Tweak multispan rendering

Consider comments and bare delimiters the same as an "empty line" for purposes of hiding rendered code output of long multispans. This results in more aggressive shortening of rendered output without losing too much context, specially in `*.stderr` tests that have "hidden" comments.

Author: estebank

tests/ui-toml/excessive_nesting/excessive_nesting.stderr

@@ -66,8 +66,7 @@ error: this block is too nested
 LL |                   if true {
    |  _________________________^
 LL | |                     if true {
-LL | |
-LL | |                     }
+...  |
 LL | |                 }
    | |_________________^
    |

tests/ui/async_yields_async.stderr

@@ -80,7 +80,6 @@ error: an async construct yields a type which is itself awaitable
 LL |       let _m = async || {
    |  _______________________-
 LL | |         println!("I'm bored");
-LL | |         // Some more stuff
 ...  |
 LL | |         CustomFutureType
    | |         ^^^^^^^^^^^^^^^^

tests/ui/branches_sharing_code/shared_at_bottom.stderr

@@ -115,8 +115,7 @@ error: all if blocks contain the same code at the end
   --> tests/ui/branches_sharing_code/shared_at_bottom.rs:183:5
    |
 LL | /         x << 2
-LL | |
-LL | |
+...  |
 LL | |     };
    | |_____^
    |
@@ -131,8 +130,7 @@ error: all if blocks contain the same code at the end
   --> tests/ui/branches_sharing_code/shared_at_bottom.rs:192:5
    |
 LL | /         x * 4
-LL | |
-LL | |
+...  |
 LL | |     }
    | |_____^
    |

tests/ui/collapsible_else_if.stderr

@@ -43,7 +43,6 @@ LL |       } else {
    |  ____________^
 LL | |         if y == "world" {
 LL | |             println!("world")
-LL | |         }
 ...  |
 LL | |         }
 LL | |     }
@@ -66,7 +65,6 @@ LL |       } else {
    |  ____________^
 LL | |         if let Some(42) = Some(42) {
 LL | |             println!("world")
-LL | |         }
 ...  |
 LL | |         }
 LL | |     }
@@ -89,7 +87,6 @@ LL |       } else {
    |  ____________^
 LL | |         if let Some(42) = Some(42) {
 LL | |             println!("world")
-LL | |         }
 ...  |
 LL | |         }
 LL | |     }
@@ -112,7 +109,6 @@ LL |       } else {
    |  ____________^
 LL | |         if x == "hello" {
 LL | |             println!("world")
-LL | |         }
 ...  |
 LL | |         }
 LL | |     }
@@ -135,7 +131,6 @@ LL |       } else {
    |  ____________^
 LL | |         if let Some(42) = Some(42) {
 LL | |             println!("world")
-LL | |         }
 ...  |
 LL | |         }
 LL | |     }

tests/ui/crashes/ice-360.stderr

@@ -2,9 +2,6 @@ error: this loop never actually loops
   --> tests/ui/crashes/ice-360.rs:5:5
    |
 LL | /     loop {
-LL | |
-LL | |
-LL | |
 ...  |
 LL | |
 LL | |     }
@@ -16,9 +13,6 @@ error: this loop could be written as a `while let` loop
   --> tests/ui/crashes/ice-360.rs:5:5
    |
 LL | /     loop {
-LL | |
-LL | |
-LL | |
 ...  |
 LL | |
 LL | |     }

tests/ui/crate_level_checks/no_std_swap.stderr

@@ -2,8 +2,7 @@ error: this looks like you are trying to swap `a` and `b`
   --> tests/ui/crate_level_checks/no_std_swap.rs:12:5
    |
 LL | /     a = b;
-LL | |
-LL | |
+...  |
 LL | |     b = a;
    | |_________^ help: try: `core::mem::swap(&mut a, &mut b)`
    |

tests/ui/doc/unbalanced_ticks.stderr

@@ -3,9 +3,7 @@ error: backticks are unbalanced
    |
 LL |   /// This is a doc comment with `unbalanced_tick marks and several words that
    |  _____^
-LL | |
-LL | | /// should be `encompassed_by` tick marks because they `contain_underscores`.
-LL | | /// Because of the initial `unbalanced_tick` pair, the error message is
+...  |
 LL | | /// very `confusing_and_misleading`.
    | |____________________________________^
    |

tests/ui/empty_line_after/doc_comments.stderr

@@ -96,10 +96,7 @@ error: empty lines after doc comment
   --> tests/ui/empty_line_after/doc_comments.rs:63:5
    |
 LL | /     /// for OldA
-LL | |     // struct OldA;
-LL | |
-LL | |     /// Docs
-LL | |     /// for OldB
+...  |
 LL | |     // struct OldB;
 LL | |
    | |_^

tests/ui/empty_line_after/outer_attribute.stderr

@@ -103,8 +103,7 @@ error: empty lines after outer attribute
   --> tests/ui/empty_line_after/outer_attribute.rs:64:1
    |
 LL | / #[allow(unused)]
-LL | |
-LL | | // This comment is isolated
+...  |
 LL | |
    | |_^
 LL |   pub fn isolated_comment() {}

tests/ui/enum_variants.stderr

@@ -13,7 +13,6 @@ error: all variants have the same prefix: `c`
 LL | / enum Foo {
 LL | |
 LL | |     cFoo,
-LL | |
 ...  |
 LL | |     cBaz,
 LL | | }
@@ -45,7 +44,6 @@ error: all variants have the same prefix: `Food`
 LL | / enum Food {
 LL | |
 LL | |     FoodGood,
-LL | |
 ...  |
 LL | |
 LL | | }