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Relax T: Sized bound on try_as_dyn / try_as_dyn_mut#156104

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Relax T: Sized bound on try_as_dyn / try_as_dyn_mut#156104
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@aobatact aobatact commented May 3, 2026 •
edited by rustbot
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trait_info_of already returns None for unsized types, so allowing T: ?Sized is sound and lets callers in generic contexts use these functions without a separate Sized bound. For unsized T, the function always returns None.

Tracking issue: #144361

Motivation

Currently, it is not possible to use try_as_dyn as "specialization-like" dispatch in
blanket impls with ?Sized type.

// Cannot add ?Sized now.
impl<T: 'static /* + ?Sized */, S: Serializer + 'static> Serialize<S> for T {
    fn serialize(&self, serializer: &mut S) -> Result<S::Ok, S::Error> {
        if let Some(spec) = try_as_dyn::<_, dyn SpecializedSer<S>>(self) {
            spec.specialized_serialize(serializer)
        } else {
            // fall back to compile-time reflection via TypeId
            ...
        }
    }
}

@rustbot rustbot added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. T-libs Relevant to the library team, which will review and decide on the PR/issue. labels May 3, 2026
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@aobatact
Relax T: Sized bound on try_as_dyn / try_as_dyn_mut
ecf527f 
`trait_info_of` already returns `None` for unsized types, so allowing
`T: ?Sized` is sound and lets callers in generic contexts use these
functions without a separate `Sized` bound. For unsized `T`, the
function always returns `None`.
@aobatact aobatact force-pushed the try-as-dyn-unsized branch from 7acc268 to ecf527f Compare May 3, 2026 14:00
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Kivooeo commented May 4, 2026

cc @oli-obk

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@bors r+ rollup

Makes sense

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rust-bors Bot commented May 4, 2026

📌 Commit ecf527f has been approved by oli-obk

It is now in the queue for this repository.

@rust-bors rust-bors Bot added S-waiting-on-bors Status: Waiting on bors to run and complete tests. Bors will change the label on completion. and removed S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. labels May 4, 2026
@oli-obk oli-obk assigned oli-obk and unassigned adwinwhite May 4, 2026
jhpratt added a commit to jhpratt/rust that referenced this pull request May 4, 2026
@jhpratt
Rollup merge of rust-lang#156104 - aobatact:try-as-dyn-unsized, r=oli…
44cd989 
…-obk

Relax `T: Sized` bound on `try_as_dyn` / `try_as_dyn_mut`

`trait_info_of` already returns `None` for unsized types, so allowing `T: ?Sized` is sound and lets callers in generic contexts use these functions without a separate `Sized` bound. For unsized `T`, the function always returns `None`.

Tracking issue: rust-lang#144361

## Motivation

Currently, it is not possible to use `try_as_dyn` as "specialization-like" dispatch in
blanket impls with `?Sized` type.

```rust
// Cannot add ?Sized now.
impl<T: 'static /* + ?Sized */, S: Serializer + 'static> Serialize<S> for T {
    fn serialize(&self, serializer: &mut S) -> Result<S::Ok, S::Error> {
        if let Some(spec) = try_as_dyn::<_, dyn SpecializedSer<S>>(self) {
            spec.specialized_serialize(serializer)
        } else {
            // fall back to compile-time reflection via TypeId
            ...
        }
    }
}
```
@jhpratt jhpratt mentioned this pull request May 4, 2026
Closed
rust-bors Bot pushed a commit that referenced this pull request May 4, 2026
@bors
Auto merge of #156140 - jhpratt:rollup-tW55BUD, r=jhpratt
2c0b32a 
Rollup of 6 pull requests

Successful merges:

