Auto merge of #84310 - RalfJung:const-fn-feature-flags, r=oli-obk

further split up const_fn feature flag

This continues the work on splitting up `const_fn` into separate feature flags:
* `const_fn_trait_bound` for `const fn` with trait bounds
* `const_fn_unsize` for unsizing coercions in `const fn` (looks like only `dyn` unsizing is still guarded here)

I don't know if there are even any things left that `const_fn` guards... at least libcore and liballoc do not need it any more.

`@oli-obk` are you currently able to do reviews?

compiler/rustc_ast/src/lib.rs

@@ -10,7 +10,8 @@
 )]
 #![feature(box_syntax)]
 #![feature(box_patterns)]
-#![feature(const_fn)] // For the `transmute` in `P::new`
+#![cfg_attr(bootstrap, feature(const_fn))] // For the `transmute` in `P::new`
+#![cfg_attr(not(bootstrap), feature(const_fn_unsize))] // For the `transmute` in `P::new`
 #![feature(const_fn_transmute)]
 #![feature(const_panic)]
 #![feature(crate_visibility_modifier)]

compiler/rustc_error_codes/src/error_codes.rs

@@ -416,7 +416,6 @@ E0716: include_str!("./error_codes/E0716.md"),
 E0718: include_str!("./error_codes/E0718.md"),
 E0719: include_str!("./error_codes/E0719.md"),
 E0720: include_str!("./error_codes/E0720.md"),
-E0723: include_str!("./error_codes/E0723.md"),
 E0724: include_str!("./error_codes/E0724.md"),
 E0725: include_str!("./error_codes/E0725.md"),
 E0727: include_str!("./error_codes/E0727.md"),
@@ -636,6 +635,7 @@ E0781: include_str!("./error_codes/E0781.md"),
     E0717, // rustc_promotable without stability attribute
 //  E0721, // `await` keyword
     E0722, // Malformed `#[optimize]` attribute
+//    E0723, unstable feature in `const` context
     E0726, // non-explicit (not `'_`) elided lifetime in unsupported position
 //  E0738, // Removed; errored on `#[track_caller] fn`s in `extern "Rust" { ... }`.
     E0757, // `#[ffi_const]` functions cannot be `#[ffi_pure]`

compiler/rustc_feature/src/active.rs

@@ -645,6 +645,12 @@ declare_features! (
     /// Allows `extern "wasm" fn`
     (active, wasm_abi, "1.53.0", Some(83788), None),
 
+    /// Allows trait bounds in `const fn`.
+    (active, const_fn_trait_bound, "1.53.0", Some(57563), None),
+
+    /// Allows unsizing coercions in `const fn`.
+    (active, const_fn_unsize, "1.53.0", Some(64992), None),
+
     // -------------------------------------------------------------------------
     // feature-group-end: actual feature gates
     // -------------------------------------------------------------------------

compiler/rustc_hir/src/lib.rs

@@ -3,7 +3,6 @@
 //! [rustc dev guide]: https://rustc-dev-guide.rust-lang.org/hir.html
 
 #![feature(crate_visibility_modifier)]
-#![feature(const_fn)] // For the unsizing cast on `&[]`
 #![feature(const_panic)]
 #![feature(extended_key_value_attributes)]
 #![feature(in_band_lifetimes)]

compiler/rustc_index/src/lib.rs

@@ -1,5 +1,4 @@
 #![feature(allow_internal_unstable)]
-#![feature(const_fn)]
 #![feature(const_panic)]
 #![feature(extend_one)]
 #![feature(iter_zip)]

compiler/rustc_infer/src/lib.rs

@@ -16,7 +16,6 @@
 #![feature(bool_to_option)]
 #![feature(box_patterns)]
 #![feature(box_syntax)]
-#![feature(const_fn)]
 #![feature(const_panic)]
 #![feature(extend_one)]
 #![feature(iter_zip)]

compiler/rustc_middle/src/lib.rs

@@ -29,7 +29,6 @@
 #![feature(bool_to_option)]
 #![feature(box_patterns)]
 #![feature(box_syntax)]
-#![feature(const_fn)]
 #![feature(const_panic)]
 #![feature(core_intrinsics)]
 #![feature(discriminant_kind)]

