Wrap long array and slice patterns.

Closes rust-lang#4530.

src/overflow.rs

@@ -77,6 +77,7 @@ pub(crate) enum OverflowableItem<'a> {
     FieldDef(&'a ast::FieldDef),
     TuplePatField(&'a TuplePatField<'a>),
     Ty(&'a ast::Ty),
+    Pat(&'a ast::Pat),
 }
 
 impl<'a> Rewrite for OverflowableItem<'a> {
@@ -116,6 +117,7 @@ impl<'a> OverflowableItem<'a> {
             OverflowableItem::FieldDef(sf) => f(*sf),
             OverflowableItem::TuplePatField(pat) => f(*pat),
             OverflowableItem::Ty(ty) => f(*ty),
+            OverflowableItem::Pat(pat) => f(*pat),
         }
     }
 
@@ -232,7 +234,7 @@ macro_rules! impl_into_overflowable_item_for_rustfmt_types {
     }
 }
 
-impl_into_overflowable_item_for_ast_node!(Expr, GenericParam, NestedMetaItem, FieldDef, Ty);
+impl_into_overflowable_item_for_ast_node!(Expr, GenericParam, NestedMetaItem, FieldDef, Ty, Pat);
 impl_into_overflowable_item_for_rustfmt_types!([MacroArg], [SegmentParam, TuplePatField]);
 
 pub(crate) fn into_overflowable_list<'a, T>(

src/patterns.rs

@@ -232,17 +232,15 @@ impl Rewrite for Pat {
             }
             PatKind::Lit(ref expr) => expr.rewrite(context, shape),
             PatKind::Slice(ref slice_pat) => {
-                let rw: Vec<String> = slice_pat
-                    .iter()
-                    .map(|p| {
-                        if let Some(rw) = p.rewrite(context, shape) {
-                            rw
-                        } else {
-                            context.snippet(p.span).to_string()
-                        }
-                    })
-                    .collect();
-                Some(format!("[{}]", rw.join(", ")))
+                overflow::rewrite_with_square_brackets(
+                    context,
+                    "",
+                    slice_pat.iter(),
+                    shape,
+                    self.span,
+                    None,
+                    None,
+                )
             }
             PatKind::Struct(ref qself, ref path, ref fields, ellipsis) => {
                 rewrite_struct_pat(qself, path, fields, ellipsis, self.span, context, shape)

tests/source/issue-4530.rs

@@ -0,0 +1,3 @@
+fn main() {
+    let [aaaaaaaaaaaaaaaaaaaaaaaaaa, bbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb, cccccccccccccccccccccccccc, ddddddddddddddddddddddddd] = panic!();
+}

tests/target/issue-4530.rs

@@ -0,0 +1,8 @@
+fn main() {
+    let [
+        aaaaaaaaaaaaaaaaaaaaaaaaaa,
+        bbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb,
+        cccccccccccccccccccccccccc,
+        ddddddddddddddddddddddddd,
+    ] = panic!();
+}