Rollup merge of #145563 - Kobzol:remove-from-from-prelude, r=petrochenkov

Remove the `From` derive macro from prelude

The new `#[derive(From)]` functionality (implemented in https://github.com/rust-lang/rust/pull/144922) caused name resolution ambiguity issues (https://github.com/rust-lang/rust/issues/145524). The reproducer looks e.g. like this:

```rust
mod foo {
    pub use derive_more::From;
}

use foo::*;

#[derive(From)] // ERROR: `From` is ambiguous
struct S(u32);
```

It's pretty unfortunate that it works like this, but I guess that there's not much to be done here, and we'll have to wait for the next edition to put the `From` macro into the prelude. That will probably require https://github.com/rust-lang/rust/pull/139493 to land.

I created a new module in core (and re-exported it in std) called `from`, where I re-exported the `From` macro. I *think* that since this is a new module, it should not have the same backwards incompatibility issue.

Happy to hear suggestions about the naming - maybe it would make sense as `core::macros::from::From`? But we already had a precedent in the `core::assert_matches` module, so I just followed suit.

Fixes: https://github.com/rust-lang/rust/issues/145524

r? ``@petrochenkov``

1 files changed, 8 insertions, 0 deletions

diff --git a/library/std/src/lib.rs b/library/std/src/lib.rs
index c27167767a2..ab417b6c72f 100644
--- a/library/std/src/lib.rs
+++ b/library/std/src/lib.rs
@@ -738,6 +738,14 @@ pub use core::{
     unreachable, write, writeln,
 };
 
+// Re-export unstable derive macro defined through core.
+#[unstable(feature = "derive_from", issue = "144889")]
+/// Unstable module containing the unstable `From` derive macro.
+pub mod from {
+    #[unstable(feature = "derive_from", issue = "144889")]
+    pub use core::from::From;
+}
+
 // Include a number of private modules that exist solely to provide
 // the rustdoc documentation for primitive types. Using `include!`
 // because rustdoc only looks for these modules at the crate level.