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.