Add experimental fallible::Vec

[ChrisDenton]

Tracking issue: #157392

There are a few things to be aware of here:

1. This is still experimental. It might be removed at any time so we have to be careful about any interdependency between this and Vec
2. Vec is a pretty crucial part of rust's standard library. We have to be extremely careful touching its code. We might regress performance or make debugging worse.

With that in mind, this PR does the minimum necessary for an MVP fallible::Vec. The only actually new thing it adds is fallible push and push_mut methods. The rest just forwards to Vec equivalents. Further APIs can be added (carefully) afterwards.

I apologise for the number of lines changed in the PR but they are mostly copy/pasted docs (I wanted them for the testable examples mostly). We should probably find a better way of duplicating them but I think this works for a minimally intrusive experiment.

[rustbot]

r? @Mark-Simulacrum

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[ChrisDenton]

 -	   = note: multiple `impl`s satisfying `Vec<_>: aux::Trait` found in the `unstable_impl_method_selection_aux` crate:
 -	           - impl aux::Trait for Vec<u32>;
 -	           - impl aux::Trait for Vec<u64>
 +	   = note: multiple `impl`s satisfying `std::vec::Vec<_>: aux::Trait` found in the `unstable_impl_method_selection_aux` crate:
 +	           - impl aux::Trait for std::vec::Vec<u32>;
 +	           - impl aux::Trait for std::vec::Vec<u64>

Ooph, that's unfortunate. Simplest thing is to rename this type. But I do wonder if that could be fixed.

[rust-log-analyzer]

The job aarch64-gnu-llvm-21-1 failed! Check out the build log: (web) (plain enhanced) (plain)

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

Saved the actual stderr to `/checkout/obj/build/aarch64-unknown-linux-gnu/test/ui/consts/too_generic_eval_ice.current/too_generic_eval_ice.current.stderr`
diff of stderr:

30    |
31    = help: the trait `PartialEq<[{integer}; 0]>` is not implemented for `[{integer}; Self::HOST_SIZE]`
32    = help: the following other types implement trait `PartialEq<Rhs>`:
+              `&[T]` implements `PartialEq<FallibleVec<U, A>>`
33              `&[T]` implements `PartialEq<Vec<U, A>>`
34              `&[T]` implements `PartialEq<[U; N]>`
35              `&[u8; N]` implements `PartialEq<ByteStr>`

36              `&[u8; N]` implements `PartialEq<ByteString>`
37              `&[u8]` implements `PartialEq<ByteStr>`
38              `&[u8]` implements `PartialEq<ByteString>`
-              `&mut [T]` implements `PartialEq<Vec<U, A>>`
-              `&mut [T]` implements `PartialEq<[U; N]>`
-            and 11 others
+              `&mut [T]` implements `PartialEq<FallibleVec<U, A>>`
+            and 14 others
42 
43 error: aborting due to 4 previous errors
44 


The actual stderr differed from the expected stderr
To update references, rerun the tests and pass the `--bless` flag
To only update this specific test, also pass `--test-args consts/too_generic_eval_ice.rs`

error in revision `current`: 1 errors occurred comparing output.
status: exit status: 1
command: env -u RUSTC_LOG_COLOR RUSTC_ICE="0" RUST_BACKTRACE="short" "/checkout/obj/build/aarch64-unknown-linux-gnu/stage2/bin/rustc" "/checkout/tests/ui/consts/too_generic_eval_ice.rs" "-Zthreads=1" "-Zsimulate-remapped-rust-src-base=/rustc/FAKE_PREFIX" "-Ztranslate-remapped-path-to-local-path=no" "-Z" "ignore-directory-in-diagnostics-source-blocks=/cargo" "-Z" "ignore-directory-in-diagnostics-source-blocks=/checkout/vendor" "--sysroot" "/checkout/obj/build/aarch64-unknown-linux-gnu/stage2" "--target=aarch64-unknown-linux-gnu" "--cfg" "current" "--check-cfg" "cfg(test,FALSE,current,next)" "--error-format" "json" "--json" "future-incompat" "-Ccodegen-units=1" "-Zui-testing" "-Zdeduplicate-diagnostics=no" "-Zwrite-long-types-to-disk=no" "-Cstrip=debuginfo" "--emit" "metadata" "-C" "prefer-dynamic" "--out-dir" "/checkout/obj/build/aarch64-unknown-linux-gnu/test/ui/consts/too_generic_eval_ice.current" "-A" "unused" "-W" "unused_attributes" "-A" "internal_features" "-A" "incomplete_features" "-A" "unused_parens" "-A" "unused_braces" "-Crpath" "-Cdebuginfo=0" "-Lnative=/checkout/obj/build/aarch64-unknown-linux-gnu/native/rust-test-helpers"
stdout: none
--- stderr -------------------------------
error: constant expression depends on a generic parameter
##[error]  --> /checkout/tests/ui/consts/too_generic_eval_ice.rs:11:13
   |
LL |         [5; Self::HOST_SIZE] == [6; 0]
   |             ^^^^^^^^^^^^^^^
   |
   = note: this may fail depending on what value the parameter takes

