Auto merge of rust-lang#117817 - fmease:deny-more-tilde-const, r=fee1-dead

Deny more `~const` trait bounds

thereby fixing a family of ICEs (delayed bugs) for `feature(const_trait_impl, effects)` code.

As discussed
r? `@fee1-dead`

Author: bors

compiler/rustc_ast_passes/messages.ftl

@@ -218,9 +218,12 @@ ast_passes_static_without_body =
     .suggestion = provide a definition for the static
 
 ast_passes_tilde_const_disallowed = `~const` is not allowed here
-    .trait = trait objects cannot have `~const` trait bounds
     .closure = closures cannot have `~const` trait bounds
     .function = this function is not `const`, so it cannot have `~const` trait bounds
+    .trait = this trait is not a `#[const_trait]`, so it cannot have `~const` trait bounds
+    .impl = this impl is not `const`, so it cannot have `~const` trait bounds
+    .object = trait objects cannot have `~const` trait bounds
+    .item = this item cannot have `~const` trait bounds
 
 ast_passes_trait_fn_const =
     functions in traits cannot be declared const

compiler/rustc_ast_passes/src/ast_validation.rs

@@ -40,6 +40,9 @@ enum SelfSemantic {
 enum DisallowTildeConstContext<'a> {
     TraitObject,
     Fn(FnKind<'a>),
+    Trait(Span),
+    Impl(Span),
+    Item,
 }
 
 struct AstValidator<'a> {
@@ -110,18 +113,6 @@ impl<'a> AstValidator<'a> {
         self.disallow_tilde_const = old;
     }
 
-    fn with_tilde_const_allowed(&mut self, f: impl FnOnce(&mut Self)) {
-        self.with_tilde_const(None, f)
-    }
-
-    fn with_banned_tilde_const(
-        &mut self,
-        ctx: DisallowTildeConstContext<'a>,
-        f: impl FnOnce(&mut Self),
-    ) {
-        self.with_tilde_const(Some(ctx), f)
-    }
-
     fn check_type_alias_where_clause_location(
         &mut self,
         ty_alias: &TyAlias,
@@ -173,7 +164,7 @@ impl<'a> AstValidator<'a> {
                 self.with_impl_trait(Some(t.span), |this| visit::walk_ty(this, t))
             }
             TyKind::TraitObject(..) => self
-                .with_banned_tilde_const(DisallowTildeConstContext::TraitObject, |this| {
+                .with_tilde_const(Some(DisallowTildeConstContext::TraitObject), |this| {
                     visit::walk_ty(this, t)
                 }),
             TyKind::Path(qself, path) => {
@@ -845,11 +836,9 @@ impl<'a> Visitor<'a> for AstValidator<'a> {
 
                     this.visit_vis(&item.vis);
                     this.visit_ident(item.ident);
-                    if let Const::Yes(_) = constness {
-                        this.with_tilde_const_allowed(|this| this.visit_generics(generics));
-                    } else {
-                        this.visit_generics(generics);
-                    }
+                    let disallowed = matches!(constness, Const::No)
+                        .then(|| DisallowTildeConstContext::Impl(item.span));
+                    this.with_tilde_const(disallowed, |this| this.visit_generics(generics));
                     this.visit_trait_ref(t);
                     this.visit_ty(self_ty);
 
