Auto merge of #88804 - Mark-Simulacrum:never-algo-v2, r=nikomatsakis,jackh726

Revise never type fallback algorithm

This is a rebase of https://github.com/rust-lang/rust/pull/84573, but dropping the stabilization of never type (and the accompanying large test diff).

Each commit builds & has tests updated alongside it, and could be reviewed in a more or less standalone fashion. But it may make more sense to review the PR as a whole, I'm not sure. It should be noted that tests being updated isn't really a good indicator of final behavior -- never_type_fallback is not enabled by default in this PR, so we can't really see the full effects of the commits here.

This combines the work by Niko, which is [documented in this gist](https://gist.github.com/nikomatsakis/7a07b265dc12f5c3b3bd0422018fa660), with some additional rules largely derived to target specific known patterns that regress with the algorithm solely derived by Niko. We build these from an intuition that:

* In general, fallback to `()` is *sound* in all cases
* But, in general, we *prefer* fallback to `!` as it accepts more code, particularly that written to intentionally use `!` (e.g., Result's with a Infallible/! variant).

When evaluating Niko's proposed algorithm, we find that there are certain cases where fallback to `!` leads to compilation failures in real-world code, and fallback to `()` fixes those errors. In order to allow for stabilization, we need to fix a good portion of these patterns.

The final rule set this PR proposes is that, by default, we fallback from `?T` to `!`, with the following exceptions:

1. `?T: Foo` and `Bar::Baz = ?T` and `(): Foo`, then fallback to `()`
2. Per [Niko's algorithm](https://gist.github.com/nikomatsakis/7a07b265dc12f5c3b3bd0422018fa660#proposal-fallback-chooses-between--and--based-on-the-coercion-graph), the "live" `?T` also fallback to `()`.

The first rule is necessary to address a fairly common pattern which boils down to something like the snippet below. Without rule 1, we do not see the closure's return type as needing a () fallback, which leads to compilation failure.

```rust
#![feature(never_type_fallback)]

trait Bar { }
impl Bar for () {  }
impl Bar for u32 {  }

fn foo<R: Bar>(_: impl Fn() -> R) {}

fn main() {
    foo(|| panic!());
}
```

r? `@jackh726`

compiler/rustc_middle/src/ty/mod.rs

@@ -2090,3 +2090,16 @@ impl<'tcx> fmt::Debug for SymbolName<'tcx> {
         fmt::Display::fmt(&self.name, fmt)
     }
 }
+
+#[derive(Debug, Default, Copy, Clone)]
+pub struct FoundRelationships {
+    /// This is true if we identified that this Ty (`?T`) is found in a `?T: Foo`
+    /// obligation, where:
+    ///
+    ///  * `Foo` is not `Sized`
+    ///  * `(): Foo` may be satisfied
+    pub self_in_trait: bool,
+    /// This is true if we identified that this Ty (`?T`) is found in a `<_ as
+    /// _>::AssocType = ?T`
+    pub output: bool,
+}

compiler/rustc_middle/src/ty/sty.rs

@@ -1672,6 +1672,14 @@ impl<'tcx> TyS<'tcx> {
         matches!(self.kind(), Infer(TyVar(_)))
     }
 
+    #[inline]
+    pub fn ty_vid(&self) -> Option<ty::TyVid> {
+        match self.kind() {
+            &Infer(TyVar(vid)) => Some(vid),
+            _ => None,
+        }
+    }
+
     #[inline]
     pub fn is_ty_infer(&self) -> bool {
         matches!(self.kind(), Infer(_))