Description
Proposal
Add a TypeFolder::Error associated type, and make all TypeFolder::fold_xxx methods return a Result<XXX, Self::Error>.
This gives more flexibility to implementors of the trait:
- if they never fail, they can set
Error = ! - if they can fail, they have the choice of panicking, or propagate an
Err, possibly holding a meaningful value
Implementation in rust-lang/rust#85469.
This is similar to #383, which added TypeVisitor::BreakTy.
This is also a part of the Traits WG's plan for a shared type library (cc rust-lang/types-team#16)
Example application
rust-lang/rust#85103: if we want to compute the layout of a type, we first need to normalize it, but normalization panics if it fails. One way to fix this is to make normalization, which uses TypeFolder, return a Result instead of panicking. This would be easy if this proposal was implemented.
Mentors or Reviewers
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 mergeon either the MCP or the PR.
- Finding a "second" suffices for internal changes. If however, you are proposing a new public-facing feature, such as a
- 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.