rustfmt silently removes code it can't parse

[fogti]

Describe the bug

Currently, when rustfmt tries to format code which e.g. uses feature(generic_associated_types), rustfmt produces no error messages, but removes code which causes previously working examples to fail compiling after formatting using rustfmt.

To Reproduce

The following diff describes the change currently performed by rustfmt:

 /// Helper trait for "stripping indention" from an object reference
 pub trait AsUnindented {
-    type Output<'ast> where Self: 'ast;
+    type Output<'ast>;
 
     /// Returns a reference to the unindented part of `Self`
     fn as_unindented<'ast>(&'ast self) -> Self::Output<'ast>;
 }

Expected behavior

Rustfmt should fail with either "Error writing files: failed to resolve mod ...: cannot parse /...rs", or a more concise error message;
and skip the offending modules (not modifying them, except maybe removing simple stuff like trailing whitespace and double-newlines, which don't require fully parsed code) and all submodules of them.

Meta

• rustfmt version: rustfmt 1.4.18-nightly (c1e9b7b 2020-06-13)
• From where did you install rustfmt?: rustup
• How do you run rustfmt: cargo fmt

[ayazhafiz]

rustfmt should bail on non-parseable inputs today; for example,

fn f( {
}

fails quickly with a number of rustc parser error messages. The problem here is probably that rustfmt just isn't try to format the generic bound of the associated type.

[ayazhafiz]

I think this was fixed (but not yet released) in https://github.com/rust-lang/rustfmt/pull/4106/files#diff-d5afe6fbd84797013d4e4b0fc11d83e9R540-R541 but we don't seem to have any tests for it