Validate ~const trait bounds on associated fns.

Previously, any associated function could have `~const` trait bounds on
generic parameters, which could lead to ICEs when these bounds were used
on associated functions of non-`#[const_trait] trait` or
non-`impl const` blocks.

Includes changes as per @fee1-dead's comments in #116210.

compiler/rustc_ast_passes/src/ast_validation.rs

@@ -52,7 +52,8 @@ struct AstValidator<'a> {
     /// Are we inside a trait impl?
     in_trait_impl: bool,
 
-    in_const_trait_impl: bool,
+    /// Are we inside a const trait defn or impl?
+    in_const_trait_or_impl: bool,
 
     has_proc_macro_decls: bool,
 
@@ -78,11 +79,19 @@ impl<'a> AstValidator<'a> {
         f: impl FnOnce(&mut Self),
     ) {
         let old = mem::replace(&mut self.in_trait_impl, is_in);
-        let old_const =
-            mem::replace(&mut self.in_const_trait_impl, matches!(constness, Some(Const::Yes(_))));
+        let old_const = mem::replace(
+            &mut self.in_const_trait_or_impl,
+            matches!(constness, Some(Const::Yes(_))),
+        );
         f(self);
         self.in_trait_impl = old;
-        self.in_const_trait_impl = old_const;
+        self.in_const_trait_or_impl = old_const;
+    }
+
+    fn with_in_trait(&mut self, is_const: bool, f: impl FnOnce(&mut Self)) {
+        let old = mem::replace(&mut self.in_const_trait_or_impl, is_const);
+        f(self);
+        self.in_const_trait_or_impl = old;
     }
 
     fn with_banned_impl_trait(&mut self, f: impl FnOnce(&mut Self)) {
@@ -933,23 +942,26 @@ impl<'a> Visitor<'a> for AstValidator<'a> {
                 }
             }
             ItemKind::Trait(box Trait { is_auto, generics, bounds, items, .. }) => {
-                if *is_auto == IsAuto::Yes {
-                    // Auto traits cannot have generics, super traits nor contain items.
-                    self.deny_generic_params(generics, item.ident.span);
-                    self.deny_super_traits(bounds, item.ident.span);
-                    self.deny_where_clause(&generics.where_clause, item.ident.span);
-                    self.deny_items(items, item.ident.span);
-                }
+                let is_const_trait = attr::contains_name(&item.attrs, sym::const_trait);
+                self.with_in_trait(is_const_trait, |this| {
+                    if *is_auto == IsAuto::Yes {
+                        // Auto traits cannot have generics, super traits nor contain items.
+                        this.deny_generic_params(generics, item.ident.span);
+                        this.deny_super_traits(bounds, item.ident.span);
+                        this.deny_where_clause(&generics.where_clause, item.ident.span);
+                        this.deny_items(items, item.ident.span);
+                    }
 
-                // Equivalent of `visit::walk_item` for `ItemKind::Trait` that inserts a bound
-                // context for the supertraits.
-                self.visit_vis(&item.vis);
-                self.visit_ident(item.ident);
-                self.visit_generics(generics);
-                self.with_tilde_const_allowed(|this| {
-                    walk_list!(this, visit_param_bound, bounds, BoundKind::SuperTraits)
+                    // Equivalent of `visit::walk_item` for `ItemKind::Trait` that inserts a bound
+                    // context for the supertraits.
+                    this.visit_vis(&item.vis);
+                    this.visit_ident(item.ident);
+                    this.visit_generics(generics);
+                    this.with_tilde_const_allowed(|this| {
+                        walk_list!(this, visit_param_bound, bounds, BoundKind::SuperTraits)
+                    });
+                    walk_list!(this, visit_assoc_item, items, AssocCtxt::Trait);
                 });
-                walk_list!(self, visit_assoc_item, items, AssocCtxt::Trait);
                 walk_list!(self, visit_attribute, &item.attrs);
                 return; // Avoid visiting again
             }
@@ -1278,7 +1290,7 @@ impl<'a> Visitor<'a> for AstValidator<'a> {
 
         let tilde_const_allowed =
             matches!(fk.header(), Some(FnHeader { constness: ast::Const::Yes(_), .. }))
-                || matches!(fk.ctxt(), Some(FnCtxt::Assoc(_)));
+                || matches!(fk.ctxt(), Some(FnCtxt::Assoc(_)) if self.in_const_trait_or_impl);
 
         let disallowed = (!tilde_const_allowed).then(|| DisallowTildeConstContext::Fn(fk));
 