compiler/rustc_mir/src/lib.rs

@@ -12,7 +12,6 @@ Rust MIR: a lowered representation of Rust.
 #![feature(bool_to_option)]
 #![feature(box_patterns)]
 #![feature(box_syntax)]
-#![feature(const_fn)]
 #![feature(const_panic)]
 #![feature(crate_visibility_modifier)]
 #![feature(decl_macro)]

compiler/rustc_mir/src/transform/check_consts/ops.rs

@@ -540,12 +540,17 @@ impl NonConstOp for UnionAccess {
 pub struct UnsizingCast;
 impl NonConstOp for UnsizingCast {
     fn status_in_item(&self, ccx: &ConstCx<'_, '_>) -> Status {
-        mcf_status_in_item(ccx)
+        if ccx.const_kind() != hir::ConstContext::ConstFn {
+            Status::Allowed
+        } else {
+            Status::Unstable(sym::const_fn_unsize)
+        }
     }
 
     fn build_error(&self, ccx: &ConstCx<'_, 'tcx>, span: Span) -> DiagnosticBuilder<'tcx> {
-        mcf_build_error(
-            ccx,
+        feature_err(
+            &ccx.tcx.sess.parse_sess,
+            sym::const_fn_unsize,
             span,
             "unsizing casts to types besides slices are not allowed in const fn",
         )
@@ -642,12 +647,17 @@ pub mod ty {
         }
 
         fn status_in_item(&self, ccx: &ConstCx<'_, '_>) -> Status {
-            mcf_status_in_item(ccx)
+            if ccx.const_kind() != hir::ConstContext::ConstFn {
+                Status::Allowed
+            } else {
+                Status::Unstable(sym::const_fn_trait_bound)
+            }
         }
 
         fn build_error(&self, ccx: &ConstCx<'_, 'tcx>, span: Span) -> DiagnosticBuilder<'tcx> {
-            mcf_build_error(
-                ccx,
+            feature_err(
+                &ccx.tcx.sess.parse_sess,
+                sym::const_fn_trait_bound,
                 span,
                 "trait bounds other than `Sized` on const fn parameters are unstable",
             )
@@ -672,21 +682,3 @@ pub mod ty {
         }
     }
 }
-
-fn mcf_status_in_item(ccx: &ConstCx<'_, '_>) -> Status {
-    if ccx.const_kind() != hir::ConstContext::ConstFn {
-        Status::Allowed
-    } else {
-        Status::Unstable(sym::const_fn)
-    }
-}
-
-fn mcf_build_error(ccx: &ConstCx<'_, 'tcx>, span: Span, msg: &str) -> DiagnosticBuilder<'tcx> {
-    let mut err = struct_span_err!(ccx.tcx.sess, span, E0723, "{}", msg);
-    err.note(
-        "see issue #57563 <https://github.com/rust-lang/rust/issues/57563> \
-             for more information",
-    );
-    err.help("add `#![feature(const_fn)]` to the crate attributes to enable");
-    err
-}

compiler/rustc_mir/src/transform/check_consts/validation.rs

@@ -1,6 +1,6 @@
 //! The `Visitor` responsible for actually checking a `mir::Body` for invalid operations.
 
-use rustc_errors::{struct_span_err, Applicability, Diagnostic, ErrorReported};
+use rustc_errors::{Applicability, Diagnostic, ErrorReported};
 use rustc_hir::def_id::DefId;
 use rustc_hir::{self as hir, HirId, LangItem};
 use rustc_index::bit_set::BitSet;
@@ -234,13 +234,11 @@ impl Validator<'mir, 'tcx> {
             if self.is_const_stable_const_fn() {
                 let hir_id = tcx.hir().local_def_id_to_hir_id(def_id);
                 if crate::const_eval::is_parent_const_impl_raw(tcx, hir_id) {
-                    struct_span_err!(
-                        self.ccx.tcx.sess,
-                        self.span,
-                        E0723,
-                        "trait methods cannot be stable const fn"
-                    )
-                    .emit();
+                    self.ccx
+                        .tcx
+                        .sess
+                        .struct_span_err(self.span, "trait methods cannot be stable const fn")
+                        .emit();
                 }
             }
 

compiler/rustc_mir_build/src/lib.rs

@@ -4,7 +4,6 @@
 #![feature(array_windows)]
 #![feature(box_patterns)]
 #![feature(box_syntax)]
-#![feature(const_fn)]
 #![feature(const_panic)]
 #![feature(control_flow_enum)]
 #![feature(crate_visibility_modifier)]

compiler/rustc_passes/src/lib.rs

@@ -5,7 +5,6 @@
 //! This API is completely unstable and subject to change.
 
