Rollup merge of #68114 - ecstatic-morse:fix-feature-gating, r=Centril

Don't require `allow_internal_unstable` unless `staged_api` is enabled.

#63770 changed `qualify_min_const_fn` to require `allow_internal_unstable` for *all* crates that used an unstable feature, regardless of whether `staged_api` was enabled or the `fn` that used that feature was stably const. In practice, this meant that every crate in the ecosystem that wanted to use nightly features added `#![feature(const_fn)]`, which skips `qualify_min_const_fn` entirely.

After this PR, crates that do not have `#![feature(staged_api)]` will only need to enable the feature they are interested in. For example, `#![feature(const_if_match)]` will be enough to enable `if` and `match` in constants. Crates with `staged_api` (e.g., `libstd`) require `#[allow_internal_unstable]` to be added to a function if it uses nightly features unless that function is also marked `#[rustc_const_unstable]`. This prevents proliferation of `#[allow_internal_unstable]` into functions that are not callable in a `const` context on stable.

r? @oli-obk (author of #63770)
cc @Centril

src/libcore/lib.rs

@@ -70,7 +70,6 @@
 #![feature(bound_cloned)]
 #![feature(cfg_target_has_atomic)]
 #![feature(concat_idents)]
-#![feature(const_fn)]
 #![feature(const_if_match)]
 #![feature(const_panic)]
 #![feature(const_fn_union)]

src/libcore/tests/lib.rs

@@ -31,7 +31,6 @@
 #![feature(slice_internals)]
 #![feature(slice_partition_dedup)]
 #![feature(int_error_matching)]
-#![feature(const_fn)]
 #![feature(array_value_iter)]
 #![feature(iter_partition_in_place)]
 #![feature(iter_is_partitioned)]

src/libproc_macro/lib.rs

@@ -21,7 +21,6 @@
 #![feature(nll)]
 #![feature(staged_api)]
 #![feature(allow_internal_unstable)]
-#![feature(const_fn)]
 #![feature(decl_macro)]
 #![feature(extern_types)]
 #![feature(in_band_lifetimes)]

src/librustc/lib.rs

@@ -31,7 +31,6 @@
 #![feature(bool_to_option)]
 #![feature(box_patterns)]
 #![feature(box_syntax)]
-#![feature(const_fn)]
 #![feature(const_transmute)]
 #![feature(core_intrinsics)]
 #![feature(drain_filter)]

src/librustc_hir/lib.rs

@@ -3,7 +3,7 @@
 //! [rustc guide]: https://rust-lang.github.io/rustc-guide/hir.html
 
 #![feature(crate_visibility_modifier)]
-#![feature(const_fn)]
+#![feature(const_fn)] // For the unsizing cast on `&[]`
 #![feature(in_band_lifetimes)]
 #![feature(specialization)]
 #![recursion_limit = "256"]

src/librustc_mir/lib.rs

@@ -13,7 +13,6 @@ Rust MIR: a lowered representation of Rust. Also: an experiment!
 #![feature(box_syntax)]
 #![feature(crate_visibility_modifier)]
 #![feature(core_intrinsics)]
-#![feature(const_fn)]
 #![feature(decl_macro)]
 #![feature(drain_filter)]
 #![feature(exhaustive_patterns)]

src/librustc_mir/transform/qualify_min_const_fn.rs

@@ -281,8 +281,22 @@ fn check_place(
     Ok(())
 }
 
-/// Returns whether `allow_internal_unstable(..., <feature_gate>, ...)` is present.
+/// Returns `true` if the given feature gate is allowed within the function with the given `DefId`.
 fn feature_allowed(tcx: TyCtxt<'tcx>, def_id: DefId, feature_gate: Symbol) -> bool {
+    // All features require that the corresponding gate be enabled,
+    // even if the function has `#[allow_internal_unstable(the_gate)]`.
+    if !tcx.features().enabled(feature_gate) {
+        return false;
+    }
+
+    // If this crate is not using stability attributes, or this function is not claiming to be a
+    // stable `const fn`, that is all that is required.
+    if !tcx.features().staged_api || tcx.has_attr(def_id, sym::rustc_const_unstable) {
+        return true;
+    }
+
+    // However, we cannot allow stable `const fn`s to use unstable features without an explicit
+    // opt-in via `allow_internal_unstable`.
     attr::allow_internal_unstable(&tcx.get_attrs(def_id), &tcx.sess.diagnostic())
         .map_or(false, |mut features| features.any(|name| name == feature_gate))
 }