 - #155543 (docs(unstable-book): Document const generics features)
 - #156043 (c-variadic: gate `va_arg` on `c_variadic_experimental_arch`)
 - #156082 (Move tests associated types)
 - #156092 (Clean up `TyCtxt::needs_crate_hash` usage and rename it to `needs_hir_hash`.)
 - #156104 (Relax `T: Sized` bound on `try_as_dyn` / `try_as_dyn_mut`)
 - #156128 (.mailmap: prefer matching just based on commit emails)
JonathanBrouwer added a commit to JonathanBrouwer/rust that referenced this pull request May 4, 2026
@JonathanBrouwer
Rollup merge of rust-lang#156104 - aobatact:try-as-dyn-unsized, r=oli…
43346f4 
…-obk

Relax `T: Sized` bound on `try_as_dyn` / `try_as_dyn_mut`

`trait_info_of` already returns `None` for unsized types, so allowing `T: ?Sized` is sound and lets callers in generic contexts use these functions without a separate `Sized` bound. For unsized `T`, the function always returns `None`.

Tracking issue: rust-lang#144361

## Motivation

Currently, it is not possible to use `try_as_dyn` as "specialization-like" dispatch in
blanket impls with `?Sized` type.

```rust
// Cannot add ?Sized now.
impl<T: 'static /* + ?Sized */, S: Serializer + 'static> Serialize<S> for T {
    fn serialize(&self, serializer: &mut S) -> Result<S::Ok, S::Error> {
        if let Some(spec) = try_as_dyn::<_, dyn SpecializedSer<S>>(self) {
            spec.specialized_serialize(serializer)
        } else {
            // fall back to compile-time reflection via TypeId
            ...
        }
    }
}
```
@JonathanBrouwer JonathanBrouwer mentioned this pull request May 4, 2026
Closed
JonathanBrouwer added a commit to JonathanBrouwer/rust that referenced this pull request May 4, 2026
@JonathanBrouwer
Rollup merge of rust-lang#156104 - aobatact:try-as-dyn-unsized, r=oli…
6beef7f 
…-obk

Relax `T: Sized` bound on `try_as_dyn` / `try_as_dyn_mut`

`trait_info_of` already returns `None` for unsized types, so allowing `T: ?Sized` is sound and lets callers in generic contexts use these functions without a separate `Sized` bound. For unsized `T`, the function always returns `None`.

Tracking issue: rust-lang#144361

## Motivation

Currently, it is not possible to use `try_as_dyn` as "specialization-like" dispatch in
blanket impls with `?Sized` type.

```rust
// Cannot add ?Sized now.
impl<T: 'static /* + ?Sized */, S: Serializer + 'static> Serialize<S> for T {
    fn serialize(&self, serializer: &mut S) -> Result<S::Ok, S::Error> {
        if let Some(spec) = try_as_dyn::<_, dyn SpecializedSer<S>>(self) {
            spec.specialized_serialize(serializer)
        } else {
            // fall back to compile-time reflection via TypeId
            ...
        }
    }
}
```
@JonathanBrouwer JonathanBrouwer mentioned this pull request May 4, 2026
Closed
rust-bors Bot pushed a commit that referenced this pull request May 4, 2026
@bors
Auto merge of #156150 - JonathanBrouwer:rollup-4mvqynT, r=JonathanBro…
6ef8ad3 
…uwer

Rollup of 10 pull requests

Successful merges:

 - #155848 ([doc]: Revert `core::io::ErrorKind` doc changes)
 - #155855 (Remove unnecessary `get_unchecked`)
 - #156062 (Added command-line argument support for `wasm32-wali-linux-musl`)
 - #155543 (docs(unstable-book): Document const generics features)
 - #156043 (c-variadic: gate `va_arg` on `c_variadic_experimental_arch`)
 - #156082 (Move tests associated types)
 - #156092 (Clean up `TyCtxt::needs_crate_hash` usage and rename it to `needs_hir_hash`.)
 - #156104 (Relax `T: Sized` bound on `try_as_dyn` / `try_as_dyn_mut`)
 - #156128 (.mailmap: prefer matching just based on commit emails)
 - #156135 (Remove most uses of def_id_to_node_id)
JonathanBrouwer added a commit to JonathanBrouwer/rust that referenced this pull request May 4, 2026
@JonathanBrouwer
Rollup merge of rust-lang#156104 - aobatact:try-as-dyn-unsized, r=oli…
34552f7 
…-obk

