add field_projections feature gate

Author: BennoLossin

compiler/rustc_feature/src/unstable.rs

@@ -504,6 +504,8 @@ declare_features! (
     (unstable, ffi_const, "1.45.0", Some(58328)),
     /// Allows the use of `#[ffi_pure]` on foreign functions.
     (unstable, ffi_pure, "1.45.0", Some(58329)),
+    /// Experimental field projections.
+    (incomplete, field_projections, "CURRENT_RUSTC_VERSION", Some(145383)),
     /// Controlling the behavior of fmt::Debug
     (unstable, fmt_debug, "1.82.0", Some(129709)),
     /// Allows using `#[align(...)]` on function items

compiler/rustc_span/src/symbol.rs

@@ -1035,6 +1035,7 @@ symbols! {
         ffi_returns_twice,
         field,
         field_init_shorthand,
+        field_projections,
         file,
         file_options,
         flags,

library/core/src/field.rs

@@ -0,0 +1 @@
+//! Field Reflection

library/core/src/lib.rs

@@ -339,6 +339,8 @@ pub mod bstr;
 pub mod cell;
 pub mod char;
 pub mod ffi;
+#[unstable(feature = "field_projections", issue = "145383")]
+pub mod field;
 #[unstable(feature = "core_io_borrowed_buf", issue = "117693")]
 pub mod io;
 pub mod iter;

library/std/src/lib.rs

@@ -508,6 +508,8 @@ pub use core::cmp;
 pub use core::convert;
 #[stable(feature = "rust1", since = "1.0.0")]
 pub use core::default;
+#[unstable(feature = "field_projections", issue = "145383")]
+pub use core::field;
 #[stable(feature = "futures_api", since = "1.36.0")]
 pub use core::future;
 #[stable(feature = "core_hint", since = "1.27.0")]

tests/ui/feature-gates/feature-gate-field-projections.rs

@@ -0,0 +1,5 @@
+#![allow(dead_code)]
+
+use std::field; //~ ERROR: use of unstable library feature `field_projections`
+
+fn main() {}

tests/ui/feature-gates/feature-gate-field-projections.stderr

@@ -0,0 +1,13 @@
+error[E0658]: use of unstable library feature `field_projections`
+  --> $DIR/feature-gate-field-projections.rs:3:5
+   |
+LL | use std::field;
+   |     ^^^^^^^^^^
+   |
+   = note: see issue #145383 <https://github.com/rust-lang/rust/issues/145383> for more information
+   = help: add `#![feature(field_projections)]` to the crate attributes to enable
+   = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0658`.