bors 116201eefe Auto merge of #97461 - eddyb:proc-macro-less-payload, r=bjorn3 ... proc_macro: don't pass a client-side function pointer through the server. Before this PR, `proc_macro::bridge::Client<F>` contained both: * the C ABI entry-point `run`, that the server can call to start the client * some "payload" `f: F` passed to that entry-point * in practice, this was always a (client-side Rust ABI) `fn` pointer to the actual function the proc macro author wrote, i.e. `#[proc_macro] fn foo(input: TokenStream) -> TokenStream` In other words, the client was passing one of its (Rust) `fn` pointers to the server, which was passing it back to the client, for the client to call (see later below for why that was ever needed). I was inspired by `@nnethercote's` attempt to remove the `get_handle_counters` field from `Client` (see https://github.com/rust-lang/rust/pull/97004#issuecomment-1139273301), which combined with removing the `f` ("payload") field, could theoretically allow for a `#[repr(transparent)]` `Client` that mostly just newtypes the C ABI entry-point `fn` pointer <sub>(and in the context of e.g. wasm isolation, that's *all* you want, since you can reason about it from outside the wasm VM, as just a 32-bit "function table index", that you can pass to the wasm VM to call that function)</sub>. <hr/> So this PR removes that "payload". But it's not a simple refactor: the reason the field existed in the first place is because monomorphizing over a function type doesn't let you call the function without having a value of that type, because function types don't implement anything like `Default`, i.e.: ```rust extern "C" fn ffi_wrapper<A, R, F: Fn(A) -> R>(arg: A) -> R { let f: F = ???; // no way to get a value of `F` f(arg) } ``` That could be solved with something like this, if it was allowed: ```rust extern "C" fn ffi_wrapper< A, R, F: Fn(A) -> R, const f: F // not allowed because the type is a generic param >(arg: A) -> R { f(arg) } ``` Instead, this PR contains a workaround in `proc_macro::bridge::selfless_reify` (see its module-level comment for more details) that can provide something similar to the `ffi_wrapper` example above, but limited to `F` being `Copy` and ZST (and requiring an `F` value to prove the caller actually can create values of `F` and it's not uninhabited or some other unsound situation). <hr/> Hopefully this time we don't have a performance regression, and this has a chance to land. cc `@mystor` `@bjorn3`		2022-05-28 16:49:52 +00:00
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src	Auto merge of #97461 - eddyb:proc-macro-less-payload, r=bjorn3	2022-05-28 16:49:52 +00:00
Cargo.toml	Add a lint for duplicated attributes.	2021-12-15 23:43:13 +01:00