This is an issue to track the functionality added by PR#60387, and also by extension this PR in cargo.

PR#60387 adds three options to rustdoc:

• --enable-per-target-ignores which adds a compiletest-like mechanism for ignoring platform on a substring basis (eg ignore-foo)
• --runtool and --runtool-arg for specifying an external program and its arguments, to which the compiled doctest binary will be passed as the final argument

The companion PR adds a flag to cargo for enabling this feature as well as cross-compiling doctests and parsing runtool tokens from a .cargo/config.

Eventually another PR for x.py can either enable this unconditionally or add a flag for testing the extensive documentation in the standard library on other platforms.

This solves cargo issue #6460