Auto merge of #114776 - fee1-dead-contrib:enable-effects-in-libcore, r=oli-obk

Enable effects for libcore

~~r? `@oli-obk~~`

forgot you are on vacation, oops

19 files changed, 173 insertions, 133 deletions

diff --git a/compiler/rustc_hir_typeck/src/expr.rs b/compiler/rustc_hir_typeck/src/expr.rs
index 70d1dad8a6f..4ad14ce3059 100644
--- a/compiler/rustc_hir_typeck/src/expr.rs
+++ b/compiler/rustc_hir_typeck/src/expr.rs
@@ -525,8 +525,14 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             _ => self.instantiate_value_path(segs, opt_ty, res, expr.span, expr.hir_id).0,
         };
 
-        if let ty::FnDef(did, ..) = *ty.kind() {
+        if let ty::FnDef(did, callee_args) = *ty.kind() {
             let fn_sig = ty.fn_sig(tcx);
+
+            // HACK: whenever we get a FnDef in a non-const context, enforce effects to get the
+            // default `host = true` to avoid inference errors later.
+            if tcx.hir().body_const_context(self.body_id).is_none() {
+                self.enforce_context_effects(expr.hir_id, qpath.span(), did, callee_args);
+            }
             if tcx.fn_sig(did).skip_binder().abi() == RustIntrinsic
                 && tcx.item_name(did) == sym::transmute
             {
diff --git a/compiler/rustc_hir_typeck/src/fn_ctxt/checks.rs b/compiler/rustc_hir_typeck/src/fn_ctxt/checks.rs
index 4def7867384..37ea94d821e 100644
--- a/compiler/rustc_hir_typeck/src/fn_ctxt/checks.rs
+++ b/compiler/rustc_hir_typeck/src/fn_ctxt/checks.rs
@@ -273,11 +273,20 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             //
             // This check is here because there is currently no way to express a trait bound for `FnDef` types only.
             if is_const_eval_select && (1..=2).contains(&idx) {
-                if let ty::FnDef(def_id, _) = checked_ty.kind() {
-                    if idx == 1 && !self.tcx.is_const_fn_raw(*def_id) {
-                        self.tcx
-                            .sess
-                            .emit_err(errors::ConstSelectMustBeConst { span: provided_arg.span });
+                if let ty::FnDef(def_id, args) = *checked_ty.kind() {
+                    if idx == 1 {
+                        if !self.tcx.is_const_fn_raw(def_id) {
+                            self.tcx.sess.emit_err(errors::ConstSelectMustBeConst {
+                                span: provided_arg.span,
+                            });
+                        } else {
+                            self.enforce_context_effects(
+                                provided_arg.hir_id,
+                                provided_arg.span,
+                                def_id,
+                                args,
+                            )
+                        }
                     }
                 } else {
                     self.tcx.sess.emit_err(errors::ConstSelectMustBeFn {
diff --git a/compiler/rustc_passes/src/lang_items.rs b/compiler/rustc_passes/src/lang_items.rs
index 476394f30cc..7e837243918 100644
--- a/compiler/rustc_passes/src/lang_items.rs
+++ b/compiler/rustc_passes/src/lang_items.rs
@@ -20,7 +20,8 @@ use rustc_hir::lang_items::{extract, GenericRequirement};
 use rustc_hir::{LangItem, LanguageItems, Target};
 use rustc_middle::ty::TyCtxt;
 use rustc_session::cstore::ExternCrate;
-use rustc_span::{symbol::kw::Empty, Span};
+use rustc_span::symbol::kw::Empty;
+use rustc_span::{sym, Span};
 
 use rustc_middle::query::Providers;
 
