feat: rustc_pass_by_value lint attribute

[mdibaiee]

Useful for thin wrapper attributes that are best passed as value instead
of reference.

Fixes #76935

[rust-highfive]

r? @lcnr

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[mdibaiee]

My PR is not fully functional yet: For some reason when I try to compile code with #[pass_by_value], I get the error below, even though I have registered it as a builtin:

(Trying to enable this on rustc_middle::ty:Ty)

error: cannot find attribute `pass_by_value` in this scope
   --> compiler/rustc_middle/src/ty/mod.rs:469:3
    |
469 | #[pass_by_value]
    |   ^^^^^^^^^^^^^

I will try to take another look a bit later, but any help is appreciated. Thank you!

P.S. this PR is subject to change as I'm trying to understand the scope better, see my question here: #76935 (comment)

[mdibaiee]

@rustbot label -T-rustdoc

[lcnr]

impl looks good to me

considering that PASS_BY_VALUE is an internal lint i would prefer #[rustc_pass_by_value] as the attribute for now.

Before merging this I would like to see some tests, both testing the Ty, TyCtxt interactions, there are some existing ui-fulldeps tests for the old TY_PASS_BY_REFERENCE lint. And probably some ordinary ui tests using custom types

[mdibaiee]

I tried to write tests for custom types, but for some reason I didn't get any errors from those custom types in my test file. I am not sure if I understand it correctly, but is that because the lint is internal, and the tests might not be considered rustc internal, and so the lint doesn't apply to them?

I did this in src/test/ui-fulldeps/internal-lints/rustc_pass_by_value.rs:

#[rustc_pass_by_value]
trait CustomTrait {
    fn test<T: CustomTrait>(
        value: T,
        reference: &T, //~ ERROR passing `CustomTrait` by reference
    );
}

#[rustc_pass_by_value]
trait CustomTraitAlias: CustomTrait {
    fn test<T: CustomTraitAlias>(
        value: T,
        reference: &T, //~ ERROR passing `CustomTraitAlias` by reference
    );
}

#[rustc_pass_by_value]
enum CustomEnum {}

impl CustomEnum {
    fn test(
        value: CustomEnum,
        reference: &CustomEnum, //~ ERROR passing `CustomEnum` by reference
    ) {
    }
}

#[rustc_pass_by_value]
struct CustomStruct {}

#[rustc_pass_by_value]
type CustomAlias<'a> = &'a CustomStruct;

impl CustomStruct {
    fn test(
        value: CustomStruct,
        reference: &CustomStruct, //~ ERROR passing `CustomStruct` by reference
    ) {
    }

    fn test_alias(
        value: CustomAlias,
        reference: &CustomAlias, //~ ERROR passing `CustomAlias` by reference
    ) {
    }
}



————

not found errors (from test file): [
    Error {
        line_num: 71,
        kind: Some(
            Error,
        ),
        msg: "passing `CustomTrait` by reference",
    },
    Error {
        line_num: 79,
        kind: Some(
            Error,
        ),
        msg: "passing `CustomTraitAlias` by reference",
    },
    Error {
        line_num: 89,
        kind: Some(
            Error,
        ),
        msg: "passing `CustomEnum` by reference",
    },
    Error {
        line_num: 103,
        kind: Some(
            Error,
        ),
        msg: "passing `CustomStruct` by reference",
    },
    Error {
        line_num: 109,
        kind: Some(
            Error,
        ),
        msg: "passing `CustomAlias` by reference",
    },
]

Is there any way to enable those internal lints in this test file? I'll take another look tomorrow and might find some solution but would appreciate any help in the meantime.

[lcnr]

some final nits, then r=me

[lcnr]

Suggested change

	cx.tcx.sess.source_map().span_to_snippet(c.span).unwrap_or_default();
	cx.tcx.sess.source_map().span_to_snippet(c.span).unwrap_or_else(|_| String::from("_"));

🤔 don't want empty strings in case the span is broken. Don't know how to even get broken strings here, so i don't think this matters too much :p

[lcnr]

r=me after this last nit, thanks for your patience

@bors delegate+

[bors]

✌️ @mdibaiee can now approve this pull request

[mdibaiee]

@lcnr thanks a million for your reviews! I will mark this with r=lcnr when CI passes 🚀

[mdibaiee]

@bors r=lcnr

[bors]

📌 Commit 2728af7 has been approved by lcnr