 #![doc(html_root_url = "https://doc.rust-lang.org/nightly/nightly-rustc/")]
-#![feature(const_fn)]
 #![feature(const_panic)]
 #![feature(crate_visibility_modifier)]
 #![feature(in_band_lifetimes)]

compiler/rustc_query_system/src/lib.rs

@@ -1,5 +1,4 @@
 #![feature(bool_to_option)]
-#![feature(const_fn)]
 #![feature(const_panic)]
 #![feature(core_intrinsics)]
 #![feature(drain_filter)]

compiler/rustc_span/src/lib.rs

@@ -16,7 +16,6 @@
 #![doc(html_root_url = "https://doc.rust-lang.org/nightly/nightly-rustc/")]
 #![feature(array_windows)]
 #![feature(crate_visibility_modifier)]
-#![feature(const_fn)]
 #![feature(const_panic)]
 #![feature(negative_impls)]
 #![feature(nll)]

compiler/rustc_span/src/symbol.rs

@@ -382,8 +382,10 @@ symbols! {
         const_fn,
         const_fn_floating_point_arithmetic,
         const_fn_fn_ptr_basics,
+        const_fn_trait_bound,
         const_fn_transmute,
         const_fn_union,
+        const_fn_unsize,
         const_generic_defaults,
         const_generics,
         const_generics_defaults,

compiler/rustc_target/src/lib.rs

@@ -9,7 +9,6 @@
 
 #![doc(html_root_url = "https://doc.rust-lang.org/nightly/nightly-rustc/")]
 #![feature(bool_to_option)]
-#![feature(const_fn)]
 #![feature(const_panic)]
 #![feature(nll)]
 #![feature(never_type)]

library/alloc/src/lib.rs

@@ -89,7 +89,8 @@
 #![feature(cfg_target_has_atomic)]
 #![feature(coerce_unsized)]
 #![feature(const_btree_new)]
-#![feature(const_fn)]
+#![cfg_attr(bootstrap, feature(const_fn))]
+#![cfg_attr(not(bootstrap), feature(const_fn_trait_bound))]
 #![feature(cow_is_borrowed)]
 #![feature(const_cow_is_borrowed)]
 #![feature(destructuring_assignment)]

library/core/src/lib.rs

@@ -81,11 +81,12 @@
 #![feature(const_refs_to_cell)]
 #![feature(const_panic)]
 #![feature(const_pin)]
-#![feature(const_fn)]
+#![cfg_attr(bootstrap, feature(const_fn))]
 #![feature(const_fn_union)]
 #![feature(const_impl_trait)]
 #![feature(const_fn_floating_point_arithmetic)]
 #![feature(const_fn_fn_ptr_basics)]
+#![cfg_attr(not(bootstrap), feature(const_fn_trait_bound))]
 #![feature(const_option)]
 #![feature(const_precise_live_drops)]
 #![feature(const_ptr_offset)]

src/ci/pgo.sh

@@ -12,7 +12,7 @@ RUSTC_BOOTSTRAP=1 ./build/$PGO_HOST/stage2/bin/rustc --edition=2018 \
 
 # Download and build a single-file stress test benchmark on perf.rust-lang.org.
 function pgo_perf_benchmark {
-    local PERF=e095f5021bf01cf3800f50b3a9f14a9683eb3e4e
+    local PERF=9442def56a39d742bf27ebcc3e0614cf117e1bc2
     local github_prefix=https://raw.githubusercontent.com/rust-lang/rustc-perf/$PERF
     local name=$1
     curl -o /tmp/$name.rs $github_prefix/collector/benchmarks/$name/src/lib.rs

src/test/ui/consts/const-eval/issue-49296.rs

@@ -1,7 +1,7 @@
 // issue-49296: Unsafe shenigans in constants can result in missing errors
 
-#![feature(const_fn)]
 #![feature(const_fn_union)]
+#![feature(const_fn_trait_bound)]
 
 const unsafe fn transmute<T: Copy, U: Copy>(t: T) -> U {
     #[repr(C)]

src/test/ui/consts/const-fn.rs

@@ -3,7 +3,8 @@
 
 // A very basic test of const fn functionality.
 