@@ -157,7 +158,14 @@ impl<'tcx> LanguageItemCollector<'tcx> {
             self.tcx.hir().get_by_def_id(item_def_id)
         {
             let (actual_num, generics_span) = match kind.generics() {
-                Some(generics) => (generics.params.len(), generics.span),
+                Some(generics) => (
+                    generics
+                        .params
+                        .iter()
+                        .filter(|p| !self.tcx.has_attr(p.def_id, sym::rustc_host))
+                        .count(),
+                    generics.span,
+                ),
                 None => (0, *item_span),
             };
 
diff --git a/library/core/src/lib.rs b/library/core/src/lib.rs
index b5458e667ed..8b04bafcda5 100644
--- a/library/core/src/lib.rs
+++ b/library/core/src/lib.rs
@@ -195,6 +195,7 @@
 //
 // Language features:
 // tidy-alphabetical-start
+#![cfg_attr(not(bootstrap), feature(effects))]
 #![feature(abi_unadjusted)]
 #![feature(adt_const_params)]
 #![feature(allow_internal_unsafe)]
diff --git a/library/core/src/ops/drop.rs b/library/core/src/ops/drop.rs
index 21c23dabfc2..34dfa9e4c51 100644
--- a/library/core/src/ops/drop.rs
+++ b/library/core/src/ops/drop.rs
@@ -202,7 +202,7 @@
 /// [nomicon]: ../../nomicon/phantom-data.html#an-exception-the-special-case-of-the-standard-library-and-its-unstable-may_dangle
 #[lang = "drop"]
 #[stable(feature = "rust1", since = "1.0.0")]
-#[const_trait]
+// FIXME(effects) #[const_trait]
 pub trait Drop {
     /// Executes the destructor for this type.
     ///
diff --git a/library/core/src/ops/function.rs b/library/core/src/ops/function.rs
index 67c8245f0bf..20f0bba4c13 100644
--- a/library/core/src/ops/function.rs
+++ b/library/core/src/ops/function.rs
@@ -72,7 +72,7 @@ use crate::marker::Tuple;
 )]
 #[fundamental] // so that regex can rely that `&str: !FnMut`
 #[must_use = "closures are lazy and do nothing unless called"]
-#[const_trait]
+// FIXME(effects) #[const_trait]
 pub trait Fn<Args: Tuple>: FnMut<Args> {
     /// Performs the call operation.
     #[unstable(feature = "fn_traits", issue = "29625")]
@@ -159,7 +159,7 @@ pub trait Fn<Args: Tuple>: FnMut<Args> {
 )]
 #[fundamental] // so that regex can rely that `&str: !FnMut`
 #[must_use = "closures are lazy and do nothing unless called"]
-#[const_trait]
+// FIXME(effects) #[const_trait]
 pub trait FnMut<Args: Tuple>: FnOnce<Args> {
     /// Performs the call operation.
     #[unstable(feature = "fn_traits", issue = "29625")]
@@ -238,7 +238,7 @@ pub trait FnMut<Args: Tuple>: FnOnce<Args> {
 )]
 #[fundamental] // so that regex can rely that `&str: !FnMut`
 #[must_use = "closures are lazy and do nothing unless called"]
-#[const_trait]
+// FIXME(effects) #[const_trait]
 pub trait FnOnce<Args: Tuple> {
     /// The returned type after the call operator is used.
     #[lang = "fn_once_output"]
diff --git a/src/librustdoc/clean/mod.rs b/src/librustdoc/clean/mod.rs
index 190b1b038b6..c4011461885 100644
--- a/src/librustdoc/clean/mod.rs
+++ b/src/librustdoc/clean/mod.rs
@@ -793,6 +793,7 @@ fn clean_ty_generics<'tcx>(
                 }
                 Some(clean_generic_param_def(param, cx))
             }
+            ty::GenericParamDefKind::Const { is_host_effect: true, .. } => None,
             ty::GenericParamDefKind::Const { .. } => Some(clean_generic_param_def(param, cx)),
         })
         .collect::<ThinVec<GenericParamDef>>();
diff --git a/src/tools/clippy/clippy_utils/src/ty/type_certainty/mod.rs b/src/tools/clippy/clippy_utils/src/ty/type_certainty/mod.rs
index dc7756533ac..4b06b12fb94 100644
--- a/src/tools/clippy/clippy_utils/src/ty/type_certainty/mod.rs
+++ b/src/tools/clippy/clippy_utils/src/ty/type_certainty/mod.rs
@@ -207,7 +207,8 @@ fn path_segment_certainty(
             // Checking `res_generics_def_id(..)` before calling `generics_of` avoids an ICE.
             if cx.tcx.res_generics_def_id(path_segment.res).is_some() {
                 let generics = cx.tcx.generics_of(def_id);
-                let lhs = if (parent_certainty.is_certain() || generics.parent_count == 0) && generics.params.is_empty()
+                let count = generics.params.len() - generics.host_effect_index.is_some() as usize;
+                let lhs = if (parent_certainty.is_certain() || generics.parent_count == 0) && count == 0
                 {
                     Certainty::Certain(None)
                 } else {
@@ -299,7 +300,7 @@ fn type_is_inferrable_from_arguments(cx: &LateContext<'_>, expr: &Expr<'_>) -> b
 
