Rustc nightly points out a wrong part of line as an error

[maekawatoshiki]

I tried this code (playground):

use std::path::{Path, PathBuf};

fn func(path: impl Into<PathBuf>, code: impl Into<String>) {}

fn main() {
    func(Path::new("hello").to_path_buf().to_string_lossy(), "world")
}

and rustc shows the following error:

error[E0277]: the trait bound `PathBuf: From<Cow<'_, str>>` is not satisfied
 --> src/main.rs:6:62
  |
6 |     func(Path::new("hello").to_path_buf().to_string_lossy(), "world")
  |     ---- required by a bound introduced by this call         ^^^^^^^ the trait `From<Cow<'_, str>>` is not implemented for `PathBuf`
  |
  = help: the following implementations were found:
            <PathBuf as From<&T>>
            <PathBuf as From<Box<Path>>>
            <PathBuf as From<Cow<'a, Path>>>
            <PathBuf as From<OsString>>
            <PathBuf as From<String>>
  = note: required because of the requirements on the impl of `Into<PathBuf>` for `Cow<'_, str>`
note: required by a bound in `func`
 --> src/main.rs:3:20
  |
3 | fn func(path: impl Into<PathBuf>, code: impl Into<String>) {}
  |                    ^^^^^^^^^^^^^ required by this bound in `func`

For more information about this error, try `rustc --explain E0277`.
error: could not compile `playground` due to previous error

Rustc says that "world" at line 6 is causing the error, but actually it is not.
I think Rustc should point out Path::new("hello").to_path_buf().to_string_lossy() instead.

Rustc version

# rustc --version --verbose
rustc 1.58.0-nightly (1af55d19c 2021-10-19)
binary: rustc
commit-hash: 1af55d19c7a9189374d89472f97dc119659bb67e
commit-date: 2021-10-19
host: x86_64-unknown-linux-gnu
release: 1.58.0-nightly
LLVM version: 13.0.0

[apiraino]

Assigning priority as discussed in the Zulip thread of the Prioritization Working Group.

@rustbot label -I-prioritize +P-medium

[hkmatsumoto]

I did a bisection and here is the result.
searched nightlies: from nightly-2021-08-01 to nightly-2021-10-20
regressed nightly: nightly-2021-09-18
searched commit range: e4828d5...9dd4ce8
regressed commit: e366210
regressed PR: #88719
cc @estebank

bisected with cargo-bisect-rustc v0.6.1

Host triple: x86_64-unknown-linux-gnu
Reproduce with:

cargo bisect-rustc --prompt --test-dir=. --start=2021-08-01 --end=2021-10-20 

[hkmatsumoto]

I feel the span of the diagnostic before the regression is also erroneous.

error[E0277]: the trait bound `PathBuf: From<Cow<'_, str>>` is not satisfied
 --> src/lib.rs:6:5
  |
6 |     func(Path::new("hello").to_path_buf().to_string_lossy(), "world")
  |     ^^^^ the trait `From<Cow<'_, str>>` is not implemented for `PathBuf`
  |
  = help: the following implementations were found:
            <PathBuf as From<&T>>
            <PathBuf as From<Box<Path>>>
            <PathBuf as From<Cow<'a, Path>>>
            <PathBuf as From<OsString>>
            <PathBuf as From<String>>
  = note: required because of the requirements on the impl of `Into<PathBuf>` for `Cow<'_, str>`
note: required by a bound in `func`
 --> src/lib.rs:3:20
  |
3 | fn func(path: impl Into<PathBuf>, code: impl Into<String>) {}
  |                    ^^^^^^^^^^^^^ required by this bound in `func`

Should ideally be something like below:

error[E0277]: the trait bound `PathBuf: From<Cow<'_, str>>` is not satisfied
 --> src/lib.rs:6:5
  |
6 |     func(Path::new("hello").to_path_buf().to_string_lossy(), "world")
  |          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `From<Cow<'_, str>>` is not implemented for `PathBuf`
  |
  = help: the following implementations were found:
            <PathBuf as From<&T>>
            <PathBuf as From<Box<Path>>>
            <PathBuf as From<Cow<'a, Path>>>
            <PathBuf as From<OsString>>
            <PathBuf as From<String>>
  = note: required because of the requirements on the impl of `Into<PathBuf>` for `Cow<'_, str>`
note: required by a bound in `func`
 --> src/lib.rs:3:20
  |
3 | fn func(path: impl Into<PathBuf>, code: impl Into<String>) {}
  |                    ^^^^^^^^^^^^^ required by this bound in `func`