panic::Location Debug implementation now prints a byte array

[tgross35]

Since #135054, the Debug implementation for std::panic::Location just prints the byte bag:

fn main() {
    dbg!(std::panic::Location::caller());
}

Stable:

[src/main.rs:2:5] std::panic::Location::caller() = Location {
    file: "src/main.rs",
    line: 2,
    col: 10,
}

Nightly:

[src/main.rs:2:5] std::panic::Location::caller() = Location {
    file_bytes_with_nul: [
        115,
        114,
        99,
        47,
        109,
        97,
        105,
        110,
        46,
        114,
        115,
        0,
    ],
    line: 2,
    col: 10,
}

https://play.rust-lang.org/?version=nightly&mode=debug&edition=2024&gist=698066c5427dd43cb12df0d2fb5b9300

Originally noticed at #142216.

[tgross35]

@cramertj is there a reason file_bytes_with_nul wasn't made a CStr in #135054? Other than .file() becoming O(n) once it becomes a thin pointer.

Might just need a handwritten debug implementation otherwise.

[tgross35]

Cc @m-ou-se from the review ^

[nealsid]

FYI, I was continuing to work on #142216 and wanted to make proposals based on after this change, so I merged it in, and, this might be a small issue but I noticed that backslashes in the path are printed twice on Windows (this has debug_refcell enabled in library/core):

PS C:\Users\neals\refcell-test> cargo +stage1 clean ; cargo +stage1 run
     Removed 28 files, 4.4MiB total
   Compiling refcell-test v0.1.0 (C:\Users\neals\refcell-test)
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.39s
     Running `target\debug\refcell-test.exe`
Hello, world! RefCell { value: 5 }

thread 'main' panicked at src\main.rs:10:15:
already borrowed: BorrowMutError { location: Location { file: "src\\main.rs", line: 7, column: 15 } }
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
error: process didn't exit successfully: `target\debug\refcell-test.exe` (exit code: 101)
PS C:\Users\neals\refcell-test> cargo +stage1 clean ; cargo +stage1 run

Without this change at HEAD, if you look at the bytes printed there is one backslash (92) in the byte array:

PS C:\Users\neals\refcell-test> cargo +stage1 clean ; cargo +stage1 run
     Removed 28 files, 4.4MiB total
   Compiling refcell-test v0.1.0 (C:\Users\neals\refcell-test)
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.61s
     Running `target\debug\refcell-test.exe`
Hello, world! RefCell { value: 5 }

thread 'main' panicked at src\main.rs:10:15:
already borrowed: BorrowMutError { location: Location { file_bytes_with_nul: [115, 114, 99, 92, 109, 97, 105, 110, 46, 114, 115, 0], line: 7, col: 15 } }
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
error: process didn't exit successfully: `target\debug\refcell-test.exe` (exit code: 101)

[tgross35]

@nealsid isn't that the same way it printed before the regression to printing bytes?

Usually the Debug implementations are intended so you could copy+paste them to source, which means strings are escaped. The Display implementation shouldn't have this.

[nealsid]

@tgross35 you're right, building at 61413ae (the parent of the commit that caused the regression) shows that it was escaped previously as well. Thanks.