src/librustc_span/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/")]
-#![feature(const_fn)]
 #![feature(crate_visibility_modifier)]
 #![feature(nll)]
 #![feature(optin_builtin_traits)]

src/librustdoc/lib.rs

@@ -12,7 +12,6 @@
 #![feature(test)]
 #![feature(ptr_offset_from)]
 #![feature(crate_visibility_modifier)]
-#![feature(const_fn)]
 #![feature(drain_filter)]
 #![feature(never_type)]
 #![feature(unicode_internals)]

src/libsyntax/lib.rs

@@ -7,7 +7,7 @@
 #![doc(html_root_url = "https://doc.rust-lang.org/nightly/", test(attr(deny(warnings))))]
 #![feature(bool_to_option)]
 #![feature(box_syntax)]
-#![feature(const_fn)]
+#![feature(const_fn)] // For the `transmute` in `P::new`
 #![feature(const_transmute)]
 #![feature(crate_visibility_modifier)]
 #![feature(label_break_value)]

src/test/ui/consts/const-mut-refs/const_mut_refs.rs

@@ -1,7 +1,6 @@
 // run-pass
 
 #![feature(const_mut_refs)]
-#![feature(const_fn)]
 
 struct Foo {
     x: usize

src/test/ui/consts/control-flow/exhaustive-c-like-enum-match.rs

@@ -3,7 +3,6 @@
 // check-pass
 
 #![feature(const_if_match)]
-#![feature(const_fn)]
 
 enum E {
     A,

src/test/ui/consts/control-flow/feature-gate-const-if-match.if_match.stderr

@@ -1,5 +1,5 @@
 error: fatal error triggered by #[rustc_error]
-  --> $DIR/feature-gate-const-if-match.rs:109:1
+  --> $DIR/feature-gate-const-if-match.rs:108:1
    |
 LL | / fn main() {
 LL | |     let _ = [0; {

src/test/ui/consts/control-flow/feature-gate-const-if-match.rs

@@ -6,7 +6,6 @@
 
 #![feature(rustc_attrs)]
 #![cfg_attr(if_match, feature(const_if_match))]
-#![feature(const_fn)]
 
 const _: i32 = if true { //[stock]~ ERROR `if` is not allowed in a `const`
     5

src/test/ui/consts/control-flow/feature-gate-const-if-match.stock.stderr

@@ -1,5 +1,5 @@
 error[E0658]: `if` is not allowed in a `const`
-  --> $DIR/feature-gate-const-if-match.rs:11:16
+  --> $DIR/feature-gate-const-if-match.rs:10:16
    |
 LL |   const _: i32 = if true {
    |  ________________^
@@ -13,7 +13,7 @@ LL | | };
    = help: add `#![feature(const_if_match)]` to the crate attributes to enable
 
 error[E0658]: `if` is not allowed in a `const`
-  --> $DIR/feature-gate-const-if-match.rs:17:16
+  --> $DIR/feature-gate-const-if-match.rs:16:16
    |
 LL |   const _: i32 = if let Some(true) = Some(false) {
    |  ________________^
@@ -27,7 +27,7 @@ LL | | };
    = help: add `#![feature(const_if_match)]` to the crate attributes to enable
 
 error[E0658]: `match` is not allowed in a `const`
-  --> $DIR/feature-gate-const-if-match.rs:23:16
+  --> $DIR/feature-gate-const-if-match.rs:22:16
    |
 LL |   const _: i32 = match 1 {
    |  ________________^
@@ -41,7 +41,7 @@ LL | | };
    = help: add `#![feature(const_if_match)]` to the crate attributes to enable
 
