Adds conditional flag for unix vs. Windows file metadata

[quietlychris]

This is a preliminary fix for #10 by adding a conditional compilation flag for the target_family between unix and windows. Please don't merge until it's been verified that this actually fixes the issue on Windows (another contributor has volunteered to do that on their machine)

[relf]

Feedback on this PR when compiling on Windows

(base) D:\rlafage\workspace\mnist>cargo build --features download
   Compiling mnist v0.5.0 (D:\rlafage\workspace\mnist)
warning: unused import: `std::os::windows::fs::MetadataExt`
  --> src\download.rs:89:17
   |
89 |             use std::os::windows::fs::MetadataExt;
   |                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   |
   = note: `#[warn(unused_imports)]` on by default

warning: use of deprecated function `std::thread::sleep_ms`: replaced by `std::thread::sleep`
   --> src\download.rs:103:29
    |
103 |                     thread::sleep_ms(10);
    |                             ^^^^^^^^
    |
    = note: `#[warn(deprecated)]` on by default

error[E0599]: no method named `size` found for struct `Metadata` in the current scope
   --> src\download.rs:101:41
    |
101 |                     current_size = meta.size() as usize;
    |                                         ^^^^ method not found in `Metadata`

For more information about this error, try `rustc --explain E0599`.
warning: `mnist` (lib) generated 2 warnings
error: could not compile `mnist` due to previous error; 2 warnings emitted

If I remove the download feature, I get:

(base) D:\rlafage\workspace\mnist>cargo build
   Compiling mnist v0.5.0 (D:\rlafage\workspace\mnist)
warning: unused variable: `base_url`
   --> src\lib.rs:404:17
    |
404 |             let base_url = if self.use_fashion_data {
    |                 ^^^^^^^^ help: if this is intentional, prefix it with an underscore: `_base_url`
    |
    = note: `#[warn(unused_variables)]` on by default

warning: panic message is not a string literal
   --> src\lib.rs:433:13
    |
433 | /             format!(
434 | |                 "Total data set length ({}) greater than maximum possible length ({}).",
435 | |                 total_length, available_length
436 | |             )
    | |_____________^
    |
    = note: `#[warn(non_fmt_panics)]` on by default
    = note: this usage of assert!() is deprecated; it will be a hard error in Rust 2021
    = note: for more information, see <https://doc.rust-lang.org/nightly/edition-guide/rust-2021/panic-macro-consistency.html>
    = note: the assert!() macro supports formatting, so there's no need for the format!() macro here
help: remove the `format!(..)` macro call
    |
433 ~
434 |                 "Total data set length ({}) greater than maximum possible length ({}).",
435 |                 total_length, available_length
436 ~
    |

warning: panic message is not a string literal
   --> src\lib.rs:562:9
    |
562 | /         format!(
563 | |             "Expected magic number {} got {}.",
564 | |             LBL_MAGIC_NUMBER, magic_number
565 | |         )
    | |_________^
    |
    = note: this usage of assert!() is deprecated; it will be a hard error in Rust 2021
    = note: for more information, see <https://doc.rust-lang.org/nightly/edition-guide/rust-2021/panic-macro-consistency.html>
    = note: the assert!() macro supports formatting, so there's no need for the format!() macro here
help: remove the `format!(..)` macro call
    |
562 ~
563 |             "Expected magic number {} got {}.",
564 |             LBL_MAGIC_NUMBER, magic_number
565 ~
    |

warning: panic message is not a string literal
   --> src\lib.rs:572:9
    |
572 | /         format!(
573 | |             "Expected data set length of {} got {}.",
574 | |             expected_length, length
575 | |         )
    | |_________^
    |
    = note: this usage of assert!() is deprecated; it will be a hard error in Rust 2021
    = note: for more information, see <https://doc.rust-lang.org/nightly/edition-guide/rust-2021/panic-macro-consistency.html>
    = note: the assert!() macro supports formatting, so there's no need for the format!() macro here
help: remove the `format!(..)` macro call
    |
572 ~
573 |             "Expected data set length of {} got {}.",
574 |             expected_length, length
575 ~
    |

warning: panic message is not a string literal
   --> src\lib.rs:599:9
    |
599 | /         format!(
600 | |             "Expected magic number {} got {}.",
601 | |             IMG_MAGIC_NUMBER, magic_number
602 | |         )
    | |_________^
    |
    = note: this usage of assert!() is deprecated; it will be a hard error in Rust 2021
    = note: for more information, see <https://doc.rust-lang.org/nightly/edition-guide/rust-2021/panic-macro-consistency.html>
    = note: the assert!() macro supports formatting, so there's no need for the format!() macro here
help: remove the `format!(..)` macro call
    |
599 ~
600 |             "Expected magic number {} got {}.",
601 |             IMG_MAGIC_NUMBER, magic_number
602 ~
    |