@@ -1363,7 +1375,7 @@ impl<'a> Visitor<'a> for AstValidator<'a> {
                 walk_list!(self, visit_ty, ty);
             }
             AssocItemKind::Fn(box Fn { sig, generics, body, .. })
-                if self.in_const_trait_impl
+                if self.in_const_trait_or_impl
                     || ctxt == AssocCtxt::Trait
                     || matches!(sig.header.constness, Const::Yes(_)) =>
             {
@@ -1510,7 +1522,7 @@ pub fn check_crate(
         features,
         extern_mod: None,
         in_trait_impl: false,
-        in_const_trait_impl: false,
+        in_const_trait_or_impl: false,
         has_proc_macro_decls: false,
         outer_impl_trait: None,
         disallow_tilde_const: None,

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

@@ -0,0 +1,28 @@
+#![feature(const_trait_impl, effects)]
+
+#[const_trait]
+trait MyTrait {
+    fn do_something(&self);
+}
+
+trait OtherTrait {
+    fn do_something_else() where Self: ~const MyTrait;
+    //~^ ERROR `~const` is not allowed here
+}
+
+struct MyStruct<T>(T);
+
+impl const MyTrait for u32 {
+    fn do_something(&self) {}
+}
+
+impl<T> MyStruct<T> {
+    pub fn foo(&self) where T: ~const MyTrait {
+        //~^ ERROR `~const` is not allowed here
+        self.0.do_something();
+    }
+}
+
+fn main() {
+    MyStruct(0u32).foo();
+}

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

@@ -0,0 +1,26 @@
+error: `~const` is not allowed here
+  --> $DIR/const-bound-on-not-const-associated-fn.rs:9:40
+   |
+LL |     fn do_something_else() where Self: ~const MyTrait;
+   |                                        ^^^^^^^^^^^^^^
+   |
+note: this function is not `const`, so it cannot have `~const` trait bounds
+  --> $DIR/const-bound-on-not-const-associated-fn.rs:9:8
+   |
+LL |     fn do_something_else() where Self: ~const MyTrait;
+   |        ^^^^^^^^^^^^^^^^^
+
+error: `~const` is not allowed here
+  --> $DIR/const-bound-on-not-const-associated-fn.rs:20:32
+   |
+LL |     pub fn foo(&self) where T: ~const MyTrait {
+   |                                ^^^^^^^^^^^^^^
+   |
+note: this function is not `const`, so it cannot have `~const` trait bounds
+  --> $DIR/const-bound-on-not-const-associated-fn.rs:20:12
+   |
+LL |     pub fn foo(&self) where T: ~const MyTrait {
+   |            ^^^
+
+error: aborting due to 2 previous errors
+

tests/ui/rfcs/rfc-2632-const-trait-impl/tilde-const-and-const-params.rs

@@ -7,7 +7,8 @@ struct Foo<const N: usize>;
 
 impl<const N: usize> Foo<N> {
     fn add<A: ~const Add42>(self) -> Foo<{ A::add(N) }> {
-        //~^ ERROR mismatched types
+        //~^ ERROR `~const` is not allowed here
+        //~| ERROR mismatched types
         Foo
     }
 }
@@ -30,7 +31,7 @@ fn bar<A: ~const Add42, const N: usize>(_: Foo<N>) -> Foo<{ A::add(N) }> {
 }
 
 fn main() {
-   let foo = Foo::<0>;
-   let foo = bar::<(), _>(foo);
-   let _foo = bar::<(), _>(foo);
+    let foo = Foo::<0>;
+    let foo = bar::<(), _>(foo);
+    let _foo = bar::<(), _>(foo);
 }