 error[E0658]: `if` is not allowed in a `static`
-  --> $DIR/feature-gate-const-if-match.rs:30:13
+  --> $DIR/feature-gate-const-if-match.rs:29:13
    |
 LL |     let x = if true { 0 } else { 1 };
    |             ^^^^^^^^^^^^^^^^^^^^^^^^
@@ -50,7 +50,7 @@ LL |     let x = if true { 0 } else { 1 };
    = help: add `#![feature(const_if_match)]` to the crate attributes to enable
 
 error[E0658]: `match` is not allowed in a `static`
-  --> $DIR/feature-gate-const-if-match.rs:32:13
+  --> $DIR/feature-gate-const-if-match.rs:31:13
    |
 LL |     let x = match x { 0 => 1, _ => 0 };
    |             ^^^^^^^^^^^^^^^^^^^^^^^^^^
@@ -59,7 +59,7 @@ LL |     let x = match x { 0 => 1, _ => 0 };
    = help: add `#![feature(const_if_match)]` to the crate attributes to enable
 
 error[E0658]: `if` is not allowed in a `static`
-  --> $DIR/feature-gate-const-if-match.rs:34:5
+  --> $DIR/feature-gate-const-if-match.rs:33:5
    |
 LL |     if let Some(x) = Some(x) { x } else { 1 }
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
@@ -68,7 +68,7 @@ LL |     if let Some(x) = Some(x) { x } else { 1 }
    = help: add `#![feature(const_if_match)]` to the crate attributes to enable
 
 error[E0658]: `if` is not allowed in a `static mut`
-  --> $DIR/feature-gate-const-if-match.rs:39:13
+  --> $DIR/feature-gate-const-if-match.rs:38:13
    |
 LL |     let x = if true { 0 } else { 1 };
    |             ^^^^^^^^^^^^^^^^^^^^^^^^
@@ -77,7 +77,7 @@ LL |     let x = if true { 0 } else { 1 };
    = help: add `#![feature(const_if_match)]` to the crate attributes to enable
 
 error[E0658]: `match` is not allowed in a `static mut`
-  --> $DIR/feature-gate-const-if-match.rs:41:13
+  --> $DIR/feature-gate-const-if-match.rs:40:13
    |
 LL |     let x = match x { 0 => 1, _ => 0 };
    |             ^^^^^^^^^^^^^^^^^^^^^^^^^^
@@ -86,7 +86,7 @@ LL |     let x = match x { 0 => 1, _ => 0 };
    = help: add `#![feature(const_if_match)]` to the crate attributes to enable
 
 error[E0658]: `if` is not allowed in a `static mut`
-  --> $DIR/feature-gate-const-if-match.rs:43:5
+  --> $DIR/feature-gate-const-if-match.rs:42:5
    |
 LL |     if let Some(x) = Some(x) { x } else { 1 }
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
@@ -95,7 +95,7 @@ LL |     if let Some(x) = Some(x) { x } else { 1 }
    = help: add `#![feature(const_if_match)]` to the crate attributes to enable
 
 error[E0658]: `if` is not allowed in a `const fn`
-  --> $DIR/feature-gate-const-if-match.rs:48:5
+  --> $DIR/feature-gate-const-if-match.rs:47:5
    |
 LL |     if true { 5 } else { 6 }
    |     ^^^^^^^^^^^^^^^^^^^^^^^^
@@ -104,7 +104,7 @@ LL |     if true { 5 } else { 6 }
    = help: add `#![feature(const_if_match)]` to the crate attributes to enable
 
 error[E0658]: `if` is not allowed in a `const fn`
-  --> $DIR/feature-gate-const-if-match.rs:52:5
+  --> $DIR/feature-gate-const-if-match.rs:51:5
    |
 LL | /     if let Some(true) = a {
 LL | |         0
@@ -117,7 +117,7 @@ LL | |     }
    = help: add `#![feature(const_if_match)]` to the crate attributes to enable
 
 error[E0658]: `match` is not allowed in a `const fn`
-  --> $DIR/feature-gate-const-if-match.rs:60:5
+  --> $DIR/feature-gate-const-if-match.rs:59:5
    |
 LL | /     match i {
 LL | |         i if i > 10 => i,
@@ -130,7 +130,7 @@ LL | |     }
    = help: add `#![feature(const_if_match)]` to the crate attributes to enable
 