Relax `T: Sized` bound on `try_as_dyn` / `try_as_dyn_mut`

`trait_info_of` already returns `None` for unsized types, so allowing `T: ?Sized` is sound and lets callers in generic contexts use these functions without a separate `Sized` bound. For unsized `T`, the function always returns `None`.

Tracking issue: rust-lang#144361

## Motivation

Currently, it is not possible to use `try_as_dyn` as "specialization-like" dispatch in
blanket impls with `?Sized` type.

```rust
// Cannot add ?Sized now.
impl<T: 'static /* + ?Sized */, S: Serializer + 'static> Serialize<S> for T {
    fn serialize(&self, serializer: &mut S) -> Result<S::Ok, S::Error> {
        if let Some(spec) = try_as_dyn::<_, dyn SpecializedSer<S>>(self) {
            spec.specialized_serialize(serializer)
        } else {
            // fall back to compile-time reflection via TypeId
            ...
        }
    }
}
```
@JonathanBrouwer JonathanBrouwer mentioned this pull request May 4, 2026
Closed
rust-bors Bot pushed a commit that referenced this pull request May 4, 2026
@bors
Auto merge of #156153 - JonathanBrouwer:rollup-yxbgv3R, r=JonathanBro…
73e4e3b 
…uwer

Rollup of 10 pull requests

Successful merges:

 - #155848 ([doc]: Revert `core::io::ErrorKind` doc changes)
 - #155855 (Remove unnecessary `get_unchecked`)
 - #155543 (docs(unstable-book): Document const generics features)
 - #155962 (`rustc`: `target_features`: allow for `cfg`-only stable `target_features`)
 - #156043 (c-variadic: gate `va_arg` on `c_variadic_experimental_arch`)
 - #156082 (Move tests associated types)
 - #156092 (Clean up `TyCtxt::needs_crate_hash` usage and rename it to `needs_hir_hash`.)
 - #156104 (Relax `T: Sized` bound on `try_as_dyn` / `try_as_dyn_mut`)
 - #156128 (.mailmap: prefer matching just based on commit emails)
 - #156135 (Remove most uses of def_id_to_node_id)
JonathanBrouwer added a commit to JonathanBrouwer/rust that referenced this pull request May 4, 2026
@JonathanBrouwer
Rollup merge of rust-lang#156104 - aobatact:try-as-dyn-unsized, r=oli…
40525b2 
…-obk

Relax `T: Sized` bound on `try_as_dyn` / `try_as_dyn_mut`

`trait_info_of` already returns `None` for unsized types, so allowing `T: ?Sized` is sound and lets callers in generic contexts use these functions without a separate `Sized` bound. For unsized `T`, the function always returns `None`.

Tracking issue: rust-lang#144361

## Motivation

Currently, it is not possible to use `try_as_dyn` as "specialization-like" dispatch in
blanket impls with `?Sized` type.

```rust
// Cannot add ?Sized now.
impl<T: 'static /* + ?Sized */, S: Serializer + 'static> Serialize<S> for T {
    fn serialize(&self, serializer: &mut S) -> Result<S::Ok, S::Error> {
        if let Some(spec) = try_as_dyn::<_, dyn SpecializedSer<S>>(self) {
            spec.specialized_serialize(serializer)
        } else {
            // fall back to compile-time reflection via TypeId
            ...
        }
    }
}
```
@JonathanBrouwer JonathanBrouwer mentioned this pull request May 4, 2026
Open
rust-bors Bot pushed a commit that referenced this pull request May 4, 2026
@bors
Auto merge of #156158 - JonathanBrouwer:rollup-fd5s7KQ, r=JonathanBro…
a41abee 
…uwer

