types of b"foo" and bytes!("foo") differ

[sp3d]

Rustc gives a message that bytes!() is deprecated:

b.rs:1:33: 1:47 warning: `bytes!` is deprecated, use `b"foo"` literals instead
b.rs:1 fn main() { let x: [u8, ..3] = *bytes!("foo"); }
                                       ^~~~~~~~~~~~~~
b.rs:1:33: 1:47 help: see http://doc.rust-lang.org/reference.html#byte-and-byte-string-literals for documentation
b.rs:1 fn main() { let x: [u8, ..3] = *bytes!("foo"); }
                                       ^~~~~~~~~~~~~~
b.rs:1:33: 1:47 help: see https://github.com/rust-lang/rust/blob/master/src/etc/2014-06-rewrite-bytes-macros.py for an automated migration
b.rs:1 fn main() { let x: [u8, ..3] = *bytes!("foo"); }
                                       ^~~~~~~~~~~~~~
b.rs:1:17: 1:18 warning: unused variable: `x`, #[warn(unused_variables)] on by default
b.rs:1 fn main() { let x: [u8, ..3] = *bytes!("foo"); }
                       ^

But the suggested replacement, as implemented by the python script, no longer compiles. Compiling the program resulting from applying the script's generated patch gives the error:

b.rs:1:32: 1:39 error: mismatched types: expected `[u8, ..3]`, found `[u8]` (expected array of 3 elements, found slice)
b.rs:1 fn main() { let x: [u8, ..3] = *b"foo"; }
                                      ^~~~~~~

The b"foo" syntax should not be suggested as a replacement for bytes!("foo") if it cannot fulfill the same roles. In particular, it seems odd to me that b"foo" syntax results in a less flexible type than bytes!("foo").

Note that [u8, ..n] can be converted to &[u8], which can be converted to the DST [u8]--but &[u8], which is what b"foo" produces, cannot be statically converted to [u8, ..n], so the change suggested by the deprecation warning results in loss of generality.

[steveklabnik]

See #18702

[ftxqxd]

Looks like a dupe of #18465.

[thestinger]

Closing as a duplicate of #18465