 error[E0658]: `if` is not allowed in a `const fn`
-  --> $DIR/feature-gate-const-if-match.rs:91:17
+  --> $DIR/feature-gate-const-if-match.rs:90:17
    |
 LL |         let x = if y { 0 } else { 1 };
    |                 ^^^^^^^^^^^^^^^^^^^^^
@@ -139,7 +139,7 @@ LL |         let x = if y { 0 } else { 1 };
    = help: add `#![feature(const_if_match)]` to the crate attributes to enable
 
 error[E0658]: `match` is not allowed in a `const fn`
-  --> $DIR/feature-gate-const-if-match.rs:93:17
+  --> $DIR/feature-gate-const-if-match.rs:92:17
    |
 LL |         let x = match x { 0 => 1, _ => 0 };
    |                 ^^^^^^^^^^^^^^^^^^^^^^^^^^
@@ -148,7 +148,7 @@ LL |         let x = match x { 0 => 1, _ => 0 };
    = help: add `#![feature(const_if_match)]` to the crate attributes to enable
 
 error[E0658]: `if` is not allowed in a `const fn`
-  --> $DIR/feature-gate-const-if-match.rs:95:9
+  --> $DIR/feature-gate-const-if-match.rs:94:9
    |
 LL |         if let Some(x) = Some(x) { x } else { 1 }
    |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
@@ -157,7 +157,7 @@ LL |         if let Some(x) = Some(x) { x } else { 1 }
    = help: add `#![feature(const_if_match)]` to the crate attributes to enable
 
 error[E0658]: `if` is not allowed in a `const`
-  --> $DIR/feature-gate-const-if-match.rs:111:17
+  --> $DIR/feature-gate-const-if-match.rs:110:17
    |
 LL |         let x = if false { 0 } else { 1 };
    |                 ^^^^^^^^^^^^^^^^^^^^^^^^^
@@ -166,7 +166,7 @@ LL |         let x = if false { 0 } else { 1 };
    = help: add `#![feature(const_if_match)]` to the crate attributes to enable
 
 error[E0658]: `match` is not allowed in a `const`
-  --> $DIR/feature-gate-const-if-match.rs:113:17
+  --> $DIR/feature-gate-const-if-match.rs:112:17
    |
 LL |         let x = match x { 0 => 1, _ => 0 };
    |                 ^^^^^^^^^^^^^^^^^^^^^^^^^^
@@ -175,7 +175,7 @@ LL |         let x = match x { 0 => 1, _ => 0 };
    = help: add `#![feature(const_if_match)]` to the crate attributes to enable
 
 error[E0658]: `if` is not allowed in a `const`
-  --> $DIR/feature-gate-const-if-match.rs:115:9
+  --> $DIR/feature-gate-const-if-match.rs:114:9
    |
 LL |         if let Some(x) = Some(x) { x } else { 1 }
    |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
@@ -184,7 +184,7 @@ LL |         if let Some(x) = Some(x) { x } else { 1 }
    = help: add `#![feature(const_if_match)]` to the crate attributes to enable
 
 error[E0658]: `if` is not allowed in a `const`
-  --> $DIR/feature-gate-const-if-match.rs:68:21
+  --> $DIR/feature-gate-const-if-match.rs:67:21
    |
 LL |     const IF: i32 = if true { 5 } else { 6 };
    |                     ^^^^^^^^^^^^^^^^^^^^^^^^
@@ -193,7 +193,7 @@ LL |     const IF: i32 = if true { 5 } else { 6 };
    = help: add `#![feature(const_if_match)]` to the crate attributes to enable
 
 error[E0658]: `if` is not allowed in a `const`
-  --> $DIR/feature-gate-const-if-match.rs:71:25
+  --> $DIR/feature-gate-const-if-match.rs:70:25
    |
 LL |     const IF_LET: i32 = if let Some(true) = None { 5 } else { 6 };
    |                         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
@@ -202,7 +202,7 @@ LL |     const IF_LET: i32 = if let Some(true) = None { 5 } else { 6 };
    = help: add `#![feature(const_if_match)]` to the crate attributes to enable
 
