Account for multiple multiline spans with empty padding

Instead of

```
LL |    fn oom(
   |  __^
   | | _|
   | ||
LL | || ) {
   | ||_-
LL | |  }
   | |__^
```

emit

```
LL | // fn oom(
LL | || ) {
   | ||_-
LL | |  }
   | |__^
   ```

Author: estebank

compiler/rustc_errors/src/emitter.rs

@@ -845,18 +845,34 @@ impl EmitterWriter {
         // 3 | |
         // 4 | | }
         //   | |_^ test
-        if let [ann] = &line.annotations[..] {
+        let mut buffer_ops = vec![];
+        let mut annotations = vec![];
+        let mut short_start = true;
+        for ann in &line.annotations {
             if let AnnotationType::MultilineStart(depth) = ann.annotation_type {
                 if source_string.chars().take(ann.start_col).all(|c| c.is_whitespace()) {
                     let style = if ann.is_primary {
                         Style::UnderlinePrimary
                     } else {
                         Style::UnderlineSecondary
                     };
-                    buffer.putc(line_offset, width_offset + depth - 1, '/', style);
-                    return vec![(depth, style)];
+                    annotations.push((depth, style));
+                    buffer_ops.push((line_offset, width_offset + depth - 1, '/', style));
+                } else {
+                    short_start = false;
+                    break;
                 }
+            } else if let AnnotationType::MultilineLine(_) = ann.annotation_type {
+            } else {
+                short_start = false;
+                break;
+            }
+        }
+        if short_start {
+            for (y, x, c, s) in buffer_ops {
+                buffer.putc(y, x, c, s);
             }
+            return annotations;
         }
 
         // We want to display like this:

src/test/ui/alloc-error/alloc-error-handler-bad-signature-1.stderr

@@ -3,10 +3,7 @@ error[E0308]: mismatched types
    |
 LL |    #[alloc_error_handler]
    |    ---------------------- in this procedural macro expansion
-LL |    fn oom(
-   |  __^
-   | | _|
-   | ||
+LL | // fn oom(
 LL | ||     info: &Layout,
 LL | || ) -> ()
    | ||_______- arguments to this function are incorrect
@@ -29,10 +26,7 @@ error[E0308]: mismatched types
    |
 LL |    #[alloc_error_handler]
    |    ---------------------- in this procedural macro expansion
-LL |    fn oom(
-   |  __^
-   | | _|
-   | ||
+LL | // fn oom(
 LL | ||     info: &Layout,
 LL | || ) -> ()
    | ||_______^ expected `!`, found `()`

src/test/ui/alloc-error/alloc-error-handler-bad-signature-2.stderr

@@ -3,10 +3,7 @@ error[E0308]: mismatched types
    |
 LL |    #[alloc_error_handler]
    |    ---------------------- in this procedural macro expansion
-LL |    fn oom(
-   |  __^
-   | | _|
-   | ||
+LL | // fn oom(
 LL | ||     info: Layout,
 LL | || ) {
    | ||_- arguments to this function are incorrect
@@ -36,10 +33,7 @@ error[E0308]: mismatched types
    |
 LL |    #[alloc_error_handler]
    |    ---------------------- in this procedural macro expansion
-LL |    fn oom(
-   |  __^
-   | | _|
-   | ||
+LL | // fn oom(
 LL | ||     info: Layout,
 LL | || ) {
    | ||_^ expected `!`, found `()`

src/test/ui/asm/x86_64/interpolated-idents.stderr

@@ -30,12 +30,7 @@ error: asm outputs are not allowed with the `noreturn` option
 LL |               asm!("", $in(x) x, $out(x) x, $lateout(x) x, $inout(x) x, $inlateout(x) x,
    |                                  ^^^^^^^^^  ^^^^^^^^^^^^^  ^^^^^^^^^^^  ^^^^^^^^^^^^^^^
 ...
-LL |       m!(in out lateout inout inlateout const sym
-   |  _____-
-   | |_____|
-   | |_____|
-   | |_____|
-   | |
+LL | /     m!(in out lateout inout inlateout const sym
 LL | |        pure nomem readonly preserves_flags
 LL | |        noreturn nostack att_syntax options);
    | |                                           -

src/test/ui/issues/issue-13497-2.stderr

@@ -1,10 +1,7 @@
 error[E0515]: cannot return value referencing local variable `rawLines`
   --> $DIR/issue-13497-2.rs:3:5
    |
-LL |        rawLines
-   |  ______^
-   | | _____|
-   | ||
+LL | //     rawLines
 LL | ||         .iter().map(|l| l.trim()).collect()
    | ||_______________-___________________________^ returns a value referencing data owned by the current function
    |  |_______________|

src/test/ui/suggestions/issue-99240-2.stderr

@@ -4,10 +4,7 @@ error[E0618]: expected function, found enum variant `Alias::Unit`
 LL |        Unit,
    |        ---- enum variant `Alias::Unit` defined here
 ...
-LL |        Alias::
-   |  ______^
-   | | _____|
-   | ||
+LL | //     Alias::
 LL | ||     Unit();
    | ||________^_- call expression requires function
    |  |________|