.rcl

@@ -2,7 +2,9 @@
   name = "Build",
 
   on = {
-    push = { branches = ["master"] },
+    push = {
+      branches = ["master"],
+    },
     workflow_dispatch = {},
   },
 

golden/fmt/list_comprehension.test

@@ -23,7 +23,7 @@ let double_comprehension = [
   h.frobnicated(),
 ];
 let b = [
-  1,
+  1
 ]; a +
 b
 

golden/fmt/magic_trailing_comma.test

@@ -1,12 +1,12 @@
-// RCL implements Black's "magic trailing comma" behavior for lists of length >= 2.
+// RCL implements Black's "magic trailing comma" behavior.
 // https://black.readthedocs.io/en/stable/the_black_code_style/current_style.html#the-magic-trailing-comma
-// If there is a trailing comma originally, and there are at least two elements,
-// then the output is tall regardless of whether it would fit wide. If there is
-// no trailing comma, the output is whatever fits.
+// If there is a trailing comma originally, then the output is tall regardless
+// regardless of whether it would fit wide. If there is no trailing comma, the
+// output is whatever fits.
 
-// Length 1, trailing comma is not relevant.
-let a = [1,];
-let b = [1];
+// Length 1.
+let a = [1];
+let b = [1,];
 let c = [
   1
 ];
@@ -14,7 +14,7 @@ let d = [
   1,
 ];
 
-// Length 2, trailing comma forces tall.
+// Length 2.
 let a = [1, 2];
 let b = [
   1, 2
@@ -25,7 +25,6 @@ let d = [
 ];
 
 // Sets.
-let z = {1,};
 let a = {1, 2};
 let b = {
   1, 2
@@ -36,7 +35,6 @@ let d = {
 };
 
 // Dicts.
-let z = {a=1,};
 let a = {a=1, b=2};
 let b = {
   a=1, b=2
@@ -47,7 +45,6 @@ let d = {
 };
 
 // Function calls.
-let z = f(1,);
 let a = f(1, 2);
 let b = f(
   1, 2
@@ -57,7 +54,7 @@ let d = f(
   1, 2,
 );
 
-// Function definitions.
+// Function definitions. Here a single argument is a special case.
 let z = (x,) => 1;
 let a = (x, y) => 1;
 let b = (
@@ -69,7 +66,6 @@ let d = (
 ) => 1;
 
 // Generic types.
-let z: List[Int,] = [];
 let a: List[Int, Int] = [];
 let b: List[
   Int, Int
@@ -80,7 +76,6 @@ let d: List[
 ] = [];
 
 // Function types.
-let z: (Int,) -> Int = null;
 let a: (Int, Int) -> Int = null;
 let b: (
   Int, Int
@@ -93,19 +88,23 @@ let d: (
 null
 
 # output:
-// RCL implements Black's "magic trailing comma" behavior for lists of length >= 2.
+// RCL implements Black's "magic trailing comma" behavior.
 // https://black.readthedocs.io/en/stable/the_black_code_style/current_style.html#the-magic-trailing-comma
-// If there is a trailing comma originally, and there are at least two elements,
-// then the output is tall regardless of whether it would fit wide. If there is
-// no trailing comma, the output is whatever fits.
+// If there is a trailing comma originally, then the output is tall regardless
+// regardless of whether it would fit wide. If there is no trailing comma, the
+// output is whatever fits.
 
-// Length 1, trailing comma is not relevant.
+// Length 1.
 let a = [1];
-let b = [1];
+let b = [
+  1,
+];
 let c = [1];
-let d = [1];
+let d = [
+  1,
+];
 
-// Length 2, trailing comma forces tall.
+// Length 2.
 let a = [1, 2];
 let b = [1, 2];
 let c = [
@@ -118,7 +117,6 @@ let d = [
 ];
 
 // Sets.
-let z = {1};
 let a = {1, 2};
 let b = {1, 2};
 let c = {
@@ -131,7 +129,6 @@ let d = {
 };
 
 // Dicts.
-let z = { a = 1 };
 let a = { a = 1, b = 2 };
 let b = { a = 1, b = 2 };
 let c = {
@@ -144,7 +141,6 @@ let d = {
 };
 
 // Function calls.
-let z = f(1);
 let a = f(1, 2);
 let b = f(1, 2);
 let c = f(
@@ -156,7 +152,7 @@ let d = f(
   2,
 );
 
-// Function definitions.
+// Function definitions. Here a single argument is a special case.
 let z = x => 1;
 let a = (x, y) => 1;
 let b = (x, y) => 1;
@@ -170,7 +166,6 @@ let d = (
 ) => 1;
 
 // Generic types.
-let z: List[Int] = [];
 let a: List[Int, Int] = [];
 let b: List[Int, Int] = [];
 let c: List[
@@ -183,7 +178,6 @@ let d: List[
 ] = [];
 
 // Function types.
-let z: (Int) -> Int = null;
 let a: (Int, Int) -> Int = null;
 let b: (Int, Int) -> Int = null;
 let c: (

src/fmt_cst.rs

@@ -67,14 +67,12 @@ impl<'a> Formatter<'a> {
     /// between the content and closing delimiter. But if we have no content
     /// (but possibly a suffix) then we need only one soft break.
     pub fn collection_opening_sep<T>(&self, list: &List<T>) -> Option<Doc<'a>> {
-        // When there is a list of at least two elements, and there is a trailing
-        // comma, then regardless of whether the list would fit in wide mode, we
-        // force it to be tall, to give the user some control over wide/tall.
-        // This is inspired by Black's "magic trailing comma":
+        // When there is a trailing comma, then regardless of whether the list
+        // would fit in wide mode, we force it to be tall, to give the user some
+        // control over wide/tall. This is inspired by Black's "magic trailing comma":
         // https://black.readthedocs.io/en/stable/the_black_code_style/current_style.html#the-magic-trailing-comma
         match list.elements.len() {
             0 => None,
-            1 => Some(Doc::SoftBreak),
             _ if list.trailing_comma => Some(Doc::HardBreak),
             _ => Some(Doc::SoftBreak),
         }