error: constant expression depends on a generic parameter
##[error]  --> /checkout/tests/ui/consts/too_generic_eval_ice.rs:11:9
   |
LL |         [5; Self::HOST_SIZE] == [6; 0]
   |         ^^^^^^^^^^^^^^^^^^^^
   |
   = note: this may fail depending on what value the parameter takes

error: constant expression depends on a generic parameter
##[error]  --> /checkout/tests/ui/consts/too_generic_eval_ice.rs:11:30
   |
LL |         [5; Self::HOST_SIZE] == [6; 0]
   |                              ^^
   |
   = note: this may fail depending on what value the parameter takes

error[E0277]: can't compare `[{integer}; Self::HOST_SIZE]` with `[{integer}; 0]`
##[error]  --> /checkout/tests/ui/consts/too_generic_eval_ice.rs:11:30
   |
LL |         [5; Self::HOST_SIZE] == [6; 0]
   |                              ^^ no implementation for `[{integer}; Self::HOST_SIZE] == [{integer}; 0]`
   |
   = help: the trait `PartialEq<[{integer}; 0]>` is not implemented for `[{integer}; Self::HOST_SIZE]`
   = help: the following other types implement trait `PartialEq<Rhs>`:
             `&[T]` implements `PartialEq<FallibleVec<U, A>>`
             `&[T]` implements `PartialEq<Vec<U, A>>`
             `&[T]` implements `PartialEq<[U; N]>`
             `&[u8; N]` implements `PartialEq<ByteStr>`
             `&[u8; N]` implements `PartialEq<ByteString>`
             `&[u8]` implements `PartialEq<ByteStr>`
             `&[u8]` implements `PartialEq<ByteString>`
             `&mut [T]` implements `PartialEq<FallibleVec<U, A>>`
           and 14 others

error: aborting due to 4 previous errors

For more information about this error, try `rustc --explain E0277`.
---
Saved the actual stderr to `/checkout/obj/build/aarch64-unknown-linux-gnu/test/ui/macros/assert-ne-no-invalid-help-issue-146204/assert-ne-no-invalid-help-issue-146204.stderr`
diff of stderr:

6    |
7    = help: the trait `PartialEq<&[u8; 4]>` is not implemented for `[u8; 4]`
8    = help: the following other types implement trait `PartialEq<Rhs>`:
+              `&[T]` implements `PartialEq<FallibleVec<U, A>>`
9              `&[T]` implements `PartialEq<Vec<U, A>>`
10              `&[T]` implements `PartialEq<[U; N]>`
11              `&[u8; N]` implements `PartialEq<ByteStr>`

12              `&[u8; N]` implements `PartialEq<ByteString>`
13              `&[u8]` implements `PartialEq<ByteStr>`
14              `&[u8]` implements `PartialEq<ByteString>`
-              `&mut [T]` implements `PartialEq<Vec<U, A>>`
-              `&mut [T]` implements `PartialEq<[U; N]>`
-            and 11 others
+              `&mut [T]` implements `PartialEq<FallibleVec<U, A>>`
+            and 14 others
18 
19 error[E0277]: can't compare `[u8; 4]` with `&[u8; 4]`
20   --> $DIR/assert-ne-no-invalid-help-issue-146204.rs:19:5

24    |
25    = help: the trait `PartialEq<&[u8; 4]>` is not implemented for `[u8; 4]`
26    = help: the following other types implement trait `PartialEq<Rhs>`:
+              `&[T]` implements `PartialEq<FallibleVec<U, A>>`
27              `&[T]` implements `PartialEq<Vec<U, A>>`
28              `&[T]` implements `PartialEq<[U; N]>`
29              `&[u8; N]` implements `PartialEq<ByteStr>`

30              `&[u8; N]` implements `PartialEq<ByteString>`
31              `&[u8]` implements `PartialEq<ByteStr>`
32              `&[u8]` implements `PartialEq<ByteString>`
-              `&mut [T]` implements `PartialEq<Vec<U, A>>`
-              `&mut [T]` implements `PartialEq<[U; N]>`
-            and 11 others
+              `&mut [T]` implements `PartialEq<FallibleVec<U, A>>`
+            and 14 others
36 
37 error[E0277]: can't compare `[u8; 4]` with `&[u8; 4]`
38   --> $DIR/assert-ne-no-invalid-help-issue-146204.rs:5:30


