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Update: Since GitHub pull request #93755 has been merged, comparison between Vecs with different allocators is now possible.

Original answer:

Vec uses the std::alloc::Global allocator by default, so Vec is in fact Vec. Since Vec and Vec are indeed distinct types, they cannot directly be compared because the PartialEq implementation is not generic for the allocator type. As @PitaJ commented, you can compare the slices instead using assert_eq!(&a[..], &b[..]) (which is also what the author of the allocator API recommends).

Today I myself faced such a problem. The problem lies in styled-components itself. Macro in styled does not work since version 5.2.2, and it is not known when it will be fixed. Simplest solution:

From [expr.prim.lambda.capture]/3:

A lambda-expression shall not have a capture-default or simple-capture in its lambda-introducer unless its innermost enclosing scope is a block scope ([basic.scope.block]) or it appears within a default member initializer and its innermost enclosing scope is the corresponding class scope ([basic.scope.class]).

Which means that captures such as [n], [&n] or [&] are not allowed in trailing return types or noexcept specifiers, but initialized captures such as [i = 1] are.

So GCC is right to reject the first two function definitions and GCC and Clang are right to reject the last one.

#[main] is an old, unstable attribute that was mostly removed from the language in 1.53.0. However, the removal missed one line, with the result you see: the attribute had no effect, but it could be used on stable Rust without an error, and conflicted with imported attributes named main. This was a bug, not intended behaviour. It has been fixed as of nightly-2022-02-10 and 1.59.0-beta.8. Your example with use tokio::main; and #[main] can now run without error.

Before it was removed, the unstable #[main] was used to specify the entry point of a program. Alex Crichton described the behaviour of it and related attributes in a 2016 comment on GitHub:

Ah yes, we've got three entry points. I.. think this is how they work:

• First, #[start], the receiver of int argc and char **argv. This is literally the symbol main (or what is called by that symbol generated in the compiler).
• Next, there's #[lang = "start"]. If no #[start] exists in the crate graph then the compiler generates a main function that calls this. This functions receives argc/argv along with a third argument that is a function pointer to the #[main] function (defined below). Importantly, #[lang = "start"] can be located in a library. For example it's located in the standard library (libstd).
• Finally, #[main], the main function for an executable. This is passed no arguments and is called by #[lang = "start"] (if it decides to). The standard library uses this to initialize itself and then call the Rust program. This, if not specified, defaults to fn main at the top.

So to answer your question, this isn't the same as #[start]. To answer your other (possibly not yet asked) question, yes we have too many entry points.

Update the Rust to satisfy the new edition 2021.

rustup default nightly && rustup update

Thanks to @ken. Yes, you can use the stable channel too!

But I love nightly personally.

The multiplication and addition are in fact pointless, as is the outer cast.

Both operators perform the usual arithmetic conversions on both operands.

For the multiplication, the left operand has type uint64 (as a result of the cast) and the right operand has type int. Since uint64 is the larger type it will be the type of the result. The operand 1 does not change value as a result of the conversion, so in multiplying by 1 the result has the same type and value as (uint64)(x).

Similarly for the addition, the operands are of type uint64 and int respectively, meaning the resulting type is uint64, and 0 does not change value after the conversion. So by adding 0 the result has the same type and value as (uint64)(x) * 1 which has the same type and value as (uint64)(x).

The cast at the end is also superfluous, as the casted expression already has type uint64. As above, the division operator performs the usual arithmetic conversions on its operands so dividing a uint64 by an int results in a uint64.

So the above macro is equivalent to:

Apparently, there is a reference to that, in a github issue - rust-lang/rust#63104:

This conflicts with the existing blanket impl in core.

Let's go step by step to see how <[_]>::into_vec(box [$($x),+]) produces a Vec:

1. [$($x),+] expands to an array of input elements: [1, 2, 3]
2. box ... puts that into a Box. box expressions are nightly-only syntax sugar for Box::new: box 5 is syntax sugar for Box::new(5) (actually it's the other way around: internally Box::new uses box, which is implemented in the compiler)
3. <[_]>::into_vec(...) calls the to_vec method on a slice containing elements that have an inferred type ([_]). Wrapping the [_] in angled brackets is needed for syntactic reasons to call an method on a slice type. And into_vec is a function that takes a boxed slice and produces a Vec:

It doesn't have any result on how the macro is used, it only serves to improve the quality of error messages when the macro is used incorrectly by telling the compiler that the output of the macro is always a single expression, not an item or multiple expressions.

The specific error that this was added to improve was for using vec![] in a pattern match, which is invalid (you can't structually match on a Vec):