Put the opening brace of impl on the next line

We put the opening brace on the next line if the following conditions hold:

1. the result before '{' ends with comments or contains newline
2. the last line of the result before '{' is not extendable (i.e. consists of
   '>' and whitespaces).

src/items.rs

@@ -33,8 +33,8 @@ use types::join_bounds;
 use utils::{colon_spaces, contains_skip, first_line_width, format_abi, format_constness,
             format_defaultness, format_mutability, format_unsafety, format_visibility,
             is_attributes_extendable, last_line_contains_single_line_comment,
-            last_line_used_width, last_line_width, mk_sp, semicolon_for_expr, starts_with_newline,
-            stmt_expr, trimmed_last_line_width};
+            last_line_extendable, last_line_used_width, last_line_width, mk_sp,
+            semicolon_for_expr, starts_with_newline, stmt_expr, trimmed_last_line_width};
 use vertical::rewrite_with_alignment;
 use visitor::FmtVisitor;
 
@@ -639,8 +639,10 @@ pub fn format_impl(
         }
         result.push_str(&where_clause_str);
 
+        let need_newline = !last_line_extendable(&result)
+            && (last_line_contains_single_line_comment(&result) || result.contains('\n'));
         match context.config.brace_style() {
-            _ if last_line_contains_single_line_comment(&result) => result.push_str(&sep),
+            _ if need_newline => result.push_str(&sep),
             BraceStyle::AlwaysNextLine => result.push_str(&sep),
             BraceStyle::PreferSameLine => result.push(' '),
             BraceStyle::SameLineWhere => {

src/utils.rs

@@ -178,7 +178,7 @@ pub fn last_line_extendable(s: &str) -> bool {
     }
     for c in s.chars().rev() {
         match c {
-            ')' | ']' | '}' | '?' => continue,
+            ')' | ']' | '}' | '?' | '>' => continue,
             '\n' => break,
             _ if c.is_whitespace() => continue,
             _ => return false,

tests/target/trait.rs

@@ -103,14 +103,13 @@ trait Foo {
 // #2331
 trait MyTrait<
     AAAAAAAAAAAAAAAAAAAA,
-BBBBBBBBBBBBBBBBBBBB,
-CCCCCCCCCCCCCCCCCCCC,
-DDDDDDDDDDDDDDDDDDDD,
+    BBBBBBBBBBBBBBBBBBBB,
+    CCCCCCCCCCCCCCCCCCCC,
+    DDDDDDDDDDDDDDDDDDDD,
 > {
     fn foo() {}
 }
 
-
 // Trait aliases
 trait FooBar = Foo + Bar;
 trait FooBar<A, B, C> = Foo + Bar;