     // Check that all type parameters appear in the functions input types.
     (0..(generics.parent_count + generics.params.len()) as u32).all(|index| {
-        fn_sig
+        Some(index as usize) == generics.host_effect_index || fn_sig
             .inputs()
             .iter()
             .any(|input_ty| contains_param(*input_ty.skip_binder(), index))
diff --git a/tests/rustdoc/rfc-2632-const-trait-impl.rs b/tests/rustdoc/rfc-2632-const-trait-impl.rs
index 5d742dc391a..7f56b2ffeb8 100644
--- a/tests/rustdoc/rfc-2632-const-trait-impl.rs
+++ b/tests/rustdoc/rfc-2632-const-trait-impl.rs
@@ -5,6 +5,8 @@
 // To future blessers: make sure that `const_trait_impl` is
 // stabilized when changing `@!has` to `@has`, and please do
 // not remove this test.
+//
+// FIXME(effects) add `const_trait` to `Fn` so we use `~const`
 #![feature(const_trait_impl)]
 #![crate_name = "foo"]
 
@@ -22,9 +24,9 @@ pub trait Tr<T> {
     // @has - '//section[@id="method.a"]/h4[@class="code-header"]/a[@class="trait"]' 'Fn'
     // @!has - '//section[@id="method.a"]/h4[@class="code-header"]/span[@class="where"]' '~const'
     // @has - '//section[@id="method.a"]/h4[@class="code-header"]/span[@class="where fmt-newline"]' ': Fn'
-    fn a<A: ~const Fn() + ~const Destruct>()
+    fn a<A: /* ~const */ Fn() + ~const Destruct>()
     where
-        Option<A>: ~const Fn() + ~const Destruct,
+        Option<A>: /* ~const */ Fn() + ~const Destruct,
     {
     }
 }
@@ -34,13 +36,13 @@ pub trait Tr<T> {
 // @has - '//section[@id="impl-Tr%3CT%3E-for-T"]/h3[@class="code-header"]/a[@class="trait"]' 'Fn'
 // @!has - '//section[@id="impl-Tr%3CT%3E-for-T"]/h3[@class="code-header"]/span[@class="where"]' '~const'
 // @has - '//section[@id="impl-Tr%3CT%3E-for-T"]/h3[@class="code-header"]/span[@class="where fmt-newline"]' ': Fn'
-impl<T: ~const Fn() + ~const Destruct> const Tr<T> for T
+impl<T: /* ~const */ Fn() + ~const Destruct> const Tr<T> for T
 where
-    Option<T>: ~const Fn() + ~const Destruct,
+    Option<T>: /* ~const */ Fn() + ~const Destruct,
 {
-    fn a<A: ~const Fn() + ~const Destruct>()
+    fn a<A: /* ~const */ Fn() + ~const Destruct>()
     where
-        Option<A>: ~const Fn() + ~const Destruct,
+        Option<A>: /* ~const */ Fn() + ~const Destruct,
     {
     }
 }
@@ -49,9 +51,9 @@ where
 // @has - '//pre[@class="rust item-decl"]/code/a[@class="trait"]' 'Fn'
 // @!has - '//pre[@class="rust item-decl"]/code/span[@class="where fmt-newline"]' '~const'
 // @has - '//pre[@class="rust item-decl"]/code/span[@class="where fmt-newline"]' ': Fn'
-pub const fn foo<F: ~const Fn() + ~const Destruct>()
+pub const fn foo<F: /* ~const */ Fn() + ~const Destruct>()
 where
-    Option<F>: ~const Fn() + ~const Destruct,
+    Option<F>: /* ~const */ Fn() + ~const Destruct,
 {
     F::a()
 }
@@ -61,9 +63,9 @@ impl<T> S<T> {
     // @has - '//section[@id="method.foo"]/h4[@class="code-header"]/a[@class="trait"]' 'Fn'
     // @!has - '//section[@id="method.foo"]/h4[@class="code-header"]/span[@class="where"]' '~const'
     // @has - '//section[@id="method.foo"]/h4[@class="code-header"]/span[@class="where fmt-newline"]' ': Fn'
-    pub const fn foo<B, C: ~const Fn() + ~const Destruct>()
+    pub const fn foo<B, C: /* ~const */ Fn() + ~const Destruct>()
     where
-        B: ~const Fn() + ~const Destruct,
+        B: /* ~const */ Fn() + ~const Destruct,
     {
         B::a()
     }
diff --git a/tests/ui/consts/fn_trait_refs.stderr b/tests/ui/consts/fn_trait_refs.stderr
index bfebf66701b..658a0b7b2a0 100644
--- a/tests/ui/consts/fn_trait_refs.stderr
+++ b/tests/ui/consts/fn_trait_refs.stderr
@@ -10,6 +10,66 @@ error[E0635]: unknown feature `const_cmp`
 LL | #![feature(const_cmp)]
    |            ^^^^^^^^^
 
