add lazy_normalization_consts feature gate

src/librustc_feature/active.rs

@@ -570,6 +570,9 @@ declare_features! (
     /// Allows capturing variables in scope using format_args!
     (active, format_args_capture, "1.46.0", Some(67984), None),
 
+    /// Lazily evaluate constants. This allows constants to depend on type parameters.
+    (active, lazy_normalization_consts, "1.46.0", Some(72219), None),
+
     // -------------------------------------------------------------------------
     // feature-group-end: actual feature gates
     // -------------------------------------------------------------------------
@@ -586,5 +589,6 @@ pub const INCOMPLETE_FEATURES: &[Symbol] = &[
     sym::raw_dylib,
     sym::const_trait_impl,
     sym::const_trait_bound_opt_out,
+    sym::lazy_normalization_consts,
     sym::specialization,
 ];

src/librustc_middle/ty/context.rs

@@ -1370,7 +1370,7 @@ impl<'tcx> TyCtxt<'tcx> {
     /// we still evaluate them eagerly.
     #[inline]
     pub fn lazy_normalization(self) -> bool {
-        self.features().const_generics
+        self.features().const_generics || self.features().lazy_normalization_consts
     }
 
     #[inline]

src/librustc_span/symbol.rs

@@ -427,6 +427,7 @@ symbols! {
         label_break_value,
         lang,
         lang_items,
+        lazy_normalization_consts,
         lateout,
         let_chains,
         lhs,

src/test/ui/lazy_normalization_consts/feature-gate-lazy_normalization_consts.rs

@@ -0,0 +1,10 @@
+pub const fn sof<T>() -> usize {
+    10
+}
+
+fn test<T>() {
+    let _: [u8; sof::<T>()];
+    //~^ ERROR the size for values of type `T`
+}
+
+fn main() {}

src/test/ui/lazy_normalization_consts/feature-gate-lazy_normalization_consts.stderr

@@ -0,0 +1,21 @@
+error[E0277]: the size for values of type `T` cannot be known at compilation time
+  --> $DIR/feature-gate-lazy_normalization_consts.rs:6:23
+   |
+LL | pub const fn sof<T>() -> usize {
+   |                  - required by this bound in `sof`
+...
+LL | fn test<T>() {
+   |         - this type parameter needs to be `std::marker::Sized`
+LL |     let _: [u8; sof::<T>()];
+   |                       ^ doesn't have a size known at compile-time
+   |
+   = help: the trait `std::marker::Sized` is not implemented for `T`
+   = note: to learn more, visit <https://doc.rust-lang.org/book/ch19-04-advanced-types.html#dynamically-sized-types-and-the-sized-trait>
+help: consider relaxing the implicit `Sized` restriction
+   |
+LL | pub const fn sof<T: ?Sized>() -> usize {
+   |                   ^^^^^^^^
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0277`.

src/test/ui/lazy_normalization_consts/issue-47814.rs

@@ -0,0 +1,16 @@
+// check-pass
+#![feature(lazy_normalization_consts)]
+#![allow(incomplete_features)]
+pub struct ArpIPv4<'a> {
+    _s: &'a u8
+}
+
+impl<'a> ArpIPv4<'a> {
+    const LENGTH: usize = 20;
+
+    pub fn to_buffer() -> [u8; Self::LENGTH] {
+        unimplemented!()
+    }
+}
+
+fn main() {}

src/test/ui/lazy_normalization_consts/issue-57739.rs

@@ -0,0 +1,17 @@
+#![feature(lazy_normalization_consts)]
+//~^ WARN the feature `lazy_normalization_consts` is incomplete
+trait ArraySizeTrait {
+    const SIZE: usize = 0;
+}
+
+impl<T: ?Sized> ArraySizeTrait for T {
+    const SIZE: usize = 1;
+}
+
+struct SomeArray<T: ArraySizeTrait> {
+    array: [u8; T::SIZE],
+    //~^ ERROR constant expression depends on a generic parameter
+    phantom: std::marker::PhantomData<T>,
+}
+
+fn main() {}

src/test/ui/lazy_normalization_consts/issue-57739.stderr

@@ -0,0 +1,19 @@
+warning: the feature `lazy_normalization_consts` is incomplete and may not be safe to use and/or cause compiler crashes
+  --> $DIR/issue-57739.rs:1:12
+   |
+LL | #![feature(lazy_normalization_consts)]
+   |            ^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: `#[warn(incomplete_features)]` on by default
+   = note: see issue #72219 <https://github.com/rust-lang/rust/issues/72219> for more information
+
+error: constant expression depends on a generic parameter
+  --> $DIR/issue-57739.rs:12:5
+   |
+LL |     array: [u8; T::SIZE],
+   |     ^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: this may fail depending on what value the parameter takes
+
+error: aborting due to previous error; 1 warning emitted
+

src/test/ui/lazy_normalization_consts/issue-73980.rs

@@ -0,0 +1,14 @@
+// check-pass
+#![feature(lazy_normalization_consts)]
+#![allow(incomplete_features)]
+
+pub struct X<P, Q>(P, Q);
+pub struct L<T: ?Sized>(T);
+
+impl<T: ?Sized> L<T> {
+    const S: usize = 1;
+}
+
+impl<T> X<T, [u8; L::<T>::S]> {}
+
+fn main() {}