[NLL] Returns are interesting for free regions
Based on #53088 - creating now to get feedback.
Closes#51175
* Make assigning to the return type interesting.
* Use "returning this value" instead of "return" in error messages.
* Prefer one of the explanations that we have a name for to a generic interesting cause in some cases.
* Treat causes that involve the destination of a call like assignments.
compile-fail-fulldeps/proc-macro/proc-macro-attributes.rs - resolution change for derive helper attributes with the same name as derive itself
run-pass/macro-comma-support.rs - indeterminate resolutions for macros in expression positions
ui/issues/issue-49074.rs - diagnostics regression, not enough recovery to report the second error
ui/object-lifetime/object-lifetime-default.stderr - unstable diagnostics?
Rollup of 8 pull requests
Successful merges:
- #52453 (improve diagnostics for tests with custom return values)
- #53271 (use ? to simplify `TransitiveRelation.maybe_map`)
- #53279 (Extend documentation of `rustc_on_unimplemented`)
- #53342 (fix error for unsized packed struct field)
- #53344 (Add doc examples for std::alloc::{alloc,alloc_zeroed}.)
- #53368 (Ignore test that fails on stage1)
- #53388 (Fix links' color)
- #53396 (Fix since of Iterator::flatten to be a proper semver)
Failed merges:
r? @ghost
