Warn on shadowing built-in types

[Wilfred]

I got a little confused by the following code:

struct Foo<T> {
    value: T
}

impl<u32> Foo<u32> {
    fn impl_func(&self) -> u32 {
        42
    }
}

rustc gives the error: 42: expected type parameter, found integral variable.

The mistake is that I've created a type parameter named u32. Could we warn on this case? I can't see any legitimate use case for shadowing u32.

[nagisa]

Should report an issue to clippy in case they don’t already have such a lint instead.

[Wilfred]

Since this code is also a compile error, I believe clippy can't help.

[mcarton]

@Wilfred Sure it can. Purely syntactic lints are run before type checking.

This had already been reported on Clippy: https://github.com/Manishearth/rust-clippy/issues/949. I implemented it: https://github.com/Manishearth/rust-clippy/pull/1199.

[Wilfred]

Aha, this is well-trodden ground then. I'm going to close as a duplicate of #35030.

Huge thanks for implementing it in clippy! I did not know clippy could help with code that can't compile.