@@ -863,10 +852,10 @@ impl<'a> Visitor<'a> for AstValidator<'a> {
                 polarity,
                 defaultness,
                 constness,
-                generics: _,
+                generics,
                 of_trait: None,
                 self_ty,
-                items: _,
+                items,
             }) => {
                 let error =
                     |annotation_span, annotation, only_trait: bool| errors::InherentImplCannot {
@@ -898,6 +887,14 @@ impl<'a> Visitor<'a> for AstValidator<'a> {
                 if let &Const::Yes(span) = constness {
                     self.err_handler().emit_err(error(span, "`const`", true));
                 }
+
+                self.visit_vis(&item.vis);
+                self.visit_ident(item.ident);
+                self.with_tilde_const(None, |this| this.visit_generics(generics));
+                self.visit_ty(self_ty);
+                walk_list!(self, visit_assoc_item, items, AssocCtxt::Impl);
+                walk_list!(self, visit_attribute, &item.attrs);
+                return; // Avoid visiting again.
             }
             ItemKind::Fn(box Fn { defaultness, sig, generics, body }) => {
                 self.check_defaultness(item.span, *defaultness);
@@ -978,8 +975,10 @@ impl<'a> Visitor<'a> for AstValidator<'a> {
                     // context for the supertraits.
                     this.visit_vis(&item.vis);
                     this.visit_ident(item.ident);
-                    this.visit_generics(generics);
-                    this.with_tilde_const_allowed(|this| {
+                    let disallowed =
+                        (!is_const_trait).then(|| DisallowTildeConstContext::Trait(item.span));
+                    this.with_tilde_const(disallowed, |this| {
+                        this.visit_generics(generics);
                         walk_list!(this, visit_param_bound, bounds, BoundKind::SuperTraits)
                     });
                     walk_list!(this, visit_assoc_item, items, AssocCtxt::Trait);
@@ -999,16 +998,11 @@ impl<'a> Visitor<'a> for AstValidator<'a> {
                 }
             }
             ItemKind::Struct(vdata, generics) => match vdata {
-                // Duplicating the `Visitor` logic allows catching all cases
-                // of `Anonymous(Struct, Union)` outside of a field struct or union.
-                //
-                // Inside `visit_ty` the validator catches every `Anonymous(Struct, Union)` it
-                // encounters, and only on `ItemKind::Struct` and `ItemKind::Union`
-                // it uses `visit_ty_common`, which doesn't contain that specific check.
                 VariantData::Struct(fields, ..) => {
                     self.visit_vis(&item.vis);
                     self.visit_ident(item.ident);
                     self.visit_generics(generics);
+                    // Permit `Anon{Struct,Union}` as field type.
                     walk_list!(self, visit_struct_field_def, fields);
                     walk_list!(self, visit_attribute, &item.attrs);
                     return;
@@ -1024,6 +1018,7 @@ impl<'a> Visitor<'a> for AstValidator<'a> {
                         self.visit_vis(&item.vis);
                         self.visit_ident(item.ident);
                         self.visit_generics(generics);
+                        // Permit `Anon{Struct,Union}` as field type.
                         walk_list!(self, visit_struct_field_def, fields);
                         walk_list!(self, visit_attribute, &item.attrs);
                         return;
@@ -1212,15 +1207,18 @@ impl<'a> Visitor<'a> for AstValidator<'a> {
                     if let Some(reason) = &self.disallow_tilde_const =>
                 {
                     let reason = match reason {
-                        DisallowTildeConstContext::TraitObject => {
-                            errors::TildeConstReason::TraitObject
-                        }
                         DisallowTildeConstContext::Fn(FnKind::Closure(..)) => {
                             errors::TildeConstReason::Closure
                         }
                         DisallowTildeConstContext::Fn(FnKind::Fn(_, ident, ..)) => {
                             errors::TildeConstReason::Function { ident: ident.span }
                         }
+                        &DisallowTildeConstContext::Trait(span) => errors::TildeConstReason::Trait { span },
+                        &DisallowTildeConstContext::Impl(span) => errors::TildeConstReason::Impl { span },
+                        DisallowTildeConstContext::TraitObject => {
+                            errors::TildeConstReason::TraitObject
+                        }
+                        DisallowTildeConstContext::Item => errors::TildeConstReason::Item,
                     };
                     self.err_handler()
                         .emit_err(errors::TildeConstDisallowed { span: bound.span(), reason });
@@ -1328,7 +1326,6 @@ impl<'a> Visitor<'a> for AstValidator<'a> {
                 || matches!(fk.ctxt(), Some(FnCtxt::Assoc(_)) if self.in_const_trait_or_impl);
 
         let disallowed = (!tilde_const_allowed).then(|| DisallowTildeConstContext::Fn(fk));
-
         self.with_tilde_const(disallowed, |this| visit::walk_fn(this, fk));
     }
 
