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Matthias Krüger 58d0730fb8
Rollup merge of - lukas-code:ptr-ptr-sub, r=compiler-errors
Allow instantiating trait object binder in ptr-to-ptr casts

For unsizing coercions between trait objects with the same principal, we already allow instantiating the for binder. For example, coercing `Box<dyn for<'a> Trait<'a>` to `Box<dyn Trait<'static>>` is allowed.

Since ptr-to-ptr casts will insert an unsizing coercion before the cast if possible, this has the consequence that the following compiles already:

```rust
// This compiles today.
fn cast<'b>(x: *mut dyn for<'a> Trait<'a>) -> *mut dyn Trait<'b> {
    // lowered as (roughly)
    // tmp: *mut dyn Trait<'?0> = Unsize(x)     // requires dyn for<'a> Trait<'a> <: dyn Trait<'?0>
    // ret: *mut dyn Trait<'b> = PtrToPtr(tmp)  // requires dyn Trait<'?0> == dyn Trait<'b>
    x as _
}
```

However, if no unsizing coercion is inserted then this currently fails to compile as one type is more general than the other. This PR will allow this code to compile, too, by changing ptr-to-ptr casts of pointers with vtable metadata to use sutyping instead of type equality.

```rust
// This will compile after this PR.
fn cast<'b>(x: *mut dyn for<'a> Trait<'a>) -> *mut Wrapper<dyn Trait<'b>> {
    // lowered as (roughly)
    // no Unsize here!
    // ret: *mut Wrapper<dyn Trait<'b>> = PtrToPtr(x)  // requires dyn for<'a> Trait<'a> == dyn Trait<'b>
    x as _
}
```

Note that it is already possible to work around the current restrictions and make the code compile before this PR by splitting the cast in two, so this shouldn't allow a new class of programs to compile:

```rust
// Workaround that compiles today.
fn cast<'b>(x: *mut dyn for<'a> Trait<'a>) -> *mut Wrapper<dyn Trait<'b>> {
    x as *mut dyn Trait<'_> as _
}
```

r? `@compiler-errors`
cc `@WaffleLapkin`
2024-09-28 09:35:12 +02:00
.github Merge pull request from GuillaumeGomez/sync_2024-08-12 2024-09-27 21:17:21 +02:00
compiler Rollup merge of - lukas-code:ptr-ptr-sub, r=compiler-errors 2024-09-28 09:35:12 +02:00
library Rollup merge of - tyilo:udp-unspecified, r=ibraheemdev 2024-09-28 09:35:09 +02:00
LICENSES Rename LICENSES/GPL-2.0.txt into LICENSES/GPL-2.0-only.txt 2024-09-06 16:01:46 +02:00
src Rollup merge of - workingjubilee:remove-space-saving-operations, r=Kobzol 2024-09-28 09:35:11 +02:00
tests Rollup merge of - lukas-code:ptr-ptr-sub, r=compiler-errors 2024-09-28 09:35:12 +02:00
.clang-format Add .clang-format 2024-06-26 05:56:00 +08:00
.editorconfig Only use max_line_length = 100 for *.rs 2023-07-10 15:18:36 -07:00
.git-blame-ignore-revs Add rustfmt 2024 reformatting to git blame ignore 2024-09-23 10:02:04 +02:00
.gitattributes Rename config.toml.example to config.example.toml 2023-03-11 14:10:00 -08:00
.gitignore gitignore: Add eglot LSP config file 2024-09-27 17:29:31 +01:00
.gitmodules Update to LLVM 19.1.0 2024-09-20 14:41:36 -07:00
.ignore Add .ignore file to make config.toml searchable in vscode 2024-06-24 10:15:16 +02:00
.mailmap Rollup merge of - BoxyUwU:boxy_mailmap, r=lqd 2024-09-02 22:35:23 +02:00
Cargo.lock Cleanup 2024-09-27 14:04:23 -04:00
Cargo.toml Cleanup 2024-09-27 14:04:23 -04:00
CODE_OF_CONDUCT.md Remove the code of conduct; instead link https://www.rust-lang.org/conduct.html 2019-10-05 22:55:19 +02:00
config.example.toml update llvm.download-ci-llvm documentation 2024-09-19 14:28:06 +03:00
configure Ensure ./configure works when configure.py path contains spaces 2024-02-16 18:57:22 +00:00
CONTRIBUTING.md fix: Update CONTRIBUTING.md recommend -> recommended 2023-11-16 23:57:09 +05:30
COPYRIGHT Update COPYRIGHT file 2022-10-30 10:23:14 -04:00
INSTALL.md Rollup merge of - onur-ozkan:use-bootstrap-instead-of-rustbuild, r=Mark-Simulacrum 2024-07-13 20:19:45 -07:00
LICENSE-APACHE Remove appendix from LICENCE-APACHE 2019-12-30 14:25:53 +00:00
LICENSE-MIT LICENSE-MIT: Remove inaccurate (misattributed) copyright notice 2017-07-26 16:51:58 -07:00
README.md Use SVG logos in the README.md. 2024-04-03 19:48:20 +02:00
RELEASES.md Rollup merge of - alexcrichton:remove-wasm32-wasip2-release-notes, r=pietroalbini 2024-09-06 07:33:58 +02:00
REUSE.toml Fix licensing information in REUSE.toml 2024-09-06 16:01:47 +02:00
rust-bors.toml Increase timeout for new bors bot 2024-03-13 08:31:07 +01:00
rustfmt.toml Use style_edition over version 2024-09-22 19:11:28 -04:00
triagebot.toml fix minor typo in triagebot config 2024-09-27 12:58:16 +09:00
x Make x capable of resolving symlinks 2023-10-14 17:53:33 +03:00
x.ps1 use & instead of start-process in x.ps1 2023-12-09 09:46:16 -05:00
x.py Fix recent python linting errors 2023-08-02 04:40:28 -04:00

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