`formatOnSave` with VSCode extension does not respect `ignore` config in rustfmt.toml

[BGR360]

Ran into this while working in the rustc repo. There are a number of paths in rustfmt.toml that are ignored, but if you open and then save any of them, then the rust-analyzer VSCode extension still formats the contents with rustfmt.

Minimal steps to reproduce

1. Create empty project with cargo new.

2. Write some poorly formatted code to src/main.rs:

    fn   main ()
    {
   println!("Hello, world!");
    }

3. Ignore src/main.rs in rustfmt.toml:

   ignore = [
       "src/main.rs",
   ]

4. Demonstrate that nightly rustfmt respects this configuration:

   $ cargo +nightly fmt -- src/main.rs
   $ cat src/main.rs
    fn   main ()
    {
   println!("Hello, world!");
    }
   

5. Enable formatOnSave using nightly rustfmt in one of two ways:

   Option 1: rust-toolchain

   In rust-toolchain:

   nightly
   

   In .vscode/settings.json:

   {
       "editor.formatOnSave": true
   }

   Option 2: overrideCommand

   In .vscode/settings.json:

   {
       "rust-analyzer.rustfmt.overrideCommand": [
           "/YOUR/HOME/.rustup/toolchains/nightly-YOUR-HOST-TRIPLE/bin/rustfmt"
       ],
       "editor.formatOnSave": true
   }

6. Open src/main.rs in VSCode and save.

Observed Behavior

The rust-analyzer extension formats the code in src/main.rs.

Expected behavior

The rust-analyzer extension respects the configuration in rustfmt.toml and
does not format the code in src/main.rs.

[jhgg]

So the root of the issue here is that we provide the file contents to rustfmt via stdin. And unfortunately, stdin does not respect ignore paths.

We could parse the rustfmt.toml ourselves, and decide whether or not the file should be ignored (basically emulate the simple logic here: https://github.com/elmarco/rustfmt/blob/master/src/ignore_path.rs#L22-L30

[Veykril]

It feels wrong to me that we need to parse(and find) the rustfmt.toml ourselves. Just like we don't want to parse(and find) the cargo config either. I wonder if it would make sense for rustfmt to accept a path via a flag and stdin input still, which then doesn't get formatted if the path matches the ignore set. Probably sounds too odd for the tool tho...

[lnicola]

If we can find rustfmt.toml, can't we use --config-path?

Beware rust-lang/rustfmt#4660.

[Veykril]

The problem is we tell rustfmt to format this specific file by just handing it over the file contents. Rustfmt has no idea about the path of the file we are formatting because we only give it the contents. As such it doesn't check(can't in fact) whether the file belongs to the ignored files.

[lnicola]

Ah, right 🤦. Even if it can read the config file, it won't know what file it's formatting.

[Veykril]

Opened rust-lang/rustfmt#5137

[sosthene-nitrokey]

This also applies if the module of the file was imported with a #[rustfmt::skip]:

src/lib.rs:

#[rustfmt::skip]
mod hello;

src/hello.rs:

 fn   world ()
 {
println!("Hello, world!");
 }

Saving in hello.rs will reformat the file though it shouldn't.