rust private

NLL diagnostics: revise `fn check_access_permissions` NLL: revise `fn check_access_permissions` so that its (still branchy) shares more code paths between the different cases, and also provide more diagnostics in more cases (though the added diagnostics still do not always meet the quality bar established by AST-borrowck) ---- Transcribing "checklist" suggested by Niko, except I am rendering it as a table to make it clear that I do not regard every item in the list to be a "must have" for landing this PR. goal | does this PR do it? -----|------------------------------ no suggestions for `ref mut` | yes suggestions for direct local assignment (`{ let x = 3; x = 4; }`) | yes (see commits at end) suggestions for direct field assignment (`{ let x = (3, 4); x.0 = 5; }` | yes (see commits at end) suggestions for upvars (`let x = 3; let c = \|\| { &mut x; }`) | yes Note that I added support for a couple of rows via changes that are not strictly part of `fn check_access_permissions`. If desired I can remove those commits from this PR and leave them for a later PR. Fix #51031 Fix #51032 (bug #51191 needs a little more investigation before closing.) Fix #51578