Inspect does not feed properly into map

[Xavientois]

I tried this code:

pub fn compile<S>(filenames: &[S]) -> String
  where
      S: AsRef<str> + AsRef<Path> + Display,
  {
      let source_strings: Vec<_> = filenames
          .iter()
          .inspect(file::validate_filename)
          .map(file::open_file)
          .map(byte_reader::read_bytes)
          .collect();

     // -- snip
  }

I expected to see this happen: For the code to compile, and for the inspect to not break the iterator like it does. It works when I write it like this:

pub fn compile<S>(filenames: &[S]) -> String
  where
      S: AsRef<str> + AsRef<Path> + Display,
  {
      let source_strings: Vec<_> = filenames
          .iter()
          .inspect(|f| file::validate_filename(f))
          .map(file::open_file)
          .map(byte_reader::read_bytes)
          .collect();

     // -- snip
  }

Instead, this happened: It gives me a compiler error:

error[E0599]: no method named `map` found for struct `Inspect<std::slice::Iter<'_, S>, fn(&&S) {validate_filename::<&&S>}>` in the current scope
    --> src/compile.rs:12:10
     |
12   |         .map(file::open_file)
     |          ^^^ method not found in `Inspect<std::slice::Iter<'_, S>, fn(&&S) {validate_filename::<&&S>}>`
     |
    ::: /Users/rrampersad/.rustup/toolchains/stable-x86_64-apple-darwin/lib/rustlib/src/rust/library/core/src/iter/adapters/mod.rs:2743:1
     |
2743 | pub struct Inspect<I, F> {
     | ------------------------ doesn't satisfy `_: Iterator`
     |
     = note: the method `map` exists but the following trait bounds were not satisfied:
             `<fn(&&S) {validate_filename::<&&S>} as FnOnce<(&&S,)>>::Output = ()`
             which is required by `Inspect<std::slice::Iter<'_, S>, fn(&&S) {validate_filename::<&&S>}>: Iterator`
             `fn(&&S) {validate_filename::<&&S>}: FnMut<(&&S,)>`
             which is required by `Inspect<std::slice::Iter<'_, S>, fn(&&S) {validate_filename::<&&S>}>: Iterator`
             `Inspect<std::slice::Iter<'_, S>, fn(&&S) {validate_filename::<&&S>}>: Iterator`
             which is required by `&mut Inspect<std::slice::Iter<'_, S>, fn(&&S) {validate_filename::<&&S>}>: Iterator`

error[E0599]: no method named `map` found for struct `Inspect<std::slice::Iter<'_, S>, fn(&&S) {validate_filename::<&&S>}>` in the current scope
    --> src/compile.rs:12:10
     |
12   |         .map(file::open_file)
     |          ^^^ method not found in `Inspect<std::slice::Iter<'_, S>, fn(&&S) {validate_filename::<&&S>}>`
     |
    ::: /Users/rrampersad/.rustup/toolchains/stable-x86_64-apple-darwin/lib/rustlib/src/rust/library/core/src/iter/adapters/mod.rs:2743:1
     |
2743 | pub struct Inspect<I, F> {
     | ------------------------ doesn't satisfy `_: Iterator`
     |
     = note: the method `map` exists but the following trait bounds were not satisfied:
             `<fn(&&S) {validate_filename::<&&S>} as FnOnce<(&&S,)>>::Output = ()`
             which is required by `Inspect<std::slice::Iter<'_, S>, fn(&&S) {validate_filename::<&&S>}>: Iterator`
             `fn(&&S) {validate_filename::<&&S>}: FnMut<(&&S,)>`
             which is required by `Inspect<std::slice::Iter<'_, S>, fn(&&S) {validate_filename::<&&S>}>: Iterator`
             `Inspect<std::slice::Iter<'_, S>, fn(&&S) {validate_filename::<&&S>}>: Iterator`
             which is required by `&mut Inspect<std::slice::Iter<'_, S>, fn(&&S) {validate_filename::<&&S>}>: Iterator`

error: aborting due to previous error

For more information about this error, try `rustc --explain E0599`.
error: aborting due to previous error

For more information about this error, try `rustc --explain E0599`.
error: could not compile `joos_1w_compiler`

To learn more, run the command again with --verbose.

Meta

rustc --version --verbose:

rustc 1.49.0 (e1884a8e3 2020-12-29)
binary: rustc
commit-hash: e1884a8e3c3e813aada8254edfa120e85bf5ffca
commit-date: 2020-12-29
host: x86_64-apple-darwin
release: 1.49.0

Signature of `validate_filename`

``` pub fn validate_filename(filename: S) where S: AsRef + Display, { ... } ```

[SNCPlay42]

Reduced:

fn validate_filename<S>(_: S) {}

fn main() {
    let _: Vec<_> = Some(()).into_iter().inspect(validate_filename).collect();
}

If I change it to

fn validate_filename<S>(_: S) {}

fn should_be_iterator<I: Iterator>(_: I) {}

fn main() {
    should_be_iterator(Some(()).into_iter().inspect(validate_filename));
}

I get this output:

error[E0308]: mismatched types
 --> src/main.rs:6:45
  |
6 |     should_be_iterator(Some(()).into_iter().inspect(validate_filename));
  |                                             ^^^^^^^ one type is more general than the other
  |
  = note: expected type `FnOnce<(&(),)>`
             found type `FnOnce<(&(),)>`

So I think this is related to/another version of #75791.

@rustbot label -E-needs-mcve