From b9c620e87f6f664d8a02226d41bdf3c86c287ba8 Mon Sep 17 00:00:00 2001
From: Waffle Lapkin <waffle.lapkin@gmail.com>
Date: Thu, 13 Nov 2025 20:13:10 +0100
Subject: [PATCH] Remove an outdated test

This... is a weird test.

It has two impls:
- `impl<T> From<Foo<T>> for Box<T>` (commented out, more on that later), and
- `impl<T> Into<Vec<T>> for Foo<T>`

The idea of that test is to show that the first impl doesn't compile, but
the second does, thus `TryFrom` should be using `Into` and not `From`
(because `Into` is more general, since the `From` impl doesn't compile).

However:
1. The types are different -- `Box` vs `Vec`, which is significant b/c
   `Box` is fundamental
2. The commented out impl actually compiles! (which wasn't detected b/c
   it's commented out :\ )

Here is a table for compilation of the impls:

|        | `Vec`        | `Box`          |
|--------|--------------|----------------|
| `From` | since 1.41.0 | never          |
| `Into` | always       | not since 1.28 |

[godbolt used to test this](https://godbolt.org/z/T38E3jGKa)

Order of events:
1. in `1.28` the `incoherent_fundamental_impls` lint becomes deny by
   default (this is *not* mentioned in the changelog yay)
2. `1.32` changed absolutely nothing, even though this version is credited
   in the test
3. the test was added (I'm not exactly sure when)
   (see https://github.com/rust-lang/rust/pull/56796)
4. in `1.41` coherence was relaxed to allow `From`+`Vec` to compile

To conclude: since `1.41` this test does nothing (and before that it was
written in a way which did not detect this change). It looks to me like
today (since `1.41`) we *could* bound `TryFrom` impl with `From` (but now
it'd be a useless breaking change of course).

Am I missing anything? Is there a useful version of this test that could
be written?
---
 tests/ui/never_type/try_from.rs | 37 ---------------------------------
 1 file changed, 37 deletions(-)
 delete mode 100644 tests/ui/never_type/try_from.rs

diff --git a/tests/ui/never_type/try_from.rs b/tests/ui/never_type/try_from.rs
deleted file mode 100644
index acde524e98f5b..0000000000000
--- a/tests/ui/never_type/try_from.rs
+++ /dev/null
@@ -1,37 +0,0 @@
-//@ run-pass
-// This test relies on `TryFrom` being blanket impl for all `T: Into`
-// and `TryInto` being blanket impl for all `U: TryFrom`
-
-// This test was added to show the motivation for doing this
-// over `TryFrom` being blanket impl for all `T: From`
-
-#![feature(never_type)]
-
-use std::convert::{TryInto, Infallible};
-
-struct Foo<T> {
-    t: T,
-}
-
-// This fails to compile due to coherence restrictions
-// as of Rust version 1.32.x, therefore it could not be used
-// instead of the `Into` version of the impl, and serves as
-// motivation for a blanket impl for all `T: Into`, instead
-// of a blanket impl for all `T: From`
-/*
-impl<T> From<Foo<T>> for Box<T> {
-    fn from(foo: Foo<T>) -> Box<T> {
-        Box::new(foo.t)
-    }
-}
-*/
-
-impl<T> Into<Vec<T>> for Foo<T> {
-    fn into(self) -> Vec<T> {
-        vec![self.t]
-    }
-}
-
-pub fn main() {
-    let _: Result<Vec<i32>, Infallible> = Foo { t: 10 }.try_into();
-}