We were able to uplift any value from `Tables` to `'static`, which is unsound.

The internal function was unsound, it could cause UB in rare cases where
the user inadvertly stored the returned object in a location that could
outlive the TyCtxt.

In order to make it safe, we now take a type context as an argument to
the internal fn, and we ensure that interned items are lifted using the
provided context.

Thus, this change ensures that the compiler can properly enforce
that the object does not outlive the type context it was lifted to.

InPlaceDstBufDrop holds onto the allocation before the shrinking happens
which means it must deallocate the destination elements but the source
allocation.

Added back previously available exports:

* Forward/Backward: used when implementing `AnalysisDomain`
* Engine: used in user's code to solve the dataflow problem
* SwitchIntEdgeEffects: used when implementing functions of the `Analysis` trait
* graphviz: potentially useful for debugging purposes

These exports are used when implementing external tools based on MIR
dataflow framework.

Closes rust-lang#120130

Update Readme

A couple minor formatting fixes and re-add missing "Installing from Source" section to the table of contents.

…imulacrum

Un-hide `iter::repeat_n`

ACP accepted in rust-lang/libs-team#120 (comment)

…nval

Make stable_mir::with_tables sound

See the first commit for the actual soundness fix. The rest is just fallout from that and is entirely safe code. Includes most of rust-lang#120120

The major difference to rust-lang#120120 is that we don't need an unsafe trait, as we can now rely on the type system (the only unsafe part, and the actual source of the unsoundness was in `with_tables`)

r? `@celinval`

…uviper

fix: Drop guard was deallocating with the incorrect size

InPlaceDstBufDrop holds onto the allocation before the shrinking happens which means it must deallocate the destination elements but the source allocation.

Thanks `@cuviper` for spotting this.

…thercote

`rustc_mir_dataflow`: Restore removed exports

Added back previously available exports:

* `Forward`/`Backward`: used when implementing `AnalysisDomain`
* `Engine`: used in user's code to solve the dataflow problem
* `SwitchIntEdgeEffects`: used when implementing functions of the `Analysis` trait
* `graphviz`: potentially useful for debugging purposes

Closes rust-lang#120130

Capture the rationale for `-Zallow-features=` in bootstrap.py

Based on the discussion in rust-lang#120096.

…r=dtolnay

Warn users about limited review for tier 2 and 3 code

Needs rust-lang/std-dev-guide#61

…s, r=dtolnay

Document some alternatives to `Vec::split_off`

One of the discussion points that came up in rust-lang#119917 is that some people use `Vec::split_off` in cases where they probably shouldn't, because the alternatives (like `mem::take`) are hard to discover.

This PR adds some suggestions to the documentation of `split_off` that should point people towards alternatives that might be more appropriate for their use-case.

I've deliberately tried to keep these changes as simple and uncontroversial as possible, so that they don't depend on how the team decides to handle the concerns raised in rust-lang#119917. That's why I haven't touched the existing documentation for `split_off`, and haven't added links to `split_off` to the documentation of other methods.