Lint to check unnecessary trait bounds

What it does

Some trait bounds aren't required, though they may be added for semantic purposes, this is rare. An example is say we have this function:

async fn<T: Send + Sync>foo() { ... }

then it no longer needs async so we change it to

fn<T: Send + Sync>foo() { ... }

but we forgot to remove the Send + Sync bounds and they slip unnoticed, this is bad because users can no longer use types that don't implement Send + Sync even though they actually can

Lint Name

unnecessary_trait_bounds

Category

correctness

Advantage

• Removes unnecessary restrictions
• Makes code more clear
• Possibly even frees a dependency if the trait is from a crate and it's not used otherwsie

Drawbacks

Basically warns on almost every trait bound in struct definitions:

struct<T: Send> Foo<T> { ... }

Whether this is good design is controversial, this could be separated to another lint but I don't see it necessary as users can simply allow it

Example

fn print<T: Debug + Display>(x: T) {
  println!("{x:?}");
}

Could be written as:

fn print<T: Debug>(x: T) {
  println!("{x:?}");
}

Start with the issue's Rust examples and define which unused trait bounds the lint should report, including whether struct definitions are in scope. Resolve the stated design concern about controversial warnings before implementing it, then add coverage demonstrating that unnecessary bounds are reported while required bounds remain accepted.