Feature request: Preserve diagnostic metadata

[mikesamuel]

The extractor should preserve diagnostic metadata so that stack traces point to the source markdown files instead of the extracted files.

When I run lit and look at the output I see

$ ./bin/lit --out-dir out examples/wc.lit
$ grep -n 'Header files to include' examples/wc.lit
37:@{Header files to include}
48:--- Header files to include
$ head -5 out/wc.c

/* wc.c */
/* Header files to include */
#include <stdio.h>
#include <fcntl.h>
#include <unistd.h>

but I would like head -6 out/wc.c to show something like

/* wc.c */
/* Header files to include */
#line 49 "examples/wc.lit"
#include <stdio.h>
#include <fcntl.h>
#include <unistd.h>

which uses the #line directive to specify the file and line for a section of generated code.

This is C preprocessor specific.
A solution that generalizes to other output languages might require a side-table of line numbers (maybe sourcemap format) that could feed into a language-specific post-processor that

• produces a JSR 000045 input for Java files
• fixes byte-code line mappings in .pyc files for python
• etc.