quickfix inserts where clause for tuple structs in wrong place and causes errors

[coderedart]

I am newbie, so i will repeat when i learnt on rust official discord while debugging this issue.
Reproduce error:

1. add derive_more and halfbrown as dependencies.

2. create a tuple struct with halfbrown::HashMap as its only field.

3. derive IntoIterator using derive_more with the #[derive(IntoIterator)]

4. vscode/RA complain that the macro needs trait bounds of IntoIterator for the inner field.

5. suggest the quickfix to put a where class specifying the requirement. I click on quickfix
   

6. quickfix introduces the where clause between the struct name and the parenthesis which declare the fields, which introduces a whole new class of errors
   

the full errors that cargo check gives me:

error: proc-macro derive panicked
 --> src/main.rs:7:10
  |
7 | #[derive(IntoIterator)]
  |          ^^^^^^^^^^^^
  |
  = help: message: derive(IntoIterator) only works when forwarding to a single field. Try putting #[into_iterator] or #[into_iterator(ignore)] on the fields in the struct

error[E0658]: parenthetical notation is only stable when used with `Fn`-family traits
 --> src/main.rs:8:72
  |
8 | pub struct UOMap<K: std::hash::Hash,V> where halfbrown::HashMap<K, V>: std::iter::IntoIterator(HashMap<K, V>);
  |                                                                        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  |
  = note: see issue #29625 <https://github.com/rust-lang/rust/issues/29625> for more information

error[E0107]: this trait takes 0 generic arguments but 1 generic argument was supplied
   --> src/main.rs:8:83
    |
8   | pub struct UOMap<K: std::hash::Hash,V> where halfbrown::HashMap<K, V>: std::iter::IntoIterator(HashMap<K, V>);
    |                                                                                   ^^^^^^^^^^^^--------------- help: remove these parenthetical generics
    |                                                                                   |
    |                                                                                   expected 0 generic arguments
    |
note: trait defined here, with 0 generic parameters
   --> /home/red/.rustup/toolchains/stable-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/core/src/iter/traits/collect.rs:204:11
    |
204 | pub trait IntoIterator {
    |           ^^^^^^^^^^^^

error[E0220]: associated type `Output` not found for `IntoIterator`
 --> src/main.rs:8:83
  |
8 | pub struct UOMap<K: std::hash::Hash,V> where halfbrown::HashMap<K, V>: std::iter::IntoIterator(HashMap<K, V>);
  |                                                                                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^ associated type `Output` not found

Some errors have detailed explanations: E0107, E0220, E0658.
For more information about an error, try `rustc --explain E0107`.

The Fix:
changing
pub struct UOMap<K: std::hash::Hash,V> where halfbrown::HashMap<K, V>: std::iter::IntoIterator(HashMap<K, V>);
to
pub struct UOMap<K: std::hash::Hash,V>(HashMap<K, V>) where halfbrown::HashMap<K, V>: std::iter::IntoIterator;
basically, insert the where clause AFTER the closing parenthesis. from what i was told on the discord, where clause needs to come just before the first {, but idk how it will be dealt with in regards to tuple struct.

rust-analyzer version: rust-analyzer version: d9b2291 2021-11-29 stable

rustc version: rustc 1.57.0 (f1edd0429 2021-11-29)

[flodiebold]

What does cargo check say before you apply the fix?

[coderedart]

error[E0277]: the trait bound `halfbrown::HashMap<K, V>: IntoIterator` is not satisfied
   --> src/main.rs:7:10
    |
7   | #[derive(IntoIterator)]
    |          ^^^^^^^^^^^^ the trait `IntoIterator` is not implemented for `halfbrown::HashMap<K, V>`
    |
note: required by a bound in `IntoIterator`
   --> /home/red/.rustup/toolchains/stable-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/core/src/iter/traits/collect.rs:204:1
    |
204 | / pub trait IntoIterator {
205 | |     /// The type of the elements being iterated over.
206 | |     #[stable(feature = "rust1", since = "1.0.0")]
207 | |     type Item;
...   |
234 | |     fn into_iter(self) -> Self::IntoIter;
235 | | }
    | |_^ required by this bound in `IntoIterator`
    = note: this error originates in the derive macro `IntoIterator` (in Nightly builds, run with -Z macro-backtrace for more info)
help: consider introducing a `where` bound, but there might be an alternative better way to express this requirement
    |
8   | pub struct UOMap<K: std::hash::Hash,V> where halfbrown::HashMap<K, V>: IntoIterator(HashMap<K, V>);
    |                                        ++++++++++++++++++++++++++++++++++++++++++++

error[E0277]: the trait bound `halfbrown::HashMap<K, V>: IntoIterator` is not satisfied
 --> src/main.rs:7:10
  |
7 | #[derive(IntoIterator)]
  |          ^^^^^^^^^^^^ the trait `IntoIterator` is not implemented for `halfbrown::HashMap<K, V>`
  |
  = note: this error originates in the derive macro `IntoIterator` (in Nightly builds, run with -Z macro-backtrace for more info)
help: consider introducing a `where` bound, but there might be an alternative better way to express this requirement
  |
8 | pub struct UOMap<K: std::hash::Hash,V> where halfbrown::HashMap<K, V>: IntoIterator(HashMap<K, V>);
  |                                        ++++++++++++++++++++++++++++++++++++++++++++

For more information about this error, try `rustc --explain E0277`.

[coderedart]

Oh, so its not a RA bug, but a compiler bug?

[flodiebold]

Yes, I was just wondering whether it might come from the derive macro, but this does indeed look like it's from rustc.

[coderedart]

should i close this issue and report the bug at rust compiler repo?

[jhgg]

Given this assist is sourced from rustc, I think we can close this issue here.