Cannot find assert in $crate::core using [no_std]

[Robbepop]

#![no_std]
macro_rules! foo {
    ($e:expr) => {
        $crate::core::assert!($e);
        $crate::core::assert_eq!($e, true);
    };
}
pub fn foo() { foo!(true); }

Compiling the above code generates this error message:

error[E0433]: failed to resolve. Could not find `assert` in `core`
  --> src/lib.rs:5:23
   |
5  |         $crate::core::assert!($e);
   |                       ^^^^^^ Could not find `assert` in `core`
...
13 |     foo!(true);
   |     ----------- in this macro invocation

However, this compiles:

#![no_std]
macro_rules! foo {
    ($e:expr) => {
        assert!($e); // no more `$crate::core::` path prefix!
        $crate::core::assert_eq!($e, true);
    };
}
pub fn foo() { foo!(true); }

Note that for the assert_eq invokation both versions $crate::core::assert_eq! and just assert_eq! works.

Is this intended behaviour or a bug?

Try it yourself at the playground.

[stephaneyfx]

This may be related to the issue where macros (it seems limited to built-in ones) don't work when called qualified with their path. Playground

fn main() {
    // These macros are built into the compiler and calling them qualified
    // with their path does not work:
    //
    // let _ = std::column!();
    // let _ = std::format_args!("blah");
    // let _ = std::concat!("abc", "def");

    // But it works with non built-in macros:
    let _ = std::format!("blah");
    let _ = std::vec![0, 1];
}

[petrochenkov]

assert is not defined in the core library, it's a built-in macro defined by the language.
assert_eq in its turn is indeed defined in core, so it can be used as core::assert_eq!(...).

It should be possible to make a proxy macro assert in core calling built-in assert, it may require some care about what rusdoc shows for that macro though.

[dtolnay]

This is a bug. The documentation of libcore shows assert being provided by core. I understand that there is nuance around built-in macros but it's clear to me that core::assert! should be accepted.

[steveklabnik]

Triage: the docs of libcore should probably be updated.

[jyn514]

Is the request for core::assert! to be accepted or for rustdoc to stop documenting core::assert? If the former, this should be tagged T-libs or T-compiler, not T-rustdoc.

[jyn514]

@dtolnay @steveklabnik you've said contradicting things: @dtolnay said core::assert! should be accepted as-is and @steveklabnik said the docs shouldn't show core::assert! as valid. Which is it? Does it need an RFC?

[dtolnay]

$crate::core::assert! needs to work. It does not require an RFC. According to OP, $crate::core::assert_eq! already works.

It's possible there was a rustdoc issue also. According to #55482 (comment), rustdoc was showing assert! in core even though it was not exposed by core in a way that was callable.

[jyn514]

Heh, after all that it turns out this compiles as-is (maybe assert was moved to core recently?). I'll add a test for it.