Enable -Znext-solver on nightly by default

[Kivooeo]

this is a draft for now to show test failure in library/alloc/src/vec, i will update description accordingly as we decide what to do with this test

[rust-log-analyzer]

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

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

tests/pass/shims/x86/rounding-error.rs ... ok
tests/pass/shims/x86/intrinsics-x86-gfni.rs ... ok

FAILED TEST: tests/pass/dyn-upcast.rs
command: MIRI_ENV_VAR_TEST="0" MIRI_TEMP="/tmp/miri-uitest-nwMZcC" RUST_BACKTRACE="1" "/checkout/obj/build/x86_64-unknown-linux-gnu/stage2-tools/x86_64-unknown-linux-gnu/release/miri" "--error-format=json" "-Dwarnings" "-Dunused" "-Ainternal_features" "-Zui-testing" "--sysroot=/checkout/obj/build/x86_64-unknown-linux-gnu/miri-sysroot" "--out-dir" "/checkout/obj/build/x86_64-unknown-linux-gnu/stage2-tools/x86_64-unknown-linux-gnu/tmp/miri_ui/0/tests/pass" "tests/pass/dyn-upcast.rs" "--edition" "2021"

error: test got exit status: 1, but expected 0
 = note: compilation failed, but was expected to succeed

error: no output was expected
Execute `./miri test --bless` to update `tests/pass/dyn-upcast.stderr` to the actual output
+++ <stderr output>
error[E0283]: type annotations needed: cannot satisfy `dyn modulo_binder::Trait<&(), &()>: modulo_binder::Supertrait<&()>`
##[error]  --> tests/pass/dyn-upcast.rs:504:10
   |
LL |         .say_hello(&0);
   |          ^^^^^^^^^
   |
   = note: cannot satisfy `dyn modulo_binder::Trait<&(), &()>: modulo_binder::Supertrait<&()>`
help: the trait `modulo_binder::Supertrait<T>` is implemented for `()`
  --> tests/pass/dyn-upcast.rs:493:5
   |
LL |     impl<T> Supertrait<T> for () {}
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
note: required by a bound in `modulo_binder::Trait::say_hello`
  --> tests/pass/dyn-upcast.rs:495:24
   |
LL |     trait Trait<T, U>: Supertrait<T> + Supertrait<U> {
   |                        ^^^^^^^^^^^^^ required by this bound in `Trait::say_hello`
LL |         fn say_hello(&self, _: &usize) {
---
error: no output was emitted
Execute `./miri test --bless` to remove `tests/pass/dyn-upcast.stdout`

full stderr:
error[E0283]: type annotations needed: cannot satisfy `dyn modulo_binder::Trait<&(), &()>: modulo_binder::Supertrait<&()>`
##[error]  --> tests/pass/dyn-upcast.rs:504:10
   |
LL |         .say_hello(&0);
   |          ^^^^^^^^^
   |
   = note: cannot satisfy `dyn modulo_binder::Trait<&(), &()>: modulo_binder::Supertrait<&()>`
help: the trait `modulo_binder::Supertrait<T>` is implemented for `()`
  --> tests/pass/dyn-upcast.rs:493:5
   |
LL |     impl<T> Supertrait<T> for () {}
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
note: required by a bound in `modulo_binder::Trait::say_hello`
  --> tests/pass/dyn-upcast.rs:495:24
   |
LL |     trait Trait<T, U>: Supertrait<T> + Supertrait<U> {
   |                        ^^^^^^^^^^^^^ required by this bound in `Trait::say_hello`
LL |         fn say_hello(&self, _: &usize) {
---

Location:
   /cargo/registry/src/index.crates.io-1949cf8c6b5b557f/ui_test-0.30.7/src/lib.rs:365

Backtrace omitted. Run with RUST_BACKTRACE=1 environment variable to display it.
Run with RUST_BACKTRACE=full to include source snippets.
error: test failed, to rerun pass `--test ui`

Caused by:
  process didn't exit successfully: `/checkout/obj/build/x86_64-unknown-linux-gnu/stage2-tools/x86_64-unknown-linux-gnu/release/build/miri/855274d73082e308/out/ui-855274d73082e308` (exit status: 1)
Command `/checkout/obj/build/x86_64-unknown-linux-gnu/stage0/bin/cargo test --target x86_64-unknown-linux-gnu -Zbinary-dep-depinfo -j 4 -Zroot-dir=/checkout --locked --color=always --profile=release --manifest-path /checkout/src/tools/miri/Cargo.toml -- [workdir=/checkout]` failed with exit code 1
Created at: src/bootstrap/src/core/build_steps/tool.rs:197:21
Executed at: src/bootstrap/src/core/build_steps/test.rs:770:19

Command has failed. Rerun with -v to see more details.
Bootstrap failed while executing `test --stage 2 miri cargo-miri`
Currently active steps:
test::Miri { target: x86_64-unknown-linux-gnu } at src/bootstrap/src/core/build_steps/test.rs:696
Build completed unsuccessfully in 0:43:35
  local time: Fri Aug  7 00:18:45 UTC 2026