-error: aborting due to 2 previous errors
+error: ~const can only be applied to `#[const_trait]` traits
+  --> $DIR/fn_trait_refs.rs:15:15
+   |
+LL |     T: ~const Fn<()> + ~const Destruct,
+   |               ^^^^^^
+
+error: ~const can only be applied to `#[const_trait]` traits
+  --> $DIR/fn_trait_refs.rs:15:15
+   |
+LL |     T: ~const Fn<()> + ~const Destruct,
+   |               ^^^^^^
+
+error: ~const can only be applied to `#[const_trait]` traits
+  --> $DIR/fn_trait_refs.rs:22:15
+   |
+LL |     T: ~const FnMut<()> + ~const Destruct,
+   |               ^^^^^^^^^
+
+error: ~const can only be applied to `#[const_trait]` traits
+  --> $DIR/fn_trait_refs.rs:22:15
+   |
+LL |     T: ~const FnMut<()> + ~const Destruct,
+   |               ^^^^^^^^^
+
+error: ~const can only be applied to `#[const_trait]` traits
+  --> $DIR/fn_trait_refs.rs:29:15
+   |
+LL |     T: ~const FnOnce<()>,
+   |               ^^^^^^^^^^
+
+error: ~const can only be applied to `#[const_trait]` traits
+  --> $DIR/fn_trait_refs.rs:29:15
+   |
+LL |     T: ~const FnOnce<()>,
+   |               ^^^^^^^^^^
+
+error: ~const can only be applied to `#[const_trait]` traits
+  --> $DIR/fn_trait_refs.rs:36:15
+   |
+LL |     T: ~const Fn<()> + ~const Destruct,
+   |               ^^^^^^
+
+error: ~const can only be applied to `#[const_trait]` traits
+  --> $DIR/fn_trait_refs.rs:36:15
+   |
+LL |     T: ~const Fn<()> + ~const Destruct,
+   |               ^^^^^^
+
+error: ~const can only be applied to `#[const_trait]` traits
+  --> $DIR/fn_trait_refs.rs:50:15
+   |
+LL |     T: ~const FnMut<()> + ~const Destruct,
+   |               ^^^^^^^^^
+
+error: ~const can only be applied to `#[const_trait]` traits
+  --> $DIR/fn_trait_refs.rs:50:15
+   |
+LL |     T: ~const FnMut<()> + ~const Destruct,
+   |               ^^^^^^^^^
+
+error: aborting due to 12 previous errors
 
