Relax the Sized requirement on the Error impl for Box.

[smmalis37]

This is already the case for Arc and ThinBox, so I'm guessing this was just an oversight? This will be insta-stable, and probably needs an FCP.

[rustbot]

r? @joshtriplett

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[rust-log-analyzer]

The job x86_64-gnu-llvm-16 failed! Check out the build log: (web) (plain)

Click to see the possible cause of the failure (guessed by this bot)

Built container sha256:9c3c93a371e5aed5c18185b24f130d95d5140dbd72a9b325e7b6b49e521a4faa
Looks like docker image is the same as before, not uploading
https://ci-caches.rust-lang.org/docker/7ebc15c01a233894034d277c8cce4e949f4e7791f66b4727c8fb6e058a0b8171d6152e1441d677cef0653843ceeee469c097b8699b2bb74249e674f6aa1a8813
sha256:9c3c93a371e5aed5c18185b24f130d95d5140dbd72a9b325e7b6b49e521a4faa
Setting extra environment values for docker:  --env ENABLE_GCC_CODEGEN=1 --env GCC_EXEC_PREFIX=/usr/lib/gcc/
[CI_JOB_NAME=x86_64-gnu-llvm-16]
##[group]Clock drift check
  local time: Sun Jan 28 21:02:14 UTC 2024
  network time: Sun, 28 Jan 2024 21:02:14 GMT
  network time: Sun, 28 Jan 2024 21:02:14 GMT
##[endgroup]
sccache: Starting the server...
##[group]Configure the build
configure: processing command line
configure: 
configure: build.configure-args := ['--build=x86_64-unknown-linux-gnu', '--llvm-root=/usr/lib/llvm-16', '--enable-llvm-link-shared', '--set', 'rust.thin-lto-import-instr-limit=10', '--set', 'change-id=99999999', '--enable-verbose-configure', '--enable-sccache', '--disable-manage-submodules', '--enable-locked-deps', '--enable-cargo-native-static', '--set', 'rust.codegen-units-std=1', '--set', 'dist.compression-profile=balanced', '--dist-compression-formats=xz', '--set', 'build.optimized-compiler-builtins', '--disable-dist-src', '--release-channel=nightly', '--enable-debug-assertions', '--enable-overflow-checks', '--enable-llvm-assertions', '--set', 'rust.verify-llvm-ir', '--set', 'rust.codegen-backends=llvm,cranelift,gcc', '--set', 'llvm.static-libstdcpp', '--enable-new-symbol-mangling']
configure: target.x86_64-unknown-linux-gnu.llvm-config := /usr/lib/llvm-16/bin/llvm-config
configure: llvm.link-shared     := True
configure: rust.thin-lto-import-instr-limit := 10
configure: change-id            := 99999999
---
   Compiling cfg-if v1.0.0
   Compiling adler v1.0.2
   Compiling rustc-demangle v0.1.23
   Compiling unwind v0.0.0 (/checkout/library/unwind)
error[E0119]: conflicting implementations of trait `From<Box<dyn core::error::Error>>` for type `Box<dyn core::error::Error>`
     |
     |
2215 | impl<'a, E: Error + 'a> From<E> for Box<dyn Error + 'a> {
     |
     = note: conflicting implementation in crate `core`:
     = note: conflicting implementation in crate `core`:
             - impl<T> From<T> for T;
     = note: upstream crates may add a new impl of trait `core::marker::FnPtr` for type `dyn core::error::Error` in future versions

error[E0119]: conflicting implementations of trait `From<Box<dyn core::error::Error + Send + Sync>>` for type `Box<dyn core::error::Error + Send + Sync>`
     |
     |
2248 | impl<'a, E: Error + Send + Sync + 'a> From<E> for Box<dyn Error + Send + Sync + 'a> {
     |
     = note: conflicting implementation in crate `core`:
     = note: conflicting implementation in crate `core`:
             - impl<T> From<T> for T;
     = note: upstream crates may add a new impl of trait `core::marker::FnPtr` for type `dyn core::error::Error + core::marker::Send + core::marker::Sync` in future versions
For more information about this error, try `rustc --explain E0119`.
error: could not compile `alloc` (lib) due to 2 previous errors
warning: build failed, waiting for other jobs to finish...
Build completed unsuccessfully in 0:00:12