@@ -1397,18 +1394,6 @@ impl<'a> Visitor<'a> for AstValidator<'a> {
         }
 
         match &item.kind {
-            AssocItemKind::Type(box TyAlias { generics, bounds, ty, .. })
-                if ctxt == AssocCtxt::Trait =>
-            {
-                self.visit_vis(&item.vis);
-                self.visit_ident(item.ident);
-                walk_list!(self, visit_attribute, &item.attrs);
-                self.with_tilde_const_allowed(|this| {
-                    this.visit_generics(generics);
-                    walk_list!(this, visit_param_bound, bounds, BoundKind::Bound);
-                });
-                walk_list!(self, visit_ty, ty);
-            }
             AssocItemKind::Fn(box Fn { sig, generics, body, .. })
                 if self.in_const_trait_or_impl
                     || ctxt == AssocCtxt::Trait
@@ -1552,7 +1537,7 @@ pub fn check_crate(
         in_const_trait_or_impl: false,
         has_proc_macro_decls: false,
         outer_impl_trait: None,
-        disallow_tilde_const: None,
+        disallow_tilde_const: Some(DisallowTildeConstContext::Item),
         is_impl_trait_banned: false,
         lint_buffer: lints,
     };

compiler/rustc_ast_passes/src/errors.rs

@@ -551,15 +551,27 @@ pub struct TildeConstDisallowed {
 
 #[derive(Subdiagnostic)]
 pub enum TildeConstReason {
-    #[note(ast_passes_trait)]
-    TraitObject,
     #[note(ast_passes_closure)]
     Closure,
     #[note(ast_passes_function)]
     Function {
         #[primary_span]
         ident: Span,
     },
+    #[note(ast_passes_trait)]
+    Trait {
+        #[primary_span]
+        span: Span,
+    },
+    #[note(ast_passes_impl)]
+    Impl {
+        #[primary_span]
+        span: Span,
+    },
+    #[note(ast_passes_object)]
+    TraitObject,
+    #[note(ast_passes_item)]
+    Item,
 }
 
 #[derive(Diagnostic)]

tests/ui/generic-const-items/const-trait-impl.rs

@@ -7,8 +7,8 @@
 #![allow(incomplete_features)]
 #![crate_type = "lib"]
 
-// FIXME(generic_const_items): Interpret `~const` as always-const.
-const CREATE<T: ~const Create>: T = T::create();
+// FIXME(generic_const_items, effects): Introduce `const` bounds to make this work.
+const CREATE<T: Create>: T = T::create();
 
 pub const K0: i32 = CREATE::<i32>;
 pub const K1: i32 = CREATE; // arg inferred

tests/ui/generic-const-items/const-trait-impl.stderr

@@ -1,8 +1,8 @@
 error[E0015]: cannot call non-const fn `<T as Create>::create` in constants
-  --> $DIR/const-trait-impl.rs:11:37
+  --> $DIR/const-trait-impl.rs:11:30
    |
-LL | const CREATE<T: ~const Create>: T = T::create();
-   |                                     ^^^^^^^^^^^
+LL | const CREATE<T: Create>: T = T::create();
+   |                              ^^^^^^^^^^^
    |
    = note: calls in constants are limited to constant functions, tuple structs and tuple variants
 

tests/ui/rfcs/rfc-2632-const-trait-impl/assoc-type-const-bound-usage.stderr

@@ -1,3 +1,11 @@
+error: `~const` is not allowed here
+  --> $DIR/assoc-type-const-bound-usage.rs:7:17
+   |
+LL |     type Assoc: ~const Foo;
+   |                 ^^^^^^^^^^
+   |
+   = note: this item cannot have `~const` trait bounds
+
 error[E0308]: mismatched types
   --> $DIR/assoc-type-const-bound-usage.rs:12:5
    |
@@ -7,6 +15,6 @@ LL |     <T as Foo>::Assoc::foo();
    = note: expected constant `host`
               found constant `true`
 
-error: aborting due to previous error
+error: aborting due to 2 previous errors
 
 For more information about this error, try `rustc --explain E0308`.

tests/ui/rfcs/rfc-2632-const-trait-impl/assoc-type.stderr

@@ -1,8 +1,16 @@
+error: `~const` is not allowed here
+  --> $DIR/assoc-type.rs:17:15
+   |
+LL |     type Bar: ~const std::ops::Add;
+   |               ^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: this item cannot have `~const` trait bounds
+
 error: ~const can only be applied to `#[const_trait]` traits
   --> $DIR/assoc-type.rs:17:22
    |
 LL |     type Bar: ~const std::ops::Add;
    |                      ^^^^^^^^^^^^^
 