Rollup of 12 pull requests

Successful merges:

 - #155848 ([doc]: Revert `core::io::ErrorKind` doc changes)
 - #155855 (Remove unnecessary `get_unchecked`)
 - #155543 (docs(unstable-book): Document const generics features)
 - #155962 (`rustc`: `target_features`: allow for `cfg`-only stable `target_features`)
 - #156043 (c-variadic: gate `va_arg` on `c_variadic_experimental_arch`)
 - #156082 (Move tests associated types)
 - #156092 (Clean up `TyCtxt::needs_crate_hash` usage and rename it to `needs_hir_hash`.)
 - #156103 (Fix E0040 suggestion for explicit `Drop::drop` UFCS calls)
 - #156104 (Relax `T: Sized` bound on `try_as_dyn` / `try_as_dyn_mut`)
 - #156128 (.mailmap: prefer matching just based on commit emails)
 - #156135 (Remove most uses of def_id_to_node_id)
 - #156152 (Update books)
GuillaumeGomez added a commit to GuillaumeGomez/rust that referenced this pull request May 5, 2026
@GuillaumeGomez
Rollup merge of rust-lang#156104 - aobatact:try-as-dyn-unsized, r=oli…
ca222b8 
…-obk

Relax `T: Sized` bound on `try_as_dyn` / `try_as_dyn_mut`

`trait_info_of` already returns `None` for unsized types, so allowing `T: ?Sized` is sound and lets callers in generic contexts use these functions without a separate `Sized` bound. For unsized `T`, the function always returns `None`.

Tracking issue: rust-lang#144361

## Motivation

Currently, it is not possible to use `try_as_dyn` as "specialization-like" dispatch in
blanket impls with `?Sized` type.

```rust
// Cannot add ?Sized now.
impl<T: 'static /* + ?Sized */, S: Serializer + 'static> Serialize<S> for T {
    fn serialize(&self, serializer: &mut S) -> Result<S::Ok, S::Error> {
        if let Some(spec) = try_as_dyn::<_, dyn SpecializedSer<S>>(self) {
            spec.specialized_serialize(serializer)
        } else {
            // fall back to compile-time reflection via TypeId
            ...
        }
    }
}
```
@GuillaumeGomez GuillaumeGomez mentioned this pull request May 5, 2026
Open
rust-bors Bot pushed a commit that referenced this pull request May 5, 2026
@bors
Auto merge of #156166 - GuillaumeGomez:rollup-nfRhhYb, r=GuillaumeGomez
cc54c7e 
Rollup of 16 pull requests

Successful merges:

 - #155848 ([doc]: Revert `core::io::ErrorKind` doc changes)
 - #155855 (Remove unnecessary `get_unchecked`)
 - #155543 (docs(unstable-book): Document const generics features)
 - #155962 (`rustc`: `target_features`: allow for `cfg`-only stable `target_features`)
 - #156043 (c-variadic: gate `va_arg` on `c_variadic_experimental_arch`)
 - #156082 (Move tests associated types)
 - #156087 (Improve `&pin` reference-pattern suggestions)
 - #156092 (Clean up `TyCtxt::needs_crate_hash` usage and rename it to `needs_hir_hash`.)
 - #156103 (Fix E0040 suggestion for explicit `Drop::drop` UFCS calls)
 - #156104 (Relax `T: Sized` bound on `try_as_dyn` / `try_as_dyn_mut`)
 - #156122 (Add a `doc_cfg` regression test to ensure foreign types impls are working as expected)
 - #156128 (.mailmap: prefer matching just based on commit emails)
 - #156135 (Remove most uses of def_id_to_node_id)
 - #156152 (Update books)
 - #156154 (tests: mark import UI tests as check-pass)
 - #156162 (Update `browser-ui-test` version to `0.23.5`)
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