 For more information about this error, try `rustc --explain E0635`.
diff --git a/tests/ui/consts/unstable-const-fn-in-libcore.rs b/tests/ui/consts/unstable-const-fn-in-libcore.rs
index 61e28117ed4..b62a74039f6 100644
--- a/tests/ui/consts/unstable-const-fn-in-libcore.rs
+++ b/tests/ui/consts/unstable-const-fn-in-libcore.rs
@@ -1,3 +1,5 @@
+// known-bug: #110395
+// FIXME check-pass
 // This is a non-regression test for const-qualification of unstable items in libcore
 // as explained in issue #67053.
 // const-qualification could miss some `const fn`s if they were unstable and the feature
@@ -15,12 +17,12 @@ impl<T> Opt<T> {
     #[rustc_const_unstable(feature = "foo", issue = "none")]
     #[stable(feature = "rust1", since = "1.0.0")]
     const fn unwrap_or_else<F: ~const FnOnce() -> T>(self, f: F) -> T {
-    //~^ ERROR destructor of
-    //~| ERROR destructor of
+    //FIXME ~^ ERROR destructor of
+    //FIXME ~| ERROR destructor of
         match self {
             Opt::Some(t) => t,
             Opt::None => f(),
-            //~^ ERROR cannot call
+            //FIXME ~^ ERROR cannot call
         }
     }
 }
diff --git a/tests/ui/consts/unstable-const-fn-in-libcore.stderr b/tests/ui/consts/unstable-const-fn-in-libcore.stderr
index 95d7b7e8537..b75f99a720f 100644
--- a/tests/ui/consts/unstable-const-fn-in-libcore.stderr
+++ b/tests/ui/consts/unstable-const-fn-in-libcore.stderr
@@ -1,34 +1,8 @@
-error[E0015]: cannot call non-const closure in constant functions
-  --> $DIR/unstable-const-fn-in-libcore.rs:22:26
-   |
-LL |             Opt::None => f(),
-   |                          ^^^
-   |
-   = note: calls in constant functions are limited to constant functions, tuple structs and tuple variants
-help: consider further restricting this bound
-   |
-LL |     const fn unwrap_or_else<F: ~const FnOnce() -> T + ~const std::ops::FnOnce<()>>(self, f: F) -> T {
-   |                                                     +++++++++++++++++++++++++++++
-
-error[E0493]: destructor of `F` cannot be evaluated at compile-time
-  --> $DIR/unstable-const-fn-in-libcore.rs:17:60
-   |
-LL |     const fn unwrap_or_else<F: ~const FnOnce() -> T>(self, f: F) -> T {
-   |                                                            ^ the destructor for this type cannot be evaluated in constant functions
-...
-LL |     }
-   |     - value is dropped here
-
-error[E0493]: destructor of `Opt<T>` cannot be evaluated at compile-time
-  --> $DIR/unstable-const-fn-in-libcore.rs:17:54
+error: ~const can only be applied to `#[const_trait]` traits
+  --> $DIR/unstable-const-fn-in-libcore.rs:19:39
    |
 LL |     const fn unwrap_or_else<F: ~const FnOnce() -> T>(self, f: F) -> T {
-   |                                                      ^^^^ the destructor for this type cannot be evaluated in constant functions
-...
-LL |     }
-   |     - value is dropped here
+   |                                       ^^^^^^^^^^^^^
 
-error: aborting due to 3 previous errors
+error: aborting due to previous error
 
-Some errors have detailed explanations: E0015, E0493.
-For more information about an error, try `rustc --explain E0015`.
diff --git a/tests/ui/impl-trait/normalize-tait-in-const.stderr b/tests/ui/impl-trait/normalize-tait-in-const.stderr
index 5fbba9a85ca..ada8fd7fa50 100644
--- a/tests/ui/impl-trait/normalize-tait-in-const.stderr
+++ b/tests/ui/impl-trait/normalize-tait-in-const.stderr
@@ -1,25 +1,8 @@
-error[E0015]: cannot call non-const closure in constant functions
-  --> $DIR/normalize-tait-in-const.rs:26:5
-   |
-LL |     fun(filter_positive());
-   |     ^^^^^^^^^^^^^^^^^^^^^^
-   |
-   = note: calls in constant functions are limited to constant functions, tuple structs and tuple variants
-help: consider further restricting this bound
-   |
-LL | const fn with_positive<F: ~const for<'a> Fn(&'a Alias<'a>) + ~const Destruct + ~const std::ops::Fn<(&Alias<'_>,)>>(fun: F) {
-   |                                                                              ++++++++++++++++++++++++++++++++++++
-
-error[E0493]: destructor of `F` cannot be evaluated at compile-time
-  --> $DIR/normalize-tait-in-const.rs:25:79
+error: ~const can only be applied to `#[const_trait]` traits
+  --> $DIR/normalize-tait-in-const.rs:25:42
    |
 LL | const fn with_positive<F: ~const for<'a> Fn(&'a Alias<'a>) + ~const Destruct>(fun: F) {
-   |                                                                               ^^^ the destructor for this type cannot be evaluated in constant functions
-LL |     fun(filter_positive());
-LL | }
-   | - value is dropped here
+   |                                          ^^^^^^^^^^^^^^^^^
 