[joshtriplett]

r? libs-api

[rust-log-analyzer]

The job x86_64-gnu-llvm-16 failed! Check out the build log: (web) (plain)

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#12 writing image sha256:06f458c6b60cab3c1025a086ae9e377f6edf557f09cff5af6d512fc4b71bd7f7 done
#12 naming to docker.io/library/rust-ci done
#12 DONE 10.1s
##[endgroup]
Setting extra environment values for docker:  --env ENABLE_GCC_CODEGEN=1 --env GCC_EXEC_PREFIX=/usr/lib/gcc/
[CI_JOB_NAME=x86_64-gnu-llvm-16]
##[group]Clock drift check
  local time: Mon Mar  4 00:33:26 UTC 2024
  network time: Mon, 04 Mar 2024 00:33:26 GMT
  network time: Mon, 04 Mar 2024 00:33:26 GMT
##[endgroup]
sccache: Starting the server...
##[group]Configure the build
configure: processing command line
configure: 
configure: build.configure-args := ['--build=x86_64-unknown-linux-gnu', '--llvm-root=/usr/lib/llvm-16', '--enable-llvm-link-shared', '--set', 'rust.thin-lto-import-instr-limit=10', '--set', 'change-id=99999999', '--enable-verbose-configure', '--enable-sccache', '--disable-manage-submodules', '--enable-locked-deps', '--enable-cargo-native-static', '--set', 'rust.codegen-units-std=1', '--set', 'dist.compression-profile=balanced', '--dist-compression-formats=xz', '--set', 'build.optimized-compiler-builtins', '--disable-dist-src', '--release-channel=nightly', '--enable-debug-assertions', '--enable-overflow-checks', '--enable-llvm-assertions', '--set', 'rust.verify-llvm-ir', '--set', 'rust.codegen-backends=llvm,cranelift,gcc', '--set', 'llvm.static-libstdcpp', '--enable-new-symbol-mangling']
configure: target.x86_64-unknown-linux-gnu.llvm-config := /usr/lib/llvm-16/bin/llvm-config
configure: llvm.link-shared     := True
configure: rust.thin-lto-import-instr-limit := 10
configure: change-id            := 99999999
---
   Compiling cfg-if v1.0.0
   Compiling adler v1.0.2
   Compiling rustc-demangle v0.1.23
   Compiling unwind v0.0.0 (/checkout/library/unwind)
error[E0119]: conflicting implementations of trait `From<Box<dyn core::error::Error>>` for type `Box<dyn core::error::Error>`
     |
     |
2238 | impl<'a, E: Error + 'a> From<E> for Box<dyn Error + 'a> {
     |
     = note: conflicting implementation in crate `core`:
     = note: conflicting implementation in crate `core`:
             - impl<T> From<T> for T;
     = note: upstream crates may add a new impl of trait `core::marker::FnPtr` for type `dyn core::error::Error` in future versions

error[E0119]: conflicting implementations of trait `From<Box<dyn core::error::Error + Send + Sync>>` for type `Box<dyn core::error::Error + Send + Sync>`
     |
     |
2271 | impl<'a, E: Error + Send + Sync + 'a> From<E> for Box<dyn Error + Send + Sync + 'a> {
     |
     = note: conflicting implementation in crate `core`:
     = note: conflicting implementation in crate `core`:
             - impl<T> From<T> for T;
     = note: upstream crates may add a new impl of trait `core::marker::FnPtr` for type `dyn core::error::Error + core::marker::Send + core::marker::Sync` in future versions
For more information about this error, try `rustc --explain E0119`.
error: could not compile `alloc` (lib) due to 2 previous errors
warning: build failed, waiting for other jobs to finish...
Build completed unsuccessfully in 0:00:11

[smmalis37]

I don't understand how this creates conflicting impls of From?

[joboet]

Because Box<dyn Error> now implements Error, there are two implementations of From<Box<dyn Error>> for Box<dyn Error>: one from the standard impl<T> From<T> for T and one from impl<E: Error> From<E> for Box<dyn Error>. This can't ever be allowed because the two implementations do different things: the first one doesn't create a new allocation, the second one adds one Box indirection.