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 :heart: :smile_cat:

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.

4 files changed, 66 insertions, 1 deletions

diff --git a/src/test/ui/function-pointer/sized-ret-with-binder.rs b/src/test/ui/function-pointer/sized-ret-with-binder.rs
new file mode 100644
index 00000000000..104ac4d222e
--- /dev/null
+++ b/src/test/ui/function-pointer/sized-ret-with-binder.rs
@@ -0,0 +1,15 @@
+// check-pass
+
+#![feature(unboxed_closures)]
+
+fn is_fn<T: for<'a> Fn<(&'a (),)>>() {}
+fn is_fn2<T: for<'a, 'b> Fn<(&'a &'b (),)>>() {}
+
+struct Outlives<'a, 'b>(std::marker::PhantomData<&'a &'b ()>);
+
+fn main() {
+    is_fn::<for<'a> fn(&'a ()) -> &'a ()>();
+    is_fn::<for<'a> fn(&'a ()) -> &'a dyn std::fmt::Debug>();
+    is_fn2::<for<'a, 'b> fn(&'a &'b ()) -> Outlives<'a, 'b>>();
+    is_fn2::<for<'a, 'b> fn(&'a &'b ()) -> (&'a (), &'a ())>();
+}
diff --git a/src/test/ui/function-pointer/unsized-ret.rs b/src/test/ui/function-pointer/unsized-ret.rs
new file mode 100644
index 00000000000..60af5769d6d
--- /dev/null
+++ b/src/test/ui/function-pointer/unsized-ret.rs
@@ -0,0 +1,14 @@
+#![feature(fn_traits)]
+#![feature(unboxed_closures)]
+
+fn foo<F: Fn<T>, T>(f: Option<F>, t: T) {
+    let y = (f.unwrap()).call(t);
+}
+
+fn main() {
+    foo::<fn() -> str, _>(None, ());
+    //~^ ERROR the size for values of type `str` cannot be known at compilation time
+
+    foo::<for<'a> fn(&'a ()) -> (dyn std::fmt::Display + 'a), _>(None, (&(),));
+    //~^ ERROR the size for values of type `(dyn std::fmt::Display + 'a)` cannot be known at compilation time
+}
diff --git a/src/test/ui/function-pointer/unsized-ret.stderr b/src/test/ui/function-pointer/unsized-ret.stderr
new file mode 100644
index 00000000000..bec3e2aa3fe
--- /dev/null
+++ b/src/test/ui/function-pointer/unsized-ret.stderr
@@ -0,0 +1,35 @@
+error[E0277]: the size for values of type `str` cannot be known at compilation time
+  --> $DIR/unsized-ret.rs:9:27
+   |
+LL |     foo::<fn() -> str, _>(None, ());
+   |     --------------------- ^^^^ doesn't have a size known at compile-time
+   |     |
+   |     required by a bound introduced by this call
+   |
+   = help: within `fn() -> str`, the trait `Sized` is not implemented for `str`
+   = note: required because it appears within the type `fn() -> str`
+note: required by a bound in `foo`
+  --> $DIR/unsized-ret.rs:4:11
+   |
+LL | fn foo<F: Fn<T>, T>(f: Option<F>, t: T) {
+   |           ^^^^^ required by this bound in `foo`
+
+error[E0277]: the size for values of type `(dyn std::fmt::Display + 'a)` cannot be known at compilation time
+  --> $DIR/unsized-ret.rs:12:66
+   |
+LL |     foo::<for<'a> fn(&'a ()) -> (dyn std::fmt::Display + 'a), _>(None, (&(),));
+   |     ------------------------------------------------------------ ^^^^ doesn't have a size known at compile-time
+   |     |
+   |     required by a bound introduced by this call
+   |
+   = 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)`
+   = note: required because it appears within the type `for<'a> fn(&'a ()) -> (dyn std::fmt::Display + 'a)`
+note: required by a bound in `foo`
+  --> $DIR/unsized-ret.rs:4:11
+   |
+LL | fn foo<F: Fn<T>, T>(f: Option<F>, t: T) {
+   |           ^^^^^ required by this bound in `foo`
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0277`.
diff --git a/src/test/ui/type-alias-impl-trait/issue-53398-cyclic-types.stderr b/src/test/ui/type-alias-impl-trait/issue-53398-cyclic-types.stderr
index d20b1cc6d85..0a34e8486a5 100644
--- a/src/test/ui/type-alias-impl-trait/issue-53398-cyclic-types.stderr
+++ b/src/test/ui/type-alias-impl-trait/issue-53398-cyclic-types.stderr
@@ -1,10 +1,11 @@
-error[E0275]: overflow evaluating the requirement `fn() -> Foo {foo}: Sized`
+error[E0275]: overflow evaluating the requirement `Foo: Sized`
   --> $DIR/issue-53398-cyclic-types.rs:5:13
    |
 LL | fn foo() -> Foo {
    |             ^^^
    |
    = help: consider increasing the recursion limit by adding a `#![recursion_limit = "256"]` attribute to your crate (`issue_53398_cyclic_types`)
+   = note: required because it appears within the type `fn() -> Foo {foo}`
 
 error: aborting due to previous error