Expand #line to support columns

The problem

The current restrictions of the #line pragma don’t bode well for embedded languages. For instance in Razor the core “transition into C#” mechanism has various implications on the #line pragma’s lacking support of a column. For instance, lets consider:

@DateTime.Now

At runtime this renders:

#line 1 "Foo.razor"

Write(DateTime.Now);

#line default

At design time this renders:

#line 1 "Foo.razor"

__o = DateTime.Now;

#line default

Now consider the implications of this at debug & edit time.

1. Errors. Any error that maps to DateTime.Now is instantly misaligned because @DateTime.Now exists at the front of the line.
2. Breakpoints. Any breakpoint can’t possibly understand how to bind to the original language document properly without understanding the difference in columns.

---

Suggestion:

• Allow line pragmas to provide a column & exist in the middle of expressions/statements and work properly at error/debug time (they currently don’t):

<p>The current time is: @DateTime.Now</p>

Write(
#line 1 25 "Foo.razor"
DateTime.Now
#line default
);

• OR allow a column, mapped offset and length to enable unmapped code to exist in the line pragma:

<p>The current time is: @DateTime.Now</p>

#line 1 25 6 12 "Foo.razor"
Write(DateTime.Now);
#line default

/cc @tmat

[tmat]

I’m working on a proposal for adding column to #line directive.

[tmat]

Yeah, I see. So the problem is really the placements of the sequence points if #line is in the middle of a statement. I guess we have two options in the compiler how to handle that - either we implement column mapping syntax or we change the handling of the #line directive when in the middle a statement. The former seems cleaner since the latter is gonna likely have a bunch of odd corner cases.