Rollup merge of #121669 - nnethercote:count-stashed-errs-again, r=estebank

Count stashed errors again

Stashed diagnostics are such a pain. Their "might be emitted, might not" semantics messes with lots of things.

#120828 and #121206 made some big changes to how they work, improving some things, but still leaving some problems, as seen by the issues caused by #121206. This PR aims to fix all of them by restricting them in a way that eliminates the "might be emitted, might not" semantics while still allowing 98% of their benefit. Details in the individual commit logs.

r? `@oli-obk`

Author: GuillaumeGomez

tests/ui/crashes/unreachable-array-or-slice.rs

@@ -0,0 +1,8 @@
+struct Foo(isize, isize, isize, isize);
+
+pub fn main() {
+    let Self::anything_here_kills_it(a, b, ..) = Foo(5, 5, 5, 5);
+    match [5, 5, 5, 5] {
+        [..] => { }
+    }
+}

tests/ui/crashes/unreachable-array-or-slice.stderr

@@ -0,0 +1,9 @@
+error[E0433]: failed to resolve: `Self` is only available in impls, traits, and type definitions
+  --> tests/ui/crashes/unreachable-array-or-slice.rs:4:9
+   |
+LL |     let Self::anything_here_kills_it(a, b, ..) = Foo(5, 5, 5, 5);
+   |         ^^^^ `Self` is only available in impls, traits, and type definitions
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0433`.