alloc: make vec! unavailable under no_global_oom_handling

The `vec!` macro has 3 rules, but two are not usable under
`no_global_oom_handling` builds of the standard library
(even with a zero size):

```rust
let _ = vec![42];    // Error: requires `exchange_malloc` lang_item.
let _ = vec![42; 0]; // Error: cannot find function `from_elem`.
```

Thus those two rules should not be available to begin with.

The remaining one, with an empty matcher, is just a shorthand for
`new()` and may not make as much sense to have alone, since the
idea behind `vec!` is to enable `Vec`s to be defined with the same
syntax as array expressions. Furthermore, the documentation can be
confusing since it shows the other rules.

Thus perhaps it is better and simpler to disable `vec!` entirely
under `no_global_oom_handling` environments, and let users call
`new()` instead:

```rust
let _: Vec<i32> = vec![];
let _: Vec<i32> = Vec::new();
```

Notwithstanding this, a `try_vec!` macro would be useful, such as
the one introduced in rust-lang/rust#95051.

If the shorthand for `new()` is deemed worth keeping on its own,
then it may be interesting to have a separate `vec!` macro with
a single rule and different, simpler documentation.

Signed-off-by: Miguel Ojeda <[email protected]>

alloc/src/macros.rs

@@ -34,7 +34,7 @@
 /// be mindful of side effects.
 ///
 /// [`Vec`]: crate::vec::Vec
-#[cfg(not(test))]
+#[cfg(all(not(no_global_oom_handling), not(test)))]
 #[macro_export]
 #[stable(feature = "rust1", since = "1.0.0")]
 #[rustc_diagnostic_item = "vec_macro"]
@@ -55,7 +55,7 @@ macro_rules! vec {
 // required for this macro definition, is not available. Instead use the
 // `slice::into_vec`  function which is only available with cfg(test)
 // NB see the slice::hack module in slice.rs for more information
-#[cfg(test)]
+#[cfg(all(not(no_global_oom_handling), test))]
 macro_rules! vec {
     () => (
         $crate::vec::Vec::new()