[wehrwolfmann]

As discussed in #1490, methods and invokables in an extern "RustQt" block
now follow the same rules as an extern "Rust" block in CXX: only the
unsafe on the fn itself is used. Previously a safe method inside an
unsafe extern "RustQt" block was declared as an unsafe fn for CXX
(unsafe_call in generator/rust/method.rs, from #1199), which silently
skipped the CXX check that a method taking a raw pointer must be unsafe.

The unsafe of the block is still used where it means something:
safe-to-call #[inherit] methods and #[inherit] signals, as they call into
the C++ base class (same as unsafe extern "C++" in CXX). The generation of
signals and inherit is unchanged.

Changes:

• cxx-qt-gen: remove ParsedMethod::unsafe_block and the forwarding in
  generate_rust_methods, update the expected outputs, and add an
  unsafe fn invokable taking a raw pointer to the invokables test
• book: new "Safety" section in extern_rustqt.md and a note in
  inheritance.md
• CHANGELOG: entry under Unreleased / Changed

This is a breaking change, only for safe methods in an
unsafe extern "RustQt" block:

• a method taking a raw pointer now fails with
  error[cxx]: pointer argument requires that the function be marked unsafe
• a method whose implementation is an unsafe fn now fails with
  error[E0133]: call to unsafe function ... is unsafe

To migrate, declare these methods as unsafe fn in the bridge. No example or
test in this repository needed changes, the existing methods taking raw
pointers (e.g. CustomParentClass::paint) are already unsafe fn.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (066d425) to head (d2f90bd).

Additional details and impacted files

@@            Coverage Diff            @@
##              main     #1500   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           75        75           
  Lines        13929     13921    -8     
=========================================
- Hits         13929     13921    -8     

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[ahayzen-kdab]

Are you able to rebase these changes when you have a chance :-)