[Issue 22691] New: [diagnostic] mismatched braces still painful

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=22691

          Issue ID: 22691
           Summary: [diagnostic] mismatched braces still painful
           Product: D
           Version: D2
          Hardware: All
                OS: All
            Status: NEW
          Severity: enhancement
          Priority: P1
         Component: dmd
          Assignee: nobody at puremagic.com
          Reporter: destructionator at gmail.com

Consider:

```
class A {
    void foo() {
        int b;
           void nested() {
               if(a) {
               }
               }
           }

           b += 5;
    }

    void bar() {

    }
}
```
error.d(10): Error: no identifier for declarator `b`
error.d(10): Error: declaration expected, not `+=`
error.d(16): Error: unrecognized declaration


The actual problem is on line 7. The first error issued is on line 10. In real
world code, there can be much, much, much more separation between the two. Even
if you recognize this from experience as a brace paste error and put it through
an auto formatter, you're still left eyeballing up for something that looks out
of place.

I'd recommend making the compiler remember the indentation level of an open
brace. If it sees a closing brace on a different level, make a note of that.
Don't need issue an error/warning yet, the compiler need not nag about style,
just remember it.

Then on the next error it actually does see, throw out the mismatched close as
a supplemental hint as a likely root cause to investigate. So here, it'd be
like

error.d(10): Error: no identifier for declarator `b`
      hint: closing brace on line 7 appears mismatched

--