feat: use resolved type instead of needing Constructor.struct_type

[asterite]

Description

Problem

Something I noticed while reviewing #7489

Summary

ConstructorExpression has a field, struct_type, that stores a resolved type from macro expansion. Instead of it we can use Type::Resolved for the actual type inside an unresolved type. This works the same way but there's one less field needed.

Additionally, this actually allows more valid programs to compile. For example this one:

use std::meta::unquote;

struct Foo<T> {}

fn main() {
    let x = comptime {
        let x = quote { Foo::<i32> };
        let q = quote { $x {} };
        unquote!(q)
    };
}

Previously when unquoting Foo::<i32> as $x the generics were lost, so the above program errored with "Type annotation needed". Now the above program compiles fine.

Additional Context

Documentation

Check one:

• No documentation needed.
• Documentation included in this PR.
• [For Experimental Features] Documentation to be submitted in a separate PR.

PR Checklist

• I have tested the changes locally.
• I have formatted the changes with Prettier and/or cargo fmt on default settings.

[github-actions]

⚠️ Performance Alert ⚠️

Possible performance regression was detected for benchmark 'Compilation Time'.
Benchmark result of this commit is worse than the previous benchmark result exceeding threshold 1.20.

Benchmark suite	Current: df1dddb	Previous: 4aba428	Ratio
rollup-base-public	6.188 s	5.012 s	1.23

This comment was automatically generated by workflow using github-action-benchmark.

CC: @TomAFrench

[jfecher]

Good catch, this annoyed me on my last PR you mentioned.
For context, we only needed struct_type: Option<TypeId> previously because we used to have a Path for the constructor type before your PR to change it to an UnresolvedType.

[asterite]

For context, we only needed struct_type: Option previously because we used to have a Path for the constructor type before your PR to change it to an UnresolvedType.

Oooh... I forgot about that. That explains the comment about "path" that I didn't understand.