Why is there an `E: Sized` bound for `Box: Error`

⚓ Rust 📅 2025-11-24 👤 surdeus 👁️ 11

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This post is auto-generated from RSS feed The Rust Programming Language Forum - Latest topics. Source: Why is there an `E: Sized` bound for `Box: Error`

There's no Sized bound for &impl Error:

github.com/rust-lang/rust

library/core/src/error.rs

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          impl<'a, T: Error + ?Sized> Error for &'a T {
              #[allow(deprecated)]
              fn cause(&self) -> Option<&dyn Error> {
                  Error::cause(&**self)
              }
          
              fn source(&self) -> Option<&(dyn Error + 'static)> {
                  Error::source(&**self)
              }
          
              fn provide<'b>(&'b self, request: &mut Request<'b>) {
                  Error::provide(&**self, request);
              }
          }

Or for Arc<impl Error>:

github.com/rust-lang/rust

library/alloc/src/sync.rs

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          impl<T: core::error::Error + ?Sized> core::error::Error for Arc<T> {
              #[allow(deprecated)]
              fn cause(&self) -> Option<&dyn core::error::Error> {
                  core::error::Error::cause(&**self)
              }
          
              fn source(&self) -> Option<&(dyn core::error::Error + 'static)> {
                  core::error::Error::source(&**self)
              }
          
              fn provide<'a>(&'a self, req: &mut core::error::Request<'a>) {
                  core::error::Error::provide(&**self, req);
              }
          }

But there is for Box<impl Error>:

github.com/rust-lang/rust

library/alloc/src/boxed.rs

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          impl<E: Error> Error for Box<E> {
              #[allow(deprecated)]
              fn cause(&self) -> Option<&dyn Error> {
                  Error::cause(&**self)
              }
          
              fn source(&self) -> Option<&(dyn Error + 'static)> {
                  Error::source(&**self)
              }
          
              fn provide<'b>(&'b self, request: &mut error::Request<'b>) {
                  Error::provide(&**self, request);
              }
          }

Just checking that there's a reason, else I assume I can create ACP

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