rust
/
rfcs
mirror of https://github.com/rust-lang/rfcs
1
0
Fork 0
Code Issues Releases Wiki Activity

Update text/0000-dyn-upcasting.md 

Co-authored-by: Jim Fasarakis-Hilliard <d.f.hilliard@gmail.com>
This commit is contained in:
Niko Matsakis 2022-12-10 06:15:30 -05:00 committed by GitHub
parent 7e3e7c2fbf
commit 54e355b94d
No known key found for this signature in database
GPG Key ID: 4AEE18F83AFDEB23
1 changed files with 1 additions and 1 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

2
text/0000-dyn-upcasting.md
View File

@ -54,7 +54,7 @@ The only upcasting coercion we permit for dyn today is to remove auto-traits; e.
One example use case comes from the [salsa](https://github.com/salsa-rs/salsa) crate. Salsa programs have a central database but they can be broken into many modules. Each module has a trait that defines its view on the final database. So for example a parser module might define a `ParserDb` trait that contains the methods the parser needs to be present. All code in the parser module then takes a `db: &mut dyn ParserDb` parameter; `dyn` traits are used to avoid monomorphization costs.
When one module uses another in Salsa, that is expressed via supertrait relationships. So if the type checker module wishes to invoke a parser, it might define its `trait TypeCheckDb: ParserDb` to have the `ParserDb` as a supertrait. The methods in the type checker then take a `db: &mut dyn TypeCheckerDb` parameter. If they wish to invoke the `ParserDb` methods, they would ideally be able to pass this `db` parameter to the parser methods and have it automatically upcast. This does not work with today's design, requiring elaborate workarounds.
When one module uses another in Salsa, that is expressed via supertrait relationships. So if the type checker module wishes to invoke a parser, it might define its `trait TypeCheckerDb: ParserDb` to have the `ParserDb` as a supertrait. The methods in the type checker then take a `db: &mut dyn TypeCheckerDb` parameter. If they wish to invoke the `ParserDb` methods, they would ideally be able to pass this `db` parameter to the parser methods and have it automatically upcast. This does not work with today's design, requiring elaborate workarounds.
# Guide-level explanation
[guide-level-explanation]: #guide-level-explanation