warning: panic message is not a string literal
   --> src\lib.rs:609:9
    |
609 | /         format!(
610 | |             "Expected data set length of {} got {}.",
611 | |             expected_length, length
612 | |         )
    | |_________^
    |
    = note: this usage of assert!() is deprecated; it will be a hard error in Rust 2021
    = note: for more information, see <https://doc.rust-lang.org/nightly/edition-guide/rust-2021/panic-macro-consistency.html>
    = note: the assert!() macro supports formatting, so there's no need for the format!() macro here
help: remove the `format!(..)` macro call
    |
609 ~
610 |             "Expected data set length of {} got {}.",
611 |             expected_length, length
612 ~
    |

warning: panic message is not a string literal
   --> src\lib.rs:620:9
    |
620 |         format!("Expected rows length of {} got {}.", ROWS, rows)
    |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |
    = note: this usage of assert!() is deprecated; it will be a hard error in Rust 2021
    = note: for more information, see <https://doc.rust-lang.org/nightly/edition-guide/rust-2021/panic-macro-consistency.html>
    = note: the assert!() macro supports formatting, so there's no need for the format!() macro here
help: remove the `format!(..)` macro call
    |
620 -         format!("Expected rows length of {} got {}.", ROWS, rows)
620 +         "Expected rows length of {} got {}.", ROWS, rows
    |

warning: panic message is not a string literal
   --> src\lib.rs:628:9
    |
628 |         format!("Expected cols length of {} got {}.", COLS, cols)
    |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |
    = note: this usage of assert!() is deprecated; it will be a hard error in Rust 2021
    = note: for more information, see <https://doc.rust-lang.org/nightly/edition-guide/rust-2021/panic-macro-consistency.html>
    = note: the assert!() macro supports formatting, so there's no need for the format!() macro here
help: remove the `format!(..)` macro call
    |
628 -         format!("Expected cols length of {} got {}.", COLS, cols)
628 +         "Expected cols length of {} got {}.", COLS, cols
    |

warning: unused return value of `Vec::<T, A>::split_off` that must be used
   --> src\lib.rs:460:9
    |
460 |         tst_img.split_off(tst_len * ROWS * COLS);
    |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |
    = note: `#[warn(unused_must_use)]` on by default
    = note: use `.truncate()` if you don't need the other half

warning: unused return value of `Vec::<T, A>::split_off` that must be used
   --> src\lib.rs:461:9
    |
461 |         tst_lbl.split_off(tst_len);
    |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |
    = note: use `.truncate()` if you don't need the other half

warning: `mnist` (lib) generated 10 warnings
    Finished dev [unoptimized + debuginfo] target(s) in 0.40s

[quietlychris]

Okay, maybe this one will fix it? I think Windows uses file_size() for metadata, not simply size(). I should really buy a cheap Windows machine at some point just to have one around...

[relf]

Unfortunatly I still get an error, now I get:

(base) D:\rlafage\workspace\mnist>cargo build --features download
   Compiling mnist v0.5.0 (D:\rlafage\workspace\mnist)
error[E0658]: attributes on expressions are experimental
   --> src\download.rs:102:21
    |
102 |                     #[cfg(target_family = "unix")]
    |                     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |
    = note: see issue #15701 <https://github.com/rust-lang/rust/issues/15701> for more information

error: removing an expression is not supported in this position
   --> src\download.rs:102:21
    |
102 |                     #[cfg(target_family = "unix")]
    |                     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

error[E0658]: attributes on expressions are experimental
   --> src\download.rs:104:21
    |
104 |                     #[cfg(target_family = "windows")]
    |                     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |
    = note: see issue #15701 <https://github.com/rust-lang/rust/issues/15701> for more information

error: removing an expression is not supported in this position
   --> src\download.rs:104:21
    |
104 |                     #[cfg(target_family = "windows")]
    |                     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

warning: use of deprecated function `std::thread::sleep_ms`: replaced by `std::thread::sleep`
   --> src\download.rs:108:29
    |
108 |                     thread::sleep_ms(10);
    |                             ^^^^^^^^
    |
    = note: `#[warn(deprecated)]` on by default

error[E0599]: no method named `size` found for struct `Metadata` in the current scope
   --> src\download.rs:103:41
    |
103 |                     current_size = meta.size() as usize;
    |                                         ^^^^ method not found in `Metadata`

Some errors have detailed explanations: E0599, E0658.
For more information about an error, try `rustc --explain E0599`.
warning: `mnist` (lib) generated 1 warning
error: could not compile `mnist` due to 5 previous errors; 1 warning emitted

I do not know if this because I use rust 1.57. Anyway I made it pass, by putting use declarations of MetadataExt at the top, create the two versions of file_size like that:

#[cfg(target_family = "unix")]
fn file_size(meta: &MetadataExt) -> usize {
    meta.size() as usize
}

#[cfg(target_family = "windows")]
fn file_size(meta: &MetadataExt) -> usize {
    meta.file_size() as usize
}

and then call

current_size = file_size(&meta);

[relf]

Ok, it compiles successfully on Windows. Thanks!

[quietlychris]

@davidMcneil If you wouldn't mind reviewing and potentially merging this whenever you get a chance, I'd appreciate it! As before, happy to discuss any changes you might think are necessary, although this is should be a pretty minor bug fix.