Fix diag span errors for bad_placeholder

[chenyukang]

Fixes #116502

[rustbot]

r? @compiler-errors

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[lqd]

This should also fix #116473, right?

[chenyukang]

This should also fix #116473, right?

no, seems the root cause is not same.

[compiler-errors]

Please explain why this actually fixes the bug -- there's no information in this PR about the root cause or the fix.

[compiler-errors]

We probably should fix this suggestion anyways

[chenyukang]

Seems the help diag you pointed out is another issue.

The root cause in the issue is we inserted duplicated span into HirPlaceholderCollector here:

rust/compiler/rustc_hir_analysis/src/collect.rs

Line 129 in a483969

self.0.push(t.span);

in the scenario of code like:

    macro_rules! m {
        () => {
            _         // This is the duplicated span ...
        };
    }
    struct S<T = m!()>(m!(), T)
    where
        T: Trait<m!()>;

This PR is trying to remove the duplicated ones, since we're mapping spans with same type type_name.

Maybe we need to use Set instead of Vec in HirPlaceholderCollector? I'm not sure about it.

[Dylan-DPC]

@chenyukang what's the status of this pr? thanks

[Dylan-DPC]

Closing this as inactive. Feel free to reöpen this pr or create a new pr if you get the time to work on this. Thanks