Ensure that fn config_path returns canonicalized paths.

[anforowicz]

This commit ensures that fn config_path in rustfmt/src/config/mod.rs always returns canonicalized paths. This is achieved by canonicalizing both: 1) paths received via CliOptions (after checking if the path exists) as well as 2) paths returned via get_toml_path (canonicalizing within fn get_toml_path).

Fixes #6178

[ytmimi]

Thanks for the PR. Would you mind double checking if there's any difference in the output when running rustfmt -v path/to/file.rs on windows before and after this change if the paths are canonicalized. I think we might already be canonicalizing them before we display the paths, but I want to double check.

Output should look something like this:

Using rustfmt config file /path/to/rustfmt.toml for /path/to/file.rs
Formatting /path/to/file.rs
Spent 0.000 secs in the parsing phase, and 0.001 secs in the formatting phase

[anforowicz]

Would you mind double checking if there's any difference in the output when running rustfmt -v path/to/file.rs on windows before and after this change if the paths are canonicalized. I think we might already be canonicalizing them before we display the paths, but I want to double check.

There are no output changes in this scenario.

Before the changes:

C:\src\rustfmt>git checkout origin/master
C:\src\rustfmt>cargo run --bin rustfmt -- -v path/to/file.rs
   Compiling rustfmt-nightly v1.7.1 (C:\src\rustfmt)
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 9.50s
     Running `target\debug\rustfmt.exe -v path/to/file.rs`
Using rustfmt config file \\?\C:\src\rustfmt\rustfmt.toml for \\?\C:\src\rustfmt\path\to\file.rs
Formatting \\?\C:\src\rustfmt\path\to\file.rs
Spent 0.012 secs in the parsing phase, and 0.002 secs in the formatting phase

After the changes:

C:\src\rustfmt>git checkout path-canon
C:\src\rustfmt>cargo run --bin rustfmt -- -v path/to/file.rs
   Compiling rustfmt-nightly v1.7.1 (C:\src\rustfmt)
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 8.84s
     Running `target\debug\rustfmt.exe -v path/to/file.rs`
Using rustfmt config file \\?\C:\src\rustfmt\rustfmt.toml for \\?\C:\src\rustfmt\path\to\file.rs
Formatting \\?\C:\src\rustfmt\path\to\file.rs
Spent 0.012 secs in the parsing phase, and 0.001 secs in the formatting phase

---

As expected the path passed to --config-path changes (gets canonicalized) after the changes.

Before the changes:

C:\src\rustfmt>git checkout origin/master
C:\src\rustfmt>cargo run --bin rustfmt -- -v --config-path=c:\test\rustfmt.toml c:\test\bar.rs
   Compiling rustfmt-nightly v1.7.1 (C:\src\rustfmt)
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 8.52s
     Running `target\debug\rustfmt.exe -v --config-path=c:\test\rustfmt.toml c:\test\bar.rs`
Using rustfmt config file c:\test\rustfmt.toml
thread 'main' panicked at C:\Users\lukasza\.cargo\registry\src\index.crates.io-6f17d22bba15001f\ignore-0.4.18\src\gitignore.rs:227:9:
path is expected to be under the root
...

After the changes:

C:\src\rustfmt>git checkout path-canon
C:\src\rustfmt>cargo run --bin rustfmt -- -v --config-path=c:\test\rustfmt.toml c:\test\bar.rs
   Compiling rustfmt-nightly v1.7.1 (C:\src\rustfmt)
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 8.44s
     Running `target\debug\rustfmt.exe -v --config-path=c:\test\rustfmt.toml c:\test\bar.rs`
Using rustfmt config file \\?\C:\test\rustfmt.toml
Formatting \\?\C:\test\bar.rs
Spent 0.010 secs in the parsing phase, and 0.005 secs in the formatting phase

[ytmimi]

Thanks for double checking. I figured that the behavior would change slightly and wanted to confirm that.

Edit: Diff Check Job passed ✅