IWYU (Include What You Use)

Which pragma should I use?

Use IWYU pragma: keep to force IWYU to keep any #include directive that would be discarded under its normal policies.
Use IWYU pragma: always_keep to force IWYU to keep a header in all includers, whether they contribute any used symbols or not.
Use IWYU pragma: export to tell IWYU that one header serves as the provider for all symbols in another, included header (e.g. facade headers). Use IWYU pragma: begin_exports/end_exports for a whole group of included headers.
Use IWYU pragma: no_include to tell IWYU that the file in which the pragma is defined should never #include a specific header (the header may already be included via some other #include.)
Use IWYU pragma: no_forward_declare to tell IWYU that the file in which the pragma is defined should never forward-declare a specific symbol (a forward declaration may already be available via some other #include.)
Use IWYU pragma: private to tell IWYU that the header in which the pragma is defined is private, and should not be included directly.
Use IWYU pragma: private, include "public.h" to tell IWYU that the header in which the pragma is defined is private, and public.h should always be included instead.
Use IWYU pragma: friend ".*favorites.*" to override IWYU pragma: private selectively, so that a set of files identified by a regex can include the file even if it's private.

The pragmas come in three different classes;

Ones that apply to a single #include directive (keep, export)
Ones that apply to a file being included (private, friend, always_keep)
Ones that apply to a file including other headers (no_include, no_forward_declare)

Some files are both included and include others, so it can make sense to mix and match.