Creating a safe_suggestion attribute for coerce suggestions

[GuillaumeGomez]

In #37388, coerce suggestions have been added. However, the system isn't very clever and will propose everything matching, wether or not it corresponds to the actual user's needs. For example:

let x: u32 = String::new();

In this case, we get len() and capacity() methods suggested. But in the example, we could think that the user wanted a conversion, and if this is the case, the suggestions are completely wrong.

In order to improve this, @jonathandturner proposed a safe_suggestion attribute. I agree with this proposition but a few details still need to be "fixed":

• Should we loop through "safe" methods only?
• How should we consider that a "safe" method matches the current user's need?
• If the safe_suggestion attribute is adopted, should we have to add it over every methods that would be considered as a safe suggestion?

I might have forget other potential issues so don't hesitate to add them if this is the case.

[GuillaumeGomez]

cc @eddyb

[Mark-Simulacrum]

As far as I know there hasn't been any progress on this.

[estebank]

Closing in favor of #59450, which has the approach that I think is more likely to work. We no longer suggest arbitrary method based on their return types.