tests/ui/rfcs/rfc-2632-const-trait-impl/tilde-const-and-const-params.stderr

@@ -1,17 +1,29 @@
 error: `~const` is not allowed here
-  --> $DIR/tilde-const-and-const-params.rs:26:11
+  --> $DIR/tilde-const-and-const-params.rs:9:15
+   |
+LL |     fn add<A: ~const Add42>(self) -> Foo<{ A::add(N) }> {
+   |               ^^^^^^^^^^^^
+   |
+note: this function is not `const`, so it cannot have `~const` trait bounds
+  --> $DIR/tilde-const-and-const-params.rs:9:8
+   |
+LL |     fn add<A: ~const Add42>(self) -> Foo<{ A::add(N) }> {
+   |        ^^^
+
+error: `~const` is not allowed here
+  --> $DIR/tilde-const-and-const-params.rs:27:11
    |
 LL | fn bar<A: ~const Add42, const N: usize>(_: Foo<N>) -> Foo<{ A::add(N) }> {
    |           ^^^^^^^^^^^^
    |
 note: this function is not `const`, so it cannot have `~const` trait bounds
-  --> $DIR/tilde-const-and-const-params.rs:26:4
+  --> $DIR/tilde-const-and-const-params.rs:27:4
    |
 LL | fn bar<A: ~const Add42, const N: usize>(_: Foo<N>) -> Foo<{ A::add(N) }> {
    |    ^^^
 
 error[E0308]: mismatched types
-  --> $DIR/tilde-const-and-const-params.rs:26:61
+  --> $DIR/tilde-const-and-const-params.rs:27:61
    |
 LL | fn bar<A: ~const Add42, const N: usize>(_: Foo<N>) -> Foo<{ A::add(N) }> {
    |                                                             ^^^^^^^^^ expected `false`, found `true`
@@ -28,6 +40,6 @@ LL |     fn add<A: ~const Add42>(self) -> Foo<{ A::add(N) }> {
    = note: expected constant `false`
               found constant `true`
 
-error: aborting due to 3 previous errors
+error: aborting due to 4 previous errors
 
 For more information about this error, try `rustc --explain E0308`.

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

@@ -6,7 +6,9 @@ trait Bar {}
 trait Foo {
     fn a();
     fn b() where Self: ~const Bar;
+    //~^ ERROR `~const` is not allowed here
     fn c<T: ~const Bar>();
+    //~^ ERROR `~const` is not allowed here
 }
 
 fn test1<T: Foo>() {

tests/ui/rfcs/rfc-2632-const-trait-impl/trait-where-clause.stderr

@@ -1,5 +1,29 @@
+error: `~const` is not allowed here
+  --> $DIR/trait-where-clause.rs:8:24
+   |
+LL |     fn b() where Self: ~const Bar;
+   |                        ^^^^^^^^^^
+   |
+note: this function is not `const`, so it cannot have `~const` trait bounds
+  --> $DIR/trait-where-clause.rs:8:8
+   |
+LL |     fn b() where Self: ~const Bar;
+   |        ^
+
+error: `~const` is not allowed here
+  --> $DIR/trait-where-clause.rs:10:13
+   |
+LL |     fn c<T: ~const Bar>();
+   |             ^^^^^^^^^^
+   |
+note: this function is not `const`, so it cannot have `~const` trait bounds
+  --> $DIR/trait-where-clause.rs:10:8
+   |
+LL |     fn c<T: ~const Bar>();
+   |        ^
+
 error[E0277]: the trait bound `T: Bar` is not satisfied
-  --> $DIR/trait-where-clause.rs:14:5
+  --> $DIR/trait-where-clause.rs:16:5
    |
 LL |     T::b();
    |     ^ the trait `Bar` is not implemented for `T`
@@ -15,13 +39,13 @@ LL | fn test1<T: Foo + Bar>() {
    |                 +++++
 
 error[E0277]: the trait bound `T: Bar` is not satisfied
-  --> $DIR/trait-where-clause.rs:16:12
+  --> $DIR/trait-where-clause.rs:18:12
    |
 LL |     T::c::<T>();
    |            ^ the trait `Bar` is not implemented for `T`
    |
 note: required by a bound in `Foo::c`
-  --> $DIR/trait-where-clause.rs:9:13
+  --> $DIR/trait-where-clause.rs:10:13
    |
 LL |     fn c<T: ~const Bar>();
    |             ^^^^^^^^^^ required by this bound in `Foo::c`
@@ -30,6 +54,6 @@ help: consider further restricting this bound
 LL | fn test1<T: Foo + Bar>() {
    |                 +++++
 
-error: aborting due to 2 previous errors
+error: aborting due to 4 previous errors
 
 For more information about this error, try `rustc --explain E0277`.