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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 #144922) caused name resolution ambiguity 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 #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: #145524 r? ``@petrochenkov``
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‎library/core/src/lib.rs‎

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@@ -226,6 +226,13 @@ pub mod assert_matches {
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pub use crate::macros::{assert_matches, debug_assert_matches};
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}
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#[unstable(feature = "derive_from", issue = "144889")]
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/// Unstable module containing the unstable `From` derive macro.
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pub mod from {
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#[unstable(feature = "derive_from", issue = "144889")]
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pub use crate::macros::builtin::From;
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}
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// We don't export this through #[macro_export] for now, to avoid breakage.
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#[unstable(feature = "autodiff", issue = "124509")]
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/// Unstable module containing the unstable `autodiff` macro.

‎library/core/src/prelude/v1.rs‎

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@@ -117,10 +117,3 @@ pub use crate::macros::builtin::deref;
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reason = "`type_alias_impl_trait` has open design concerns"
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)]
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pub use crate::macros::builtin::define_opaque;
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#[unstable(
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feature = "derive_from",
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issue = "144889",
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reason = "`derive(From)` is unstable"
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)]
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pub use crate::macros::builtin::From;

‎library/std/src/lib.rs‎

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

‎tests/ui/deriving/deriving-all-codegen.rs‎

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@@ -18,6 +18,8 @@
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#![allow(deprecated)]
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#![feature(derive_from)]
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use std::from::From;
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// Empty struct.
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#[derive(Clone, Copy, Debug, Default, Hash, PartialEq, Eq, PartialOrd, Ord)]
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struct Empty;
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// `clone` implemention that just does `*self`.
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#[derive(Clone, Copy, Debug, Default, Hash, PartialEq, Eq, PartialOrd, Ord)]
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struct Big {
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b1: u32, b2: u32, b3: u32, b4: u32, b5: u32, b6: u32, b7: u32, b8: u32,
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b1: u32,
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b2: u32,
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b3: u32,
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b4: u32,
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b5: u32,
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b6: u32,
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b7: u32,
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b8: u32,
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}
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// It is more efficient to compare scalar types before non-scalar types.
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// A single-variant enum.
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#[derive(Clone, Debug, Hash, PartialEq, Eq, PartialOrd, Ord)]
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enum Enum1 {
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Single { x: u32 }
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Single { x: u32 },
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}
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// A C-like, fieldless enum with a single variant.
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P,
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Q,
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R(u32),
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S { d1: Option<u32>, d2: Option<i32> },
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S {
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d1: Option<u32>,
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d2: Option<i32>,
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},
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}
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// When comparing enum variant it is more efficient to compare scalar types before non-scalar types.

‎tests/ui/deriving/deriving-all-codegen.stdout‎

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@@ -23,6 +23,8 @@ extern crate std;
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#[prelude_import]
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use std::prelude::rust_2021::*;
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use std::from::From;
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// Empty struct.
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struct Empty;
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#[automatically_derived]

‎tests/ui/deriving/deriving-from-wrong-target.rs‎

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//@ edition: 2021
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//@ check-fail
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#![feature(derive_from)]
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#![allow(dead_code)]
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use std::from::From;
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#[derive(From)]
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//~^ ERROR `#[derive(From)]` used on a struct with no fields
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struct S1;

‎tests/ui/deriving/deriving-from-wrong-target.stderr‎

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error: `#[derive(From)]` used on a struct with no fields
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--> $DIR/deriving-from-wrong-target.rs:7:10
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--> $DIR/deriving-from-wrong-target.rs:8:10
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|
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LL | #[derive(From)]
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| ^^^^
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= note: `#[derive(From)]` can only be used on structs with exactly one field
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error: `#[derive(From)]` used on a struct with no fields
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--> $DIR/deriving-from-wrong-target.rs:11:10
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--> $DIR/deriving-from-wrong-target.rs:12:10
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LL | #[derive(From)]
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| ^^^^
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= note: `#[derive(From)]` can only be used on structs with exactly one field
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error: `#[derive(From)]` used on a struct with multiple fields
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--> $DIR/deriving-from-wrong-target.rs:15:10
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--> $DIR/deriving-from-wrong-target.rs:16:10
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|
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LL | #[derive(From)]
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| ^^^^
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= note: `#[derive(From)]` can only be used on structs with exactly one field
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error: `#[derive(From)]` used on a struct with multiple fields
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--> $DIR/deriving-from-wrong-target.rs:19:10
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--> $DIR/deriving-from-wrong-target.rs:20:10
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LL | #[derive(From)]
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| ^^^^
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= note: `#[derive(From)]` can only be used on structs with exactly one field
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error: `#[derive(From)]` used on an enum
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--> $DIR/deriving-from-wrong-target.rs:26:10
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--> $DIR/deriving-from-wrong-target.rs:27:10
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LL | #[derive(From)]
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= note: `#[derive(From)]` can only be used on structs with exactly one field
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error[E0277]: the size for values of type `T` cannot be known at compilation time
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--> $DIR/deriving-from-wrong-target.rs:30:10
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--> $DIR/deriving-from-wrong-target.rs:31:10
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|
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LL | #[derive(From)]
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| ^^^^ doesn't have a size known at compile-time
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error[E0277]: the size for values of type `T` cannot be known at compilation time
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--> $DIR/deriving-from-wrong-target.rs:30:10
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--> $DIR/deriving-from-wrong-target.rs:31:10
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|
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LL | #[derive(From)]
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| ^^^^ doesn't have a size known at compile-time
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| - this type parameter needs to be `Sized`
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|
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note: required because it appears within the type `SUnsizedField<T>`
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--> $DIR/deriving-from-wrong-target.rs:33:8
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--> $DIR/deriving-from-wrong-target.rs:34:8
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LL | struct SUnsizedField<T: ?Sized> {
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| ^^^^^^^^^^^^^
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error[E0277]: the size for values of type `T` cannot be known at compilation time
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--> $DIR/deriving-from-wrong-target.rs:34:11
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--> $DIR/deriving-from-wrong-target.rs:35:11
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LL | struct SUnsizedField<T: ?Sized> {
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| - this type parameter needs to be `Sized`

‎tests/ui/deriving/deriving-from.rs‎

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#![feature(derive_from)]
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use core::from::From;
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#[derive(From)]
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struct TupleSimple(u32);
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‎tests/ui/feature-gates/feature-gate-derive-from.rs‎

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//@ edition: 2021
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use std::from::From; //~ ERROR use of unstable library feature `derive_from
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#[derive(From)] //~ ERROR use of unstable library feature `derive_from`
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struct Foo(u32);

‎tests/ui/feature-gates/feature-gate-derive-from.stderr‎

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@@ -8,6 +8,16 @@ LL | #[derive(From)]
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= help: add `#![feature(derive_from)]` to the crate attributes to enable
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= note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
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error: aborting due to 1 previous error
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error[E0658]: use of unstable library feature `derive_from`
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--> $DIR/feature-gate-derive-from.rs:1:5
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|
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LL | use std::from::From;
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| ^^^^^^^^^^^^^^^
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|
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= note: see issue #144889 <https://github.com/rust-lang/rust/issues/144889> for more information
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= help: add `#![feature(derive_from)]` to the crate attributes to enable
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= note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
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error: aborting due to 2 previous errors
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For more information about this error, try `rustc --explain E0658`.

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