13 Read the source
Often when you encounter an error and you do not immediately understand why or what it is saying, the first thing you should try is to search for the error message. You can search the error directly on google, or search on specific sites like https://community.rstudio.com or https://stackoverflow.com.
Another option is to GitHub search to shed light on your problem.
13.1 GitHub search
GitHub allows you to search code, repositories, and issues which can often reveal useful insights into problems.
Doing a generic search is often fruitful, but you can often get more pertinent results with a more targeted approach.
13.2 Where things exist in the R source
The SVN repository used by the R core team to develop R is mirrored on GitHub by Winston Chang at https://github.com/wch/r-source. This means that all the code used by your local R session (including compiled code) is searchable.
The R source uses a complicated layout and contains the source of all the code in base R (src/main) as well as the set of packages included in base R, such as stats, graphics, utils and others (src/library/*). It also contains all of the documentation included in R including Writing R extensions, R internals and R admin guides (doc/manual).
Try the activity usethis::use_course("rstd.io/wtf-read-source")
to search the R source for code and documentation.
13.3 Where things exist in CRAN packages
You can find the development home of most R packages by looking at the URL field in the package DESCRIPTION, as can be seen on the CRAN landing page (e.g. devtools landing page). The BugReports field will give you a direct link to the issue page where you should report any issues found with the package.
In addition all code for CRAN packages is mirrored on GitHub by Gábor Csárdi at https://github.com/cran, which means all the code for CRAN packages is also searchable.
All R code in packages is kept in R/. In addition if the package is using roxygen the source code will also contain roxygen comments (#') with the function level documentation.
If a package is not using roxygen (often older packages or those in base R) the documentation will be in .Rd files in the man/ directory. (These files also exist in roxygen packages, but are generated automatically and should not be edited by hand).
If the package uses compiled code it will be in src/ regardless of what language the compiled code is written in.
Long-form documentation in the form of vignettes are stored in vignettes/.
Try the activity usethis::use_course("rstd.io/wtf-read-source")
to search and find source code and documentation in CRAN packages.