Confusing type inference around integer literals

[XAMPPRocky]

This might be a duplicate of #39255, I'm not sure. But it's not clear why the type is correctly inferred for u8 but isn't for u32.

I tried this code:

#[derive(Default)]
pub struct Builder {
    per_page: Option<u8>,
    page: Option<u32>,
}

impl Builder {
    pub fn per_page(mut self, per_page: impl Into<u8>) -> Self {
        self.per_page = Some(per_page.into());
        self
    }

    pub fn page(mut self, page: impl Into<u32>) -> Self {
        self.page = Some(page.into());
        self
    }
}

fn main() {
    let builder = Builder::default()
        .per_page(25)
        // Doesn't work
        .page(25);
        // Works
        .page(25u32);
}

I expected to see this happen: This code compiles successfully.

Instead, this happened:

   Compiling playground v0.0.1 (/playground)
error[E0277]: the trait bound `u32: std::convert::From<i32>` is not satisfied
  --> src/main.rs:24:51
   |
24 |     let builder = Builder::default().per_page(25).page(25);
   |                                                   ^^^^ the trait `std::convert::From<i32>` is not implemented for `u32`
   |
   = help: the following implementations were found:
             <u32 as std::convert::From<bool>>
             <u32 as std::convert::From<char>>
             <u32 as std::convert::From<std::net::Ipv4Addr>>
             <u32 as std::convert::From<std::num::NonZeroU32>>
           and 2 others
   = note: required because of the requirements on the impl of `std::convert::Into<u32>` for `i32`

error: aborting due to previous error

For more information about this error, try `rustc --explain E0277`.
error: could not compile `playground`.

To learn more, run the command again with —verbose.

Meta

rustc --version --verbose:

rustc 1.43.0 (4fb7144ed 2020-04-20)
binary: rustc
commit-hash: 4fb7144ed159f94491249e86d5bbd033b5d60550
commit-date: 2020-04-20
host: x86_64-apple-darwin
release: 1.43.0
LLVM version: 9.0

[jonas-schievink]

This is because there is only one impl Into<u8> for X that applies when X is an integer type variable, which is the blanket From impl, which then calls the reflective impl<T> From<T> for T impl, so the compiler knows that the type has to be u8.

u32 has impls of From<u16> and From<u8>, in addition to the reflective From<u32> impl, so the compiler doesn't know which integer type to use.

It's different from #39255 since this issue doesn't rely on the i32 fallback, instead running into multiple applicable impls, but I believe both are expected behavior, so closing.