Alternative approach for constructing the AST and mod/file mapping

[calebcartwright]

There's been a couple issues of late with syntax/parse errors stemming from files that rustfmt was configured to ignore. The underlying cause has been traced to how the complete AST is constructed for the crate up front and the mod nodes in the AST are then associated to the corresponding files.

Based on discussion in some other threads, it should be possible to construct the AST and create the file/mod mapping via a different approach that addresses the issues encountered currently (and also avoids having to parse and construct an AST for files that rustfmt is just going to ignore anyway).

There's a lot more detail from @topecongiro on the underlying cause with the current approach and the new approach in #3920 (comment)

[calebcartwright]

One challenge we may run into with using the ignore list to conditionally construct the AST/file mapping for a crate would be rustfmt potentially ignoring unintentional/unanticipated files that were only imported in an ignored file.

For example:

lib.rs

mod foo;

foo.rs

#[path = "bar.rs"]
mod bar;

fn abc() {
    bar::def();
}

bar.rs

fn def() {
println!("foo bar");
}

and a rustfmt.toml config:

ignore = [ "foo.rs" ]

With this example, I believe the new approach for constructing the ast/file mapping with the current ignore setting would result in rustmt ignoring both bar.rs and foo.rs which I suspect could be confusing/surprising for some users.

That has me wondering if perhaps we should introduce a new config option to allow users to differentiate between files that rustfmt should ignore entirely (i.e. not even try to parse) vs. files that rustfmt can still parse (to be able to discover other mods/files that are imported) but should not format.

So with the above example, if the rustfmt config file was using the theoretical new config option:

parser-ignore = [ "foo.rs" ]

then neither foo.rs nor bar.rs would be formatted but with

ignore = [ "foo.rs" ]

bar.rs would still be formatted

Thoughts?