Add TyKind::Alias to keep type alias information

[GuillaumeGomez]

Proposal

Motivation

In rustdoc, we have this old issue where foreign type aliases in foreign functions declaration are discarded, only the aliased type remaining.

You can see it by documenting: https://github.com/svartalf/rust-issue-15823

The issue also appears for diagnostics where type aliases can be discarded, making them confusing for developers.

Proposal

After talking about this problem with @oli-obk, they suggested that a new TyAlias variant should be added to the TyKind enum. Similar to associated type projections, this will get resolved to the aliased type during normalization, but for rustdoc and diagnostics we retain the information that this was about a type alias.

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.

cc @rust-lang/compiler @rust-lang/compiler-contributors

[oli-obk]

@rustbot second

[apiraino]

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