Fix stack overflow when checking for structural recursion
This pull request aims to fix#74224 and fix#84611. The current logic for detecting ADTs with structural recursion is flawed because it only looks at the root type, and then for exact matches. What I mean by this is that for examples such as:
```rust
struct A<T> {
x: T,
y: A<A<T>>,
}
struct B {
z: A<usize>
}
fn main() {}
```
When checking `A`, the compiler correctly determines that it has an infinite size (because the "root" type is `A`, and `A` occurs, albeit with different type arguments, as a nested type in `A`).
However, when checking `B`, it also recurses into `A`, but now `B` is the root type, and it only checks for _exact_ matches of `A`, but since `A` never precisely contains itself (only `A<A<T>>`, `A<A<A<T>>>`, etc.), an endless recursion ensues until the stack overflows.
In this PR, I have attempted to fix this behavior by implementing a two-phase checking: When checking `B`, my code first checks `A` _separately_ and stops if `A` already turns out to be infinite. If not (such as for `Option<T>`), the second phase checks whether the root type (`B`) is ever nested inside itself, e.g.:
```rust
struct Foo { x: Option<Option<Foo>> }
```
Special care needs to be taken for mutually recursive types, e.g.:
```rust
struct A<T> {
z: T,
x: B<T>,
}
struct B<T> {
y: A<T>
}
```
Here, both `A` and `B` both _are_ `SelfRecursive` and _contain_ a recursive type. The current behavior, which I have maintained, is to treat both `A` and `B` as `SelfRecursive`, and accordingly report errors for both.
Part of #68490.
Care has been taken to leave the old consts where appropriate, for testing backcompat regressions, module shadowing, etc. The intrinsics docs were accidentally referring to some methods on f64 as std::f64, which I changed due to being contrary with how we normally disambiguate the shadow module from the primitive. In one other place I changed std::u8 to std::ops since it was just testing path handling in macros.
For places which have legitimate uses of the old consts, deprecated attributes have been optimistically inserted. Although currently unnecessary, they exist to emphasize to any future deprecation effort the necessity of these specific symbols and prevent them from being accidentally removed.
add `unused_braces` lint
Add the lint `unused_braces` which is warn by default.
`unused_parens` is also extended and now checks anon consts.
closes#68387
r? @varkor
This purges uses of uninitialized where possible from test cases. Some
are merely moved over to the equally bad pattern of
MaybeUninit::uninit().assume_init() but with an annotation that this is
"the best we can do".
