Delete current polymorphization implementation

[saethlin]

Proposal

I am writing this because I keep tripping over the polymorphization implementation while working on post-mono MIR optimizations. I want there to be a point in the compiler where entire MIR bodies are fully monomorphized and then they stay monomorphized. The current polymorphization implementation breaks that mental model, in at least two ways:

• Anything that has been touched by polymorphization must not be re-monomorphized. This is surprisingly fiddly to uphold, because the current monomorphization strategy is that each rustc crate defines its own fn monomorphize and uses it to monomorphize individual consts, operands, etc.
• This comment is entirely correct; polymorphization means we cannot use TypingEnv::fully_monomorphized even though this is after monomorphization. https://github.com/rust-lang/rust/blob/b19329a37cedf2027517ae22c87cf201f93d776e/compiler/rustc_ty_utils/src/abi.rs#L48-L63

So there's this rake, and I keep stepping on it, and it's quite annoying.

I've done a GitHub code search for the flag and I cannot find any actual users: https://github.com/search?type=code&q=-Zpolymorphize+NOT+repo%3Arust-lang%2Frust++NOT+repo%3Amatthiaskrgr%2Ficemaker+NOT+repo%3Amatthiaskrgr%2Fglacier+NOT+repo%3Amatthiaskrgr%2Fglacier2+NOT+repo%3Arust-lang%2Fzulip_archive+NOT+path%3A%C2%B7tests%2Fui%2F*+NOT+path%3Atests%2Fui%2F*%2F*+NOT+path%3Atests%2Fui%2F*%2F*%2F*+NOT+path%3A%C2%B7src%2Ftest%2Fui%2F*+NOT+path%3Asrc%2Ftest%2Fui%2F*%2F*+NOT+path%3Asrc%2Ftest%2Fui%2F*%2F*%2F*+NOT+repo%3Adavidtwco%2Fzulip_archive+NOT+path%3Atests%2Fcrashes%2F*.rs&p=1

It doesn't make a whole lot of sense to be spending maintenance effort on a feature that may have users we can't see, but they are so few that none are public on GitHub.

There are a few more details about the current state and history in the feature's tracking issue, including a link to a sketch of a redesign: rust-lang/rust#124962

Mentors or Reviewers

If you have a reviewer or mentor in mind for this work, mention them
here. You can put your own name here if you are planning to mentor the
work.

Process

The main points of the Major Change Process are as follows:

• File an issue describing the proposal.
• A compiler team member or contributor who is knowledgeable in the area can second by writing @rustbot second.
  • Finding a "second" suffices for internal changes. If however, you are proposing a new public-facing feature, such as a -C flag, then full team check-off is required.
  • Compiler team members can initiate a check-off via @rfcbot fcp merge on either the MCP or the PR.
• Once an MCP is seconded, the Final Comment Period begins. If no objections are raised after 10 days, the MCP is considered approved.

You can read more about Major Change Proposals on forge.

Comments

This issue is not meant to be used for technical discussion. There is a Zulip stream for that. Use this issue to leave procedural comments, such as volunteering to review, indicating that you second the proposal (or third, etc), or raising a concern that you would like to be addressed.

[rustbot]

This issue is not meant to be used for technical discussion. There is a Zulip stream for that. Use this issue to leave procedural comments, such as volunteering to review, indicating that you second the proposal (or third, etc), or raising a concern that you would like to be addressed.

Concerns or objections to the proposal should be discussed on Zulip and formally registered here by adding a comment with the following syntax:

@rustbot concern reason-for-concern 
<description of the concern> 

Concerns can be lifted with:

@rustbot resolve reason-for-concern 

See documentation at https://forge.rust-lang.org

cc @rust-lang/compiler

[compiler-errors]

I'm reasonably confident that a new polymorphization implementation should be rewritten from scratch. From the codegen-adjacent type-system side, removing polymorphism would remove some hacks that I've been wanting to get rid of.

@rustbot second

[apiraino]

@rustbot label -final-comment-period +major-change-accepted