extra: change XXX to FIXME and elaborate on comments
Testsuite: Test for #7013. Closes #7013
rustc: More helpful error message when using a struct type like a function

Closes #6702
testsuite: Test for #6132

Closes #6132
rustc: Remove FIXME (see #4949)
testsuite: Test for #4447

Closes #4447
testsuite: Un-xfail test for #5917
rustc: More helpful error message when using a struct type like a function

Closes #6702