The actual stderr differed from the expected stderr
To update references, rerun the tests and pass the `--bless` flag
To only update this specific test, also pass `--test-args macros/assert-ne-no-invalid-help-issue-146204.rs`

error: 1 errors occurred comparing output.
status: exit status: 1
command: env -u RUSTC_LOG_COLOR RUSTC_ICE="0" RUST_BACKTRACE="short" "/checkout/obj/build/aarch64-unknown-linux-gnu/stage2/bin/rustc" "/checkout/tests/ui/macros/assert-ne-no-invalid-help-issue-146204.rs" "-Zthreads=1" "-Zsimulate-remapped-rust-src-base=/rustc/FAKE_PREFIX" "-Ztranslate-remapped-path-to-local-path=no" "-Z" "ignore-directory-in-diagnostics-source-blocks=/cargo" "-Z" "ignore-directory-in-diagnostics-source-blocks=/checkout/vendor" "--sysroot" "/checkout/obj/build/aarch64-unknown-linux-gnu/stage2" "--target=aarch64-unknown-linux-gnu" "--check-cfg" "cfg(test,FALSE)" "--error-format" "json" "--json" "future-incompat" "-Ccodegen-units=1" "-Zui-testing" "-Zdeduplicate-diagnostics=no" "-Zwrite-long-types-to-disk=no" "-Cstrip=debuginfo" "--emit" "metadata" "-C" "prefer-dynamic" "--out-dir" "/checkout/obj/build/aarch64-unknown-linux-gnu/test/ui/macros/assert-ne-no-invalid-help-issue-146204" "-A" "unused" "-W" "unused_attributes" "-A" "internal_features" "-A" "incomplete_features" "-A" "unused_parens" "-A" "unused_braces" "-Crpath" "-Cdebuginfo=0" "-Lnative=/checkout/obj/build/aarch64-unknown-linux-gnu/native/rust-test-helpers"
stdout: none
--- stderr -------------------------------
error[E0277]: can't compare `[u8; 4]` with `&[u8; 4]`
##[error]  --> /checkout/tests/ui/macros/assert-ne-no-invalid-help-issue-146204.rs:16:5
   |
LL |     assert_ne!(buf, b"----");
   |     ^^^^^^^^^^^^^^^^^^^^^^^^ no implementation for `[u8; 4] == &[u8; 4]`
   |
   = help: the trait `PartialEq<&[u8; 4]>` is not implemented for `[u8; 4]`
   = help: the following other types implement trait `PartialEq<Rhs>`:
             `&[T]` implements `PartialEq<FallibleVec<U, A>>`
             `&[T]` implements `PartialEq<Vec<U, A>>`
             `&[T]` implements `PartialEq<[U; N]>`
             `&[u8; N]` implements `PartialEq<ByteStr>`
             `&[u8; N]` implements `PartialEq<ByteString>`
             `&[u8]` implements `PartialEq<ByteStr>`
             `&[u8]` implements `PartialEq<ByteString>`
             `&mut [T]` implements `PartialEq<FallibleVec<U, A>>`
           and 14 others

error[E0277]: can't compare `[u8; 4]` with `&[u8; 4]`
##[error]  --> /checkout/tests/ui/macros/assert-ne-no-invalid-help-issue-146204.rs:19:5
   |
LL |     assert_eq!(buf, b"----");
   |     ^^^^^^^^^^^^^^^^^^^^^^^^ no implementation for `[u8; 4] == &[u8; 4]`
   |
   = help: the trait `PartialEq<&[u8; 4]>` is not implemented for `[u8; 4]`
   = help: the following other types implement trait `PartialEq<Rhs>`:
             `&[T]` implements `PartialEq<FallibleVec<U, A>>`
             `&[T]` implements `PartialEq<Vec<U, A>>`
             `&[T]` implements `PartialEq<[U; N]>`
             `&[u8; N]` implements `PartialEq<ByteStr>`
             `&[u8; N]` implements `PartialEq<ByteString>`
             `&[u8]` implements `PartialEq<ByteStr>`
             `&[u8]` implements `PartialEq<ByteString>`
             `&mut [T]` implements `PartialEq<FallibleVec<U, A>>`
           and 14 others

error[E0277]: can't compare `[u8; 4]` with `&[u8; 4]`
##[error]  --> /checkout/tests/ui/macros/assert-ne-no-invalid-help-issue-146204.rs:5:30
   |
LL |                 if *left_val == *right_val {
   |                              ^^ no implementation for `[u8; 4] == &[u8; 4]`
...
LL |     local_assert_ne!(buf, b"----");
   |     ------------------------------ in this macro invocation
   |
   = help: the trait `PartialEq<&[u8; 4]>` is not implemented for `[u8; 4]`
   = note: this error originates in the macro `local_assert_ne` (in Nightly builds, run with -Z macro-backtrace for more info)
help: consider dereferencing here
   |
LL |                 if *left_val == **right_val {
   |                                 +

error: aborting due to 3 previous errors

For more information about this error, try `rustc --explain E0277`.