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Auto merge of #100096 - compiler-errors:fn-return-must-be-sized, r=jackh726
a fn pointer doesn't implement `Fn`/`FnMut`/`FnOnce` if its return type isn't sized I stumbled upon #83915 which hasn't received much attention recently, and I wanted to revive it since this is one existing soundness hole that seems pretty easy to fix. I'm not actually sure that the [alternative approach described here](https://github.com/rust-lang/rust/pull/83915#issuecomment-823643322) is sufficient, given the `src/test/ui/function-pointer/unsized-ret.rs` example I provided below. Rebasing the branch mentioned in that comment and testing that UI test, it seems that we actually end up only observing that `str: !Sized` during monomorphization, whereupon we ICE. Even if we were to fix that ICE, ideally we'd be raising an error that a fn pointer is being used badly during _typecheck_ instead of monomorphization, hence adapting the original approach in #83915. I am happy to close this if people would prefer we rebase the original PR and land that -- I am partly opening to be annoying and get people thinking about this unsoundness again ❤️ 😸 cc: `@estebank` and `@nikomatsakis` r? types Here's a link to the thread: https://rust-lang.zulipchat.com/#narrow/stream/144729-t-types/topic/PR.20.2383915/near/235421351 for more context.
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commit
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@ -618,7 +618,36 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
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)
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.map_bound(|(trait_ref, _)| trait_ref);
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let nested = self.confirm_poly_trait_refs(obligation, trait_ref)?;
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let mut nested = self.confirm_poly_trait_refs(obligation, trait_ref)?;
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// Confirm the `type Output: Sized;` bound that is present on `FnOnce`
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let cause = obligation.derived_cause(BuiltinDerivedObligation);
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// The binder on the Fn obligation is "less" important than the one on
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// the signature, as evidenced by how we treat it during projection.
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// The safe thing to do here is to liberate it, though, which should
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// have no worse effect than skipping the binder here.
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let liberated_fn_ty = self.infcx.replace_bound_vars_with_placeholders(obligation.self_ty());
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let output_ty = self
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.infcx
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.replace_bound_vars_with_placeholders(liberated_fn_ty.fn_sig(self.tcx()).output());
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let output_ty = normalize_with_depth_to(
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self,
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obligation.param_env,
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cause.clone(),
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obligation.recursion_depth,
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output_ty,
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&mut nested,
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);
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let tr = ty::Binder::dummy(ty::TraitRef::new(
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self.tcx().require_lang_item(LangItem::Sized, None),
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self.tcx().mk_substs_trait(output_ty, &[]),
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));
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nested.push(Obligation::new(
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cause,
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obligation.param_env,
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tr.to_poly_trait_predicate().to_predicate(self.tcx()),
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));
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Ok(ImplSourceFnPointerData { fn_ty: self_ty, nested })
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}
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15
src/test/ui/function-pointer/sized-ret-with-binder.rs
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15
src/test/ui/function-pointer/sized-ret-with-binder.rs
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@ -0,0 +1,15 @@
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// check-pass
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#![feature(unboxed_closures)]
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fn is_fn<T: for<'a> Fn<(&'a (),)>>() {}
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fn is_fn2<T: for<'a, 'b> Fn<(&'a &'b (),)>>() {}
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struct Outlives<'a, 'b>(std::marker::PhantomData<&'a &'b ()>);
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fn main() {
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is_fn::<for<'a> fn(&'a ()) -> &'a ()>();
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is_fn::<for<'a> fn(&'a ()) -> &'a dyn std::fmt::Debug>();
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is_fn2::<for<'a, 'b> fn(&'a &'b ()) -> Outlives<'a, 'b>>();
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is_fn2::<for<'a, 'b> fn(&'a &'b ()) -> (&'a (), &'a ())>();
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}
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14
src/test/ui/function-pointer/unsized-ret.rs
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src/test/ui/function-pointer/unsized-ret.rs
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@ -0,0 +1,14 @@
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#![feature(fn_traits)]
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#![feature(unboxed_closures)]
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fn foo<F: Fn<T>, T>(f: Option<F>, t: T) {
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let y = (f.unwrap()).call(t);
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}
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fn main() {
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foo::<fn() -> str, _>(None, ());
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//~^ ERROR the size for values of type `str` cannot be known at compilation time
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foo::<for<'a> fn(&'a ()) -> (dyn std::fmt::Display + 'a), _>(None, (&(),));
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//~^ ERROR the size for values of type `(dyn std::fmt::Display + 'a)` cannot be known at compilation time
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}
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35
src/test/ui/function-pointer/unsized-ret.stderr
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35
src/test/ui/function-pointer/unsized-ret.stderr
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@ -0,0 +1,35 @@
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error[E0277]: the size for values of type `str` cannot be known at compilation time
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--> $DIR/unsized-ret.rs:9:27
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LL | foo::<fn() -> str, _>(None, ());
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| --------------------- ^^^^ doesn't have a size known at compile-time
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| |
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| required by a bound introduced by this call
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= help: within `fn() -> str`, the trait `Sized` is not implemented for `str`
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= note: required because it appears within the type `fn() -> str`
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note: required by a bound in `foo`
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--> $DIR/unsized-ret.rs:4:11
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LL | fn foo<F: Fn<T>, T>(f: Option<F>, t: T) {
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| ^^^^^ required by this bound in `foo`
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error[E0277]: the size for values of type `(dyn std::fmt::Display + 'a)` cannot be known at compilation time
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--> $DIR/unsized-ret.rs:12:66
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LL | foo::<for<'a> fn(&'a ()) -> (dyn std::fmt::Display + 'a), _>(None, (&(),));
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| ------------------------------------------------------------ ^^^^ doesn't have a size known at compile-time
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| |
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| required by a bound introduced by this call
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= help: within `for<'a> fn(&'a ()) -> (dyn std::fmt::Display + 'a)`, the trait `for<'a> Sized` is not implemented for `(dyn std::fmt::Display + 'a)`
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= note: required because it appears within the type `for<'a> fn(&'a ()) -> (dyn std::fmt::Display + 'a)`
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note: required by a bound in `foo`
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--> $DIR/unsized-ret.rs:4:11
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LL | fn foo<F: Fn<T>, T>(f: Option<F>, t: T) {
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| ^^^^^ required by this bound in `foo`
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error: aborting due to 2 previous errors
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For more information about this error, try `rustc --explain E0277`.
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@ -1,10 +1,11 @@
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error[E0275]: overflow evaluating the requirement `fn() -> Foo {foo}: Sized`
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error[E0275]: overflow evaluating the requirement `Foo: Sized`
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--> $DIR/issue-53398-cyclic-types.rs:5:13
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LL | fn foo() -> Foo {
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| ^^^
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= help: consider increasing the recursion limit by adding a `#![recursion_limit = "256"]` attribute to your crate (`issue_53398_cyclic_types`)
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= note: required because it appears within the type `fn() -> Foo {foo}`
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error: aborting due to previous error
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