 error[E0658]: `match` is not allowed in a `const`
-  --> $DIR/feature-gate-const-if-match.rs:74:24
+  --> $DIR/feature-gate-const-if-match.rs:73:24
    |
 LL |     const MATCH: i32 = match 0 { 1 => 2, _ => 0 };
    |                        ^^^^^^^^^^^^^^^^^^^^^^^^^^
@@ -211,7 +211,7 @@ LL |     const MATCH: i32 = match 0 { 1 => 2, _ => 0 };
    = help: add `#![feature(const_if_match)]` to the crate attributes to enable
 
 error[E0658]: `if` is not allowed in a `const`
-  --> $DIR/feature-gate-const-if-match.rs:79:21
+  --> $DIR/feature-gate-const-if-match.rs:78:21
    |
 LL |     const IF: i32 = if true { 5 } else { 6 };
    |                     ^^^^^^^^^^^^^^^^^^^^^^^^
@@ -220,7 +220,7 @@ LL |     const IF: i32 = if true { 5 } else { 6 };
    = help: add `#![feature(const_if_match)]` to the crate attributes to enable
 
 error[E0658]: `if` is not allowed in a `const`
-  --> $DIR/feature-gate-const-if-match.rs:82:25
+  --> $DIR/feature-gate-const-if-match.rs:81:25
    |
 LL |     const IF_LET: i32 = if let Some(true) = None { 5 } else { 6 };
    |                         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
@@ -229,7 +229,7 @@ LL |     const IF_LET: i32 = if let Some(true) = None { 5 } else { 6 };
    = help: add `#![feature(const_if_match)]` to the crate attributes to enable
 
 error[E0658]: `match` is not allowed in a `const`
-  --> $DIR/feature-gate-const-if-match.rs:85:24
+  --> $DIR/feature-gate-const-if-match.rs:84:24
    |
 LL |     const MATCH: i32 = match 0 { 1 => 2, _ => 0 };
    |                        ^^^^^^^^^^^^^^^^^^^^^^^^^^
@@ -238,7 +238,7 @@ LL |     const MATCH: i32 = match 0 { 1 => 2, _ => 0 };
    = help: add `#![feature(const_if_match)]` to the crate attributes to enable
 
 error[E0019]: constant contains unimplemented expression type
-  --> $DIR/feature-gate-const-if-match.rs:115:21
+  --> $DIR/feature-gate-const-if-match.rs:114:21
    |
 LL |         if let Some(x) = Some(x) { x } else { 1 }
    |                     ^

src/test/ui/consts/control-flow/short-circuit-let.rs

@@ -4,7 +4,6 @@
 
 #![feature(const_if_match)]
 #![feature(const_panic)]
-#![feature(const_fn)]
 
 const X: i32 = {
     let mut x = 0;

src/test/ui/consts/control-flow/single_variant_match_ice.rs

@@ -1,6 +1,6 @@
 // check-pass
 
-#![feature(const_if_match, const_fn)]
+#![feature(const_if_match)]
 
 enum Foo {
     Prob,

src/test/ui/internal/internal-unstable-const.rs

@@ -0,0 +1,10 @@
+#![feature(staged_api)]
+#![feature(const_if_match)]
+
+#[stable(feature = "rust1", since = "1.0.0")]
+#[rustc_const_stable(feature = "rust1", since = "1.0.0")]
+const fn foo() -> i32 {
+    if true { 4 } else { 5 } //~ loops and conditional expressions are not stable in const fn
+}
+
+fn main() {}

src/test/ui/internal/internal-unstable-const.stderr

@@ -0,0 +1,12 @@
+error[E0723]: loops and conditional expressions are not stable in const fn
+  --> $DIR/internal-unstable-const.rs:7:5
+   |
+LL |     if true { 4 } else { 5 }
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: for more information, see issue https://github.com/rust-lang/rust/issues/57563
+   = help: add `#![feature(const_fn)]` to the crate attributes to enable
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0723`.