Merge unused_tuple_struct_fields into dead_code

[shepmaster]

This implicitly upgrades the lint from allow to warn and places it into the unused lint group.

Discussion on Zulip

[rustbot]

Some changes occurred to MIR optimizations

cc @rust-lang/wg-mir-opt

Some changes occurred in src/librustdoc/clean/types.rs

cc @camelid

[compiler-errors]

I'd happily take a PR that proactively fixes all of the compiler warnings. Most of them can just be removed, except for the TyCtxt one, which can be replaced with PhantomData.

[shepmaster]

Yeah, my plan is to find all of the current issues and submit it as a separate PR, it’s just easier to find it here first.

I was attempting to do that without causing undue noise (thanks rustbot 👀).

[traviscross]

Regarding the T-lang nomination, one of the open questions here is whether this should be part of the dead_code lint now that we're making it warn-by-default.

The motivation for making this part of dead_code is that unused fields of normal structs warn under dead_code, so this would be consistent. E.g.:

warning: field `a` is never read
 --> src/lib.rs:1:12
  |
1 | struct A { a: () }
  |        -   ^
  |        |
  |        field in this struct
  |
  = note: `#[warn(dead_code)]` on by default

On 2022-01-25, T-lang requested this lint be separated from dead_code so that it could be introduced gradually and so people could disable it specifically. The motivation seems to have been the belief that this lint could be "harder to fix than removing named fields, because removing a positional field changes the indices of the following fields".

However, after that decision, on 2022-01-26, @pnkfelix suggested a change to the lint that would allow people to resolve the warning without changing field indices:

...I thought: "well, we should let people include () fields (i.e. not warn about them), and perhaps even have the lint diagnostic here suggest either removing the field or making it have () type. That would keep the indices unchanged."

This change was implemented and is suggested as part of the error message, e.g.:

error: field is never read: `1`
  --> $DIR/tuple-struct-field.rs:6:21
   |
LL | struct Wrapper(i32, [u8; LEN], String);
   |                     ^^^^^^^^^
   |
help: change the field to unit type to suppress this warning while preserving the field numbering
   |
LL | struct Wrapper(i32, (), String);
   | 

In discussion in T-compiler in Zulip, the feeling seemed to be that this should be part of dead_code. Given the change suggested by @pnkfelix and implemented in the lint that resolves the issue of preserving field indices, do we now agree?

[bors]

☔ The latest upstream changes (presumably #118370) made this pull request unmergeable. Please resolve the merge conflicts.

[shepmaster]

a PR that proactively fixes all of the compiler warnings.

Opened in #118382

[traviscross]

@rustbot labels -I-lang-nominated

This was discussed in the T-lang triage call today, and the consensus was that it's OK for this to become part of the dead_code lint.

[shepmaster]

r? compiler

[WaffleLapkin]

Just a bit of a nitpick. Feel free to change labels to waiting on review once CI is green

[WaffleLapkin]

Isn't this just is_positional = is_pos or is_positional |= is_pos?

[WaffleLapkin]

Also can't we just do a single check is variant is tuple-like or not, instead of trying to move this out of the loop?..

[shepmaster]

Isn't this [...] is_positional |= is_pos?

Yeah, I thought about that but wanted to make the diff smaller / easier to see how it changed. Happy to update to this.

do a single check is variant is tuple-like or not

That seems plausible, but I don't know my way around the compiler enough to do so. Got any hints?

instead of trying to move this out of the loop

I thought you were suggesting that we should move it out of the loop — I must not understand what you mean.

[WaffleLapkin]

I thought you were suggesting that we should move it out of the loop — I must not understand what you mean.

Right, I see why this is confusing.

"trying to move this out of the loop"

this refers to the is_pos which is moved into is_positional, which is outside of the loop.