-#![feature(const_fn, const_indexing)]
+#![feature(const_indexing)]
+#![feature(const_fn_trait_bound)]
 
 const fn add(x: u32, y: u32) -> u32 {
     x + y

src/test/ui/consts/const_fn_trait_bound.gated.stderr

@@ -0,0 +1,8 @@
+error: fatal error triggered by #[rustc_error]
+  --> $DIR/const_fn_trait_bound.rs:17:1
+   |
+LL | fn main() {}
+   | ^^^^^^^^^
+
+error: aborting due to previous error
+

src/test/ui/consts/const_fn_trait_bound.rs

@@ -0,0 +1,17 @@
+// gate-test-const_fn_trait_bound
+
+// revisions: stock gated
+
+#![feature(rustc_attrs)]
+#![cfg_attr(gated, feature(const_fn_trait_bound))]
+
+const fn test1<T: std::ops::Add>() {}
+//[stock]~^ trait bounds
+const fn test2(_x: &dyn Send) {}
+//[stock]~^ trait bounds
+const fn test3() -> &'static dyn Send { loop {} }
+//[stock]~^ trait bounds
+
+
+#[rustc_error]
+fn main() {} //[gated]~ fatal error triggered by #[rustc_error]

src/test/ui/consts/const_fn_trait_bound.stock.stderr

@@ -0,0 +1,30 @@
+error[E0658]: trait bounds other than `Sized` on const fn parameters are unstable
+  --> $DIR/const_fn_trait_bound.rs:8:16
+   |
+LL | const fn test1<T: std::ops::Add>() {}
+   |                ^
+   |
+   = note: see issue #57563 <https://github.com/rust-lang/rust/issues/57563> for more information
+   = help: add `#![feature(const_fn_trait_bound)]` to the crate attributes to enable
+
+error[E0658]: trait bounds other than `Sized` on const fn parameters are unstable
+  --> $DIR/const_fn_trait_bound.rs:10:16
+   |
+LL | const fn test2(_x: &dyn Send) {}
+   |                ^^
+   |
+   = note: see issue #57563 <https://github.com/rust-lang/rust/issues/57563> for more information
+   = help: add `#![feature(const_fn_trait_bound)]` to the crate attributes to enable
+
+error[E0658]: trait bounds other than `Sized` on const fn parameters are unstable
+  --> $DIR/const_fn_trait_bound.rs:12:21
+   |
+LL | const fn test3() -> &'static dyn Send { loop {} }
+   |                     ^^^^^^^^^^^^^^^^^
+   |
+   = note: see issue #57563 <https://github.com/rust-lang/rust/issues/57563> for more information
+   = help: add `#![feature(const_fn_trait_bound)]` to the crate attributes to enable
+
+error: aborting due to 3 previous errors
+
+For more information about this error, try `rustc --explain E0658`.

src/test/ui/consts/const_fn_unsize.gated.stderr

@@ -0,0 +1,8 @@
+error: fatal error triggered by #[rustc_error]
+  --> $DIR/const_fn_unsize.rs:16:1
+   |
+LL | fn main() {}
+   | ^^^^^^^^^
+
+error: aborting due to previous error
+

src/test/ui/consts/const_fn_unsize.rs

@@ -0,0 +1,16 @@
+// gate-test-const_fn_unsize
+
+// revisions: stock gated
+
+#![feature(rustc_attrs)]
+#![cfg_attr(gated, feature(const_fn_unsize))]
+
+use std::ptr::NonNull;
+
+const fn test() {
+    let _x = NonNull::<[i32; 0]>::dangling() as NonNull<[i32]>;
+    //[stock]~^ unsizing cast
+}
+
+#[rustc_error]
+fn main() {} //[gated]~ fatal error triggered by #[rustc_error]

src/test/ui/consts/const_fn_unsize.stock.stderr

@@ -0,0 +1,12 @@
+error[E0658]: unsizing casts to types besides slices are not allowed in const fn
+  --> $DIR/const_fn_unsize.rs:11:14
+   |
+LL |     let _x = NonNull::<[i32; 0]>::dangling() as NonNull<[i32]>;
+   |              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: see issue #64992 <https://github.com/rust-lang/rust/issues/64992> for more information
+   = help: add `#![feature(const_fn_unsize)]` to the crate attributes to enable
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0658`.