doc: document the custom compiler instrumentation flags in CONTRIBUTING.md (#7963)

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@ -165,3 +165,35 @@ which tests have failed and then fix them by making more changes. Just use
`git push pullrequest` to publish your changes. The CI tests will `git push pullrequest` to publish your changes. The CI tests will
run with your updated code. Use `hub ci-status --verbose` to monitor run with your updated code. Use `hub ci-status --verbose` to monitor
their status. their status.
## Flags
V allows you to pass custom flags using `-d my_flag` that can then be checked
at compile time (see the documentation about
[compile-time if](https://github.com/vlang/v/blob/master/doc/docs.md#compile-time-if)).
There are numerous flags that can be passed when building the compiler
with `v self` or when creating a copy of the compiler, that will help
you when debugging.
Beware that the flags must be passed when building the compiler,
not the program, so do for example: `v -d time_parsing cmd/v` or
`v -d trace_checker self`.
Some flags can make the compiler very verbose, so it is recommended
to create a copy of the compiler rather than replacing it with `v self`.
| Flag | Usage |
|------|-------|
| `debugautostr` | Prints informations about `.str()` method auto-generated by the compiler during C generation |
| `debugscanner` | Prints debug information during the scanning phase |
| `debug_codegen` | Prints automatically generated V code during the scanning phase |
| `debug_interface_table` | Prints generated interfaces during C generation |
| `debug_interface_type_implements` | Prints debug information when checking that a type implements in interface |
| `print_vweb_template_expansions` | Prints vweb compiled HTML files |
| `time_checking` | Prints the time spent checking files and other related informations |
| `time_parsing` | Prints the time spent parsing files and other related informations |
| `trace_ccoptions` | Prints options passed down to the C compiler |
| `trace_checker` | Prints informations about the statements being checked |
| `trace_gen` | Prints strings written to the generated C file. Beware, this flag is very verbose |
| `trace_parser` | Prints informations about parsed statements and expressions |
| `trace_thirdparty_obj_files` | Prints informations about built thirdparty obj files |
| `trace_use_cache` | Prints informations about cache use |