-error: aborting due to 2 previous errors
+error: aborting due to previous error
 
-Some errors have detailed explanations: E0015, E0493.
-For more information about an error, try `rustc --explain E0015`.
diff --git a/tests/ui/rfcs/rfc-2632-const-trait-impl/const-closure-parse-not-item.rs b/tests/ui/rfcs/rfc-2632-const-trait-impl/const-closure-parse-not-item.rs
index 2c99d8bf1c6..15f062edf0e 100644
--- a/tests/ui/rfcs/rfc-2632-const-trait-impl/const-closure-parse-not-item.rs
+++ b/tests/ui/rfcs/rfc-2632-const-trait-impl/const-closure-parse-not-item.rs
@@ -1,4 +1,5 @@
-// check-pass
+// known-bug: #110395
+// FIXME check-pass
 
 #![feature(const_trait_impl, const_closures)]
 #![allow(incomplete_features)]
diff --git a/tests/ui/rfcs/rfc-2632-const-trait-impl/const-closure-parse-not-item.stderr b/tests/ui/rfcs/rfc-2632-const-trait-impl/const-closure-parse-not-item.stderr
new file mode 100644
index 00000000000..f25390a9070
--- /dev/null
+++ b/tests/ui/rfcs/rfc-2632-const-trait-impl/const-closure-parse-not-item.stderr
@@ -0,0 +1,8 @@
+error: ~const can only be applied to `#[const_trait]` traits
+  --> $DIR/const-closure-parse-not-item.rs:7:32
+   |
+LL | const fn test() -> impl ~const Fn() {
+   |                                ^^^^
+
+error: aborting due to previous error
+
diff --git a/tests/ui/rfcs/rfc-2632-const-trait-impl/const-closure-trait-method-fail.stderr b/tests/ui/rfcs/rfc-2632-const-trait-impl/const-closure-trait-method-fail.stderr
index 96ffca6519a..4c45b0e56c6 100644
--- a/tests/ui/rfcs/rfc-2632-const-trait-impl/const-closure-trait-method-fail.stderr
+++ b/tests/ui/rfcs/rfc-2632-const-trait-impl/const-closure-trait-method-fail.stderr
@@ -1,15 +1,8 @@
-error[E0015]: cannot call non-const closure in constant functions
-  --> $DIR/const-closure-trait-method-fail.rs:15:5
+error: ~const can only be applied to `#[const_trait]` traits
+  --> $DIR/const-closure-trait-method-fail.rs:14:39
    |
-LL |     x(())
-   |     ^^^^^
-   |
-   = note: calls in constant functions are limited to constant functions, tuple structs and tuple variants
-help: consider further restricting this bound
-   |
-LL | const fn need_const_closure<T: ~const FnOnce(()) -> i32 + ~const std::ops::FnOnce<((),)>>(x: T) -> i32 {
-   |                                                         ++++++++++++++++++++++++++++++++
+LL | const fn need_const_closure<T: ~const FnOnce(()) -> i32>(x: T) -> i32 {
+   |                                       ^^^^^^^^^^^^^^^^^
 
 error: aborting due to previous error
 
-For more information about this error, try `rustc --explain E0015`.
diff --git a/tests/ui/rfcs/rfc-2632-const-trait-impl/const-closure-trait-method.stderr b/tests/ui/rfcs/rfc-2632-const-trait-impl/const-closure-trait-method.stderr
index fd0c2911814..a8ef244ea30 100644
--- a/tests/ui/rfcs/rfc-2632-const-trait-impl/const-closure-trait-method.stderr
+++ b/tests/ui/rfcs/rfc-2632-const-trait-impl/const-closure-trait-method.stderr
@@ -1,15 +1,8 @@
-error[E0015]: cannot call non-const closure in constant functions
-  --> $DIR/const-closure-trait-method.rs:15:5
+error: ~const can only be applied to `#[const_trait]` traits
+  --> $DIR/const-closure-trait-method.rs:14:39
    |
-LL |     x(())
-   |     ^^^^^
-   |
-   = note: calls in constant functions are limited to constant functions, tuple structs and tuple variants
-help: consider further restricting this bound
-   |
-LL | const fn need_const_closure<T: ~const FnOnce(()) -> i32 + ~const std::ops::FnOnce<((),)>>(x: T) -> i32 {
-   |                                                         ++++++++++++++++++++++++++++++++
+LL | const fn need_const_closure<T: ~const FnOnce(()) -> i32>(x: T) -> i32 {
+   |                                       ^^^^^^^^^^^^^^^^^
 
