--file-lines (range formatting) seems to be broken in a couple of ways

[sunshowers]

Hey -- I'm trying to integrate RLS and rustfmt with our IDE, and it looks like range formatting is broken in a couple of ways. Here's a repro with 19bd5ea, with these settings:

condense_wildcard_suffixes = true
reorder_imported_names = true
reorder_imports = true
reorder_imports_in_group = true
use_try_shorthand = true

1. Download lz4.rs from this gist somewhere.
2. rustfmt lz4.rs --file-lines '[{"file": "lz4.rs", "range": [20, 27]}]' --write-mode display
3. What you get back looks like lz4_output.rs.

Note a couple of things:

1. Imports were reordered even though none of the imports were in the range.
2. Line 32 got formatted even though it isn't in the range.

[nrc]

Looks like we are 'rounding up' to the whole module, rather than just the impl, so we end up reformatting a lot more than we should.

[sunshowers]

Hmm, I don't know much about rustfmt's internal logic, but would it make sense to round up to the surrounding block?

[nrc]

would it make sense to round up to the surrounding block?

Depends what you mean by block.

So, just for a bit of context: we have to parse the whole file, so when we are rounding, we are rounding AST nodes, not text or tokens. Currently Rustfmt can only operate on module-level items (functions, impls, etc) so we have to round up to the nearest one of those. (In fact, I think we may always round up to the nearest module, which would explain this bug). In the future we should be able to format items within traits/impls as well.