Move the Error trait into core (without fn backtrace)

[yaahc]

current status: it works!

[rust-highfive]

r? @dtolnay

(rust-highfive has picked a reviewer for you, use r? to override)

[yaahc]

r? @spastorino

[lqd]

Since the recent negative impls PR hasn't reached the beta compiler yet, this will also need a few #[cfg(not(bootstrap))] to go past these first errors. And likely more: if I understand correctly, a lot of std's impl Errors (the trait impls, and inherent impls like the various impl dyn Errors) also need to be moved to either core or alloc, right ?

[spastorino]

@spastorino before adding the impl !Error for &str line I got this error

error[E0119]: conflicting implementations of trait core::convert::From<&str> for type boxed::Box<dyn core::error::Error + core::marker::Send + core::marker::Sync>
--> library/alloc/src/boxed.rs:1982:1
|
1880 | impl<'a, E: Error + Send + Sync + 'a> From for Box<dyn Error + Send + Sync + 'a> {
| ----------------------------------------------------------------------------------- first implementation here
...
1982 | impl<'a> From<&str> for Box<dyn Error + Send + Sync + 'a> {
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ conflicting implementation for boxed::Box<dyn core::error::Error + core::marker::Send + core::marker::Sync>
|
= note: upstream crates may add a new impl of trait core::error::Error for type &str in future versions

error[E0119]: conflicting implementations of trait core::convert::From<&str> for type boxed::Box<dyn core::error::Error>
--> library/alloc/src/boxed.rs:2005:1
|
1848 | impl<'a, E: Error + 'a> From for Box<dyn Error + 'a> {
| ------------------------------------------------------- first implementation here
...
2005 | impl From<&str> for Box {
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ conflicting implementation for boxed::Box<dyn core::error::Error>
|
= note: upstream crates may add a new impl of trait core::error::Error for type &str in future versions

For more information about this error, try rustc --explain E0119.
error: could not compile alloc due to 2 previous errors
Build completed unsuccessfully in 0:00:00

And now I get this error:

error[E0119]: conflicting implementations of trait core::convert::From<&str> for type boxed::Box<dyn core::error::Error + core::marker::Send + core::marker::Sync>
--> library/alloc/src/boxed.rs:1982:1
|
1880 | impl<'a, E: Error + Send + Sync + 'a> From for Box<dyn Error + Send + Sync + 'a> {
| ----------------------------------------------------------------------------------- first implementation here
...
1982 | impl<'a> From<&str> for Box<dyn Error + Send + Sync + 'a> {
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ conflicting implementation for boxed::Box<dyn core::error::Error + core::marker::Send + core::marker::Sync>

error[E0119]: conflicting implementations of trait core::convert::From<&str> for type boxed::Box<dyn core::error::Error>
--> library/alloc/src/boxed.rs:2005:1
|
1848 | impl<'a, E: Error + 'a> From for Box<dyn Error + 'a> {
| ------------------------------------------------------- first implementation here
...
2005 | impl From<&str> for Box {
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ conflicting implementation for boxed::Box<dyn core::error::Error>

I was talking with Niko about negative impls today and I fear that this may be a case that's similar to what Yoshua was dealing with. More info https://rust-lang.zulipchat.com/#narrow/stream/144729-wg-traits/topic/negative.20impls.20in.20coherence

We may need to do something like https://smallcultfollowing.com/babysteps/blog/2018/02/09/maximally-minimal-specialization-always-applicable-impls/ from what we've talked with Niko. We'd be working on that and will let you know once we land a fix.

[JohnCSimon]

Returning to author to address broken tests
@rustbot label: +S-waiting-on-author -S-waiting-on-review

[yaahc]

Turns out that the unstable attributes don't actually work the way I expected them to. I had expected this to be an error the same way the it is in the above test but it actually compiles just fine...

[Dylan-DPC]

@yaahc any updates?

[yaahc]

This is superceded by #98072 and a subsequent PR to move Error into core once #95956 has landed