 error: aborting due to previous error
 
-For more information about this error, try `rustc --explain E0015`.
diff --git a/tests/ui/rfcs/rfc-2632-const-trait-impl/const-closures.stderr b/tests/ui/rfcs/rfc-2632-const-trait-impl/const-closures.stderr
index abf2a2dc511..6d61b23e4b7 100644
--- a/tests/ui/rfcs/rfc-2632-const-trait-impl/const-closures.stderr
+++ b/tests/ui/rfcs/rfc-2632-const-trait-impl/const-closures.stderr
@@ -1,39 +1,26 @@
-error[E0015]: cannot call non-const closure in constant functions
-  --> $DIR/const-closures.rs:12:5
+error: ~const can only be applied to `#[const_trait]` traits
+  --> $DIR/const-closures.rs:8:19
    |
-LL |     f() * 7
-   |     ^^^
-   |
-   = note: calls in constant functions are limited to constant functions, tuple structs and tuple variants
-help: consider further restricting this bound
-   |
-LL |         F: ~const FnOnce() -> u8 + ~const std::ops::Fn<()>,
-   |                                  +++++++++++++++++++++++++
+LL |         F: ~const FnOnce() -> u8,
+   |                   ^^^^^^^^^^^^^^
 
-error[E0015]: cannot call non-const closure in constant functions
-  --> $DIR/const-closures.rs:24:5
-   |
-LL |     f() + f()
-   |     ^^^
+error: ~const can only be applied to `#[const_trait]` traits
+  --> $DIR/const-closures.rs:9:19
    |
-   = note: calls in constant functions are limited to constant functions, tuple structs and tuple variants
-help: consider further restricting this bound
-   |
-LL | const fn answer<F: ~const Fn() -> u8 + ~const std::ops::Fn<()>>(f: &F) -> u8 {
-   |                                      +++++++++++++++++++++++++
+LL |         F: ~const FnMut() -> u8,
+   |                   ^^^^^^^^^^^^^
 
-error[E0015]: cannot call non-const closure in constant functions
-  --> $DIR/const-closures.rs:24:11
-   |
-LL |     f() + f()
-   |           ^^^
+error: ~const can only be applied to `#[const_trait]` traits
+  --> $DIR/const-closures.rs:10:19
    |
-   = note: calls in constant functions are limited to constant functions, tuple structs and tuple variants
-help: consider further restricting this bound
+LL |         F: ~const Fn() -> u8,
+   |                   ^^^^^^^^^^
+
+error: ~const can only be applied to `#[const_trait]` traits
+  --> $DIR/const-closures.rs:23:27
    |
-LL | const fn answer<F: ~const Fn() -> u8 + ~const std::ops::Fn<()>>(f: &F) -> u8 {
-   |                                      +++++++++++++++++++++++++
+LL | const fn answer<F: ~const Fn() -> u8>(f: &F) -> u8 {
+   |                           ^^^^^^^^^^
 
-error: aborting due to 3 previous errors
+error: aborting due to 4 previous errors
 
-For more information about this error, try `rustc --explain E0015`.
diff --git a/tests/ui/rfcs/rfc-2632-const-trait-impl/effects/infer-fallback.rs b/tests/ui/rfcs/rfc-2632-const-trait-impl/effects/infer-fallback.rs
new file mode 100644
index 00000000000..2f474d978d3
--- /dev/null
+++ b/tests/ui/rfcs/rfc-2632-const-trait-impl/effects/infer-fallback.rs
@@ -0,0 +1,11 @@
+// check-pass
+#![feature(const_trait_impl, effects)]
+
+const fn a() {}
+
+fn foo<F: FnOnce()>(a: F) {}
+
+fn main() {
+    let _ = a;
+    foo(a);
+}