Rollup merge of #151281 - fee1-dead-contrib:push-zmqtzvuvlmuk, r=oli-obk

constify `Iterator`, take IV

Like its predecessors (rust-lang/rust#92433, rust-lang/rust#102225, rust-lang/rust#106541), this PR allows one to make `Iterator` implementations `const`, and thus enables the ability to have `for` loops in `const` contexts. I've also included constifying `Option as IntoIterator`, `option::IntoIter as Iterator` as a minimal dogfooding example.

But unlike its predecessors, it uses a new attribute (not unsound anymore!) that prevents any `Iterator` extension methods from being called. This is intentionally made minimal for an initial approval, the fun stuff like `.fold`, `Range as Iterator` could be done later.

cc @rust-lang/wg-const-eval, cc @oli-obk to review the compiler parts

cc rust-lang/rust#92476

Author: JonathanBrouwer

src/SUMMARY.md

@@ -199,7 +199,7 @@
     - [Inference details](./opaque-types-impl-trait-inference.md)
     - [Return Position Impl Trait In Trait](./return-position-impl-trait-in-trait.md)
     - [Region inference restrictions](./borrow-check/opaque-types-region-inference-restrictions.md)
-- [Const condition checking](./effects.md)
+- [Const traits and const condition checking](./effects.md)
 - [Pattern and exhaustiveness checking](./pat-exhaustive-checking.md)
 - [Unsafety checking](./unsafety-checking.md)
 - [MIR dataflow](./mir/dataflow.md)

src/effects.md

@@ -1,4 +1,4 @@
-# Effects and const condition checking
+# Effects, const traits, and const condition checking
 
 ## The `HostEffect` predicate
 
@@ -154,3 +154,18 @@ be dropped at compile time.
 
 [old solver]: https://doc.rust-lang.org/nightly/nightly-rustc/src/rustc_trait_selection/traits/effects.rs.html
 [new trait solver]: https://doc.rust-lang.org/nightly/nightly-rustc/src/rustc_next_trait_solver/solve/effect_goals.rs.html
+
+## More on const traits
+
+To be expanded later.
+
+### The `#[rustc_non_const_trait_method]` attribute
+
+This is intended for internal (standard library) usage only. With this attribute
+applied to a trait method, the compiler will not check the default body of this
+method for ability to run in compile time. Users of the trait will also not be
+allowed to use this trait method in const contexts. This attribute is primarily
+used for constifying large traits such as `Iterator` without having to make all
+its methods `const` at the same time.
+
+This attribute should not be present while stabilizing the trait as `const`.