Support creating default Vec within a custom allocator

[Nokel81]

This PR adds support for calling Default::default for some Vec<T, A> as long as the A also implements Default.

[rustbot]

r? @Mark-Simulacrum

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[Nokel81]

@rustbot modify label: +I-libs-api-nominated

[rust-log-analyzer]

The job pr-check-1 failed! Check out the build log: (web) (plain enhanced) (plain)

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

[RUSTC-TIMING] quick_error test:false 0.070
    Checking fnv v1.0.7
[RUSTC-TIMING] fnv test:false 0.062
    Checking rusty-fork v0.3.1
error[E0283]: type annotations needed for `Vec<NodeOrToken<SyntaxNode<RustLanguage>, SyntaxToken<...>>, _>`
   --> src/tools/rust-analyzer/crates/ide-db/src/syntax_helpers/tree_diff.rs:89:13
    |
 89 |         let mut look_ahead_scratch = Vec::default();
    |             ^^^^^^^^^^^^^^^^^^^^^^   --- type must be known at this point
    |
    = note: the type must implement `Allocator`
---
              std::sync::Arc<T, A>
note: required by a bound in `Vec`
   --> library/alloc/src/vec/mod.rs:438:78
    |
438 | pub struct Vec<T, #[unstable(feature = "allocator_api", issue = "32838")] A: Allocator = Global> {
    |                                                                              ^^^^^^^^^ required by this bound in `Vec`
    = note: the full name for the type has been written to '/checkout/obj/build/x86_64-unknown-linux-gnu/stage2-tools/i686-pc-windows-gnu/release/deps/ide_db-402cfb8f638e4fa4.long-type-7506825192990240553.txt'
    = note: consider using `--verbose` to print the full type name to the console
help: consider giving `look_ahead_scratch` an explicit type, where the type for type parameter `A` is specified
    |
 89 |         let mut look_ahead_scratch: Vec<_, A> = Vec::default();
    |                                   +++++++++++

[RUSTC-TIMING] build_script_build test:false 0.374
    Checking xshell v0.2.7
[RUSTC-TIMING] rusty_fork test:false 0.292

[bjorn3]

This breaks type inference unfortunately.

[Nokel81]

This breaks type inference unfortunately.

Is that because of the lack of defaults within impl blocks? Should I switch to using a form of specialization?

[bjorn3]

Type inference unfortunately ignores defaults entirely. This is also why for example HashMap::new() doesn't allow picking a hasher. I don't see how specialization can help here at all.

[Nokel81]

Type inference unfortunately ignores defaults entirely. This is also why for example HashMap::new() doesn't allow picking a hasher. I don't see how specialization can help here at all.

From reading the docs I was thinking that it would be possible to mark impl <T> const Default for Vec<T> (or maybe for Vec<T, Global> since that is what it means) as the "default" version for type inference and then still have impl <T, A: Allocator + [const] Default> const Default for Vec<T, A> as an impl that exists but is ignored when doing type inference.

[bjorn3]

I don't believe that is possible, but I did be happy to be proven wrong.

[Mark-Simulacrum]

@rustbot author

Feel free to mark as waiting on review if you find a solution (I don't have the bandwidth to help search for one unfortunately). Maybe asking on Zulip is a reasonable starting point.

[rustbot]

Reminder, once the PR becomes ready for a review, use @rustbot ready.

[nia-e]

Per discussion in the last @rust-lang/libs-api meeting, this would be nice to add but we don't think it's possible without extensive breakage due to how inference currently works. This will be brought up with the edition team as there may be some way to allow this - it would certainly be useful! - but unfortunately this PR is going to be blocked on that at a minimum. Thanks ^^