-error: aborting due to previous error
+error: aborting due to 2 previous errors
 

tests/ui/rfcs/rfc-2632-const-trait-impl/effects/const-bound-on-not-const-associated-fn.rs renamed to tests/ui/rfcs/rfc-2632-const-trait-impl/const-bound-on-not-const-associated-fn.rs

@@ -1,4 +1,4 @@
-#![feature(const_trait_impl, effects)]
+#![feature(const_trait_impl)]
 
 #[const_trait]
 trait MyTrait {

tests/ui/rfcs/rfc-2632-const-trait-impl/effects/const-bound-on-not-const-associated-fn.stderr renamed to tests/ui/rfcs/rfc-2632-const-trait-impl/const-bound-on-not-const-associated-fn.stderr

@@ -1,19 +1,10 @@
-error[E0493]: destructor of `T` cannot be evaluated at compile-time
-  --> $DIR/const-drop.rs:19:32
+error: `~const` is not allowed here
+  --> $DIR/const-drop.rs:67:38
    |
-LL | const fn a<T: ~const Destruct>(_: T) {}
-   |                                ^      - value is dropped here
-   |                                |
-   |                                the destructor for this type cannot be evaluated in constant functions
-
-error[E0493]: destructor of `S<'_>` cannot be evaluated at compile-time
-  --> $DIR/const-drop.rs:24:13
+LL |     pub struct ConstDropWithBound<T: ~const SomeTrait>(pub core::marker::PhantomData<T>);
+   |                                      ^^^^^^^^^^^^^^^^
    |
-LL |     let _ = S(&mut c);
-   |             ^^^^^^^^^- value is dropped here
-   |             |
-   |             the destructor for this type cannot be evaluated in constant functions
+   = note: this item cannot have `~const` trait bounds
 
-error: aborting due to 2 previous errors
+error: aborting due to previous error
 
-For more information about this error, try `rustc --explain E0493`.

tests/ui/rfcs/rfc-2632-const-trait-impl/const-drop.precise.stderr

@@ -63,6 +63,7 @@ mod t {
         fn foo() {}
     }
 
+    // FIXME(effects): This should be a `const` bound instead of a `~const` one.
     pub struct ConstDropWithBound<T: ~const SomeTrait>(pub core::marker::PhantomData<T>);
 
     impl<T: ~const SomeTrait> const Drop for ConstDropWithBound<T> {

tests/ui/rfcs/rfc-2632-const-trait-impl/const-drop.rs

@@ -1,19 +1,10 @@
-error[E0493]: destructor of `T` cannot be evaluated at compile-time
-  --> $DIR/const-drop.rs:19:32
+error: `~const` is not allowed here
+  --> $DIR/const-drop.rs:67:38
    |
-LL | const fn a<T: ~const Destruct>(_: T) {}
-   |                                ^      - value is dropped here
-   |                                |
-   |                                the destructor for this type cannot be evaluated in constant functions
-
-error[E0493]: destructor of `S<'_>` cannot be evaluated at compile-time
-  --> $DIR/const-drop.rs:24:13
+LL |     pub struct ConstDropWithBound<T: ~const SomeTrait>(pub core::marker::PhantomData<T>);
+   |                                      ^^^^^^^^^^^^^^^^
    |
-LL |     let _ = S(&mut c);
-   |             ^^^^^^^^^- value is dropped here
-   |             |
-   |             the destructor for this type cannot be evaluated in constant functions
+   = note: this item cannot have `~const` trait bounds
 
-error: aborting due to 2 previous errors
+error: aborting due to previous error
 
-For more information about this error, try `rustc --explain E0493`.

tests/ui/rfcs/rfc-2632-const-trait-impl/const-drop.stock.stderr

@@ -18,11 +18,7 @@ trait Bar {
     fn bar();
 }
 
-// bgr360: I was only able to exercise the code path that raises the
-// "missing ~const qualifier" error by making this base impl non-const, even
-// though that doesn't really make sense to do. As seen below, if the base impl
-// is made const, rustc fails earlier with an overlapping impl failure.
-impl<T> Bar for T
+impl<T> const Bar for T
 where
     T: ~const Foo,
 {