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Merged
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Nov 12, 2023
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Deny more ~const trait bounds #117817

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5 changes: 4 additions & 1 deletion compiler/rustc_ast_passes/messages.ftl
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Original file line number Diff line number Diff line change
Expand Up @@ -214,9 +214,12 @@ 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
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@fee1-dead fee1-dead Nov 12, 2023 •
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This is quite ambiguous and we'd probably always want to have a reason.. OTOH, we don't want to allow tilde const for these "unknown" cases. Would be nice if this was turned into a span_bug! so that a more specific context is always defined.

No need for this PR to have this though.


ast_passes_trait_fn_const =
functions in traits cannot be declared const
Expand Down
75 changes: 30 additions & 45 deletions compiler/rustc_ast_passes/src/ast_validation.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -40,6 +40,9 @@ enum SelfSemantic {
enum DisallowTildeConstContext<'a> {
TraitObject,
Fn(FnKind<'a>),
Trait(Span),
Impl(Span),
Item,
}

struct AstValidator<'a> {
Expand Down Expand Up @@ -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,
Expand Down Expand Up @@ -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) => {
Expand Down Expand Up @@ -822,11 +813,9 @@ 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);

Expand All @@ -840,10 +829,10 @@ 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 {
Expand Down Expand Up @@ -875,6 +864,14 @@ 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));
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@fmease fmease Nov 12, 2023 •
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Currently permitting ~const trait bounds on inherent impls not to regress rfc-2632-const-trait-impl/tilde_const_on_impl_bound.rs. Not sure if the following code should compile at some point:

struct Struct;
#[const_trait] trait Trait {}
impl<T: ~const Trait> Struct {
    const fn f(_: T) {}
}

If I were to straight up deny ~const here, it'd obviously fix #117004. I haven't tested yet if the ~const lowering code can properly lower the code above.

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We should probably deny this first. We don't really need this to work and making it work certainly adds to the overhead. Perhaps a followup PR? Or I can do that. Thanks for working on this! :)

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I think we should deny ~const here. If we want to be able to use Trait in const fn f, I think the user should need to write:

struct Struct;
#[const_trait] trait Trait {}
impl<T: Trait> Struct {
    const fn f(_: T) where T: ~const Trait {}
}

at least in my opinion.

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);
Expand Down Expand Up @@ -955,8 +952,10 @@ 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);
Expand All @@ -976,16 +975,11 @@ 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;
Expand All @@ -1001,6 +995,7 @@ 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;
Expand Down Expand Up @@ -1189,15 +1184,18 @@ 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 });
Expand Down Expand Up @@ -1305,7 +1303,6 @@ 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));
}

Expand Down Expand Up @@ -1374,18 +1371,6 @@ 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
Expand Down Expand Up @@ -1537,7 +1522,7 @@ 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,
};
Expand Down
16 changes: 14 additions & 2 deletions compiler/rustc_ast_passes/src/errors.rs
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Expand Up @@ -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)]
Expand Down
4 changes: 2 additions & 2 deletions tests/ui/generic-const-items/const-trait-impl.rs
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Expand Up @@ -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
Expand Down
6 changes: 3 additions & 3 deletions tests/ui/generic-const-items/const-trait-impl.stderr
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@@ -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

Expand Down
10 changes: 9 additions & 1 deletion tests/ui/rfcs/rfc-2632-const-trait-impl/assoc-type-const-bound-usage.stderr
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@@ -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
|
Expand All @@ -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`.
10 changes: 9 additions & 1 deletion tests/ui/rfcs/rfc-2632-const-trait-impl/assoc-type.stderr
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@@ -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

2 changes: 1 addition & 1 deletion ...const-bound-on-not-const-associated-fn.rs → ...const-bound-on-not-const-associated-fn.rs
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@@ -1,4 +1,4 @@
#![feature(const_trait_impl, effects)]
#![feature(const_trait_impl)]

#[const_trait]
trait MyTrait {
Expand Down
0 ...t-bound-on-not-const-associated-fn.stderr → ...t-bound-on-not-const-associated-fn.stderr
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21 changes: 6 additions & 15 deletions tests/ui/rfcs/rfc-2632-const-trait-impl/const-drop.precise.stderr
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@@ -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`.
1 change: 1 addition & 0 deletions tests/ui/rfcs/rfc-2632-const-trait-impl/const-drop.rs
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Expand Up @@ -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> {
Expand Down
21 changes: 6 additions & 15 deletions tests/ui/rfcs/rfc-2632-const-trait-impl/const-drop.stock.stderr
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@@ -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`.
9 changes: 0 additions & 9 deletions tests/ui/rfcs/rfc-2632-const-trait-impl/issue-90052.rs
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This file was deleted.

14 changes: 0 additions & 14 deletions tests/ui/rfcs/rfc-2632-const-trait-impl/issue-90052.stderr
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6 changes: 1 addition & 5 deletions ...c-2632-const-trait-impl/specialization/const-default-bound-non-const-specialized-bound.rs
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Expand Up @@ -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,
{
Expand Down
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