Provide placeholder generics for traits in "no method found for type parameter" suggestions

[dianne]

In the diagnostics for the error no method named `method` found for type parameter `T` in the current scope [E0599], the compiler will suggest adding bounds on T for traits that define a method named method. However, these suggestions didn't include any generic arguments, so applying them would result in a missing generics for trait or missing lifetime specifier error. This PR adds placeholder arguments to the suggestion in such cases. Specifically, I tried to base the placeholders off of what's done in suggestions for when generics are missing or too few are provided:

• The placeholder for a parameter without a default is the name of the parameter.
• Placeholders are not provided for parameters with defaults.

Placeholder arguments are enclosed in /* and */, and the applicability of the suggestion is downgraded to Applicability::HasPlaceholders if any placeholders are provided.

Fixes #132407

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[fmease]

Could you surround the placeholder arguments with /* and */ to highlight the fact that they are placeholders? E.g., Trait2</* '_, A, B */>. Otherwise users may take them literally and wonder whether the suggestion is wrong if it leads to more errors. It's not a hypothetical btw, people have reported such suggestions as bugs.

This would also be consistent with the diagnostic suggestion for E0220 (associated type not found). See 02a2f02 were I impl'ed that.

[fmease]

You should be able to simplify this by using identity_for_item over generics_of. Compare:

rust/compiler/rustc_hir_analysis/src/hir_ty_lowering/errors.rs

Lines 253 to 263 in ef972a3

	let trait_args = &ty::GenericArgs::identity_for_item(tcx, best_trait)[1..];
	let mut trait_ref = trait_name.clone();
	let applicability = if let [arg, args @ ..] = trait_args {
	use std::fmt::Write;
	write!(trait_ref, "</* {arg}").unwrap();
	args.iter().try_for_each(|arg| write!(trait_ref, ", {arg}")).unwrap();
	trait_ref += " */>";
	Applicability::HasPlaceholders
	} else {
	Applicability::MaybeIncorrect
	};

[fmease]

Ah, disregard, that wouldn't consider generic parameter defaults.

[dianne]

The placeholders should now be surrounded by /* and */. I'll open a PR to do that for E0107 too, if that's alright. That's what I was using as a reference, and it looks easy to fix.

[fmease]

Thanks! Sorry for the slight delay. I was just dumbfounded that we don't have an API for that already somewhere. I guess one could use GenericArgs::identity_for_item plus Generics::own_args_no_defaults but er, that would perform much worse in this case.

I have small stylistic suggestions but nothing major. Lastly, could you squash your commits into one?

[dianne]

Done. Thanks!
@rustbot ready

[fmease]

@bors r+ rollup (diagnostics)

[bors]

📌 Commit 02add7d has been approved by fmease

It is now in the queue for this repository.