file-lines do not reformat parts of a multi-line statement

[athei]

Imagine the following statement in which one line is wrongly indented:

let diff = repo
.diff_tree_to_workdir(Some(&head), Some(&mut diff_options))
    .unwrap();

I now specify only the second line in my --file-lines.

What I expect to happen
The second line becomes correctly indented to match the same column as the .unwrap().

What actually happens
Nothing. The line is ignored because it is part of a larger statement.

Why this is a problem
I have a use case where a script uses git and --file-lines to only format lines which where changed in a specific commit. This script fails when someone just edits parts of a larger statement because the line is not formatted then. Adding more context to the range is also not viable because then lines are formatted that weren't touched in the commit.

#3397

[singron]

I noticed rustfmt refused to reindent a single line of a multiline match expression unless the --file-lines range included both the start of the match and the misindented line.

Here is a minimal reproduction:

fn main() {
    match 1 { // Line 2 is the start of the match
        1 => (),
        2 => {
let _ = (); // Line 5 isn't indented
        }
    }
}

# This should reindent line 5, but it does nothing
rustfmt +nightly --unstable-features --file-lines '[{"file": "a.rs", "range": [5, 5]}]' a.rs --check
# This does reindent line 5
rustfmt +nightly --unstable-features --file-lines '[{"file": "a.rs", "range": [2, 5]}]' a.rs --check

version rustfmt 1.4.12-nightly (9f53665f 2020-02-10)