> So "From" has to return something that might be an error, in some way. Just so that the Try... variants can be generated
Not quite. From can't fail, but TryFrom for example could fail.
Lets try a couple very concrete examples, From<u16> for i32 exists. Turning any 16-bit unsigned integer into a 32-bit signed integer works easily. As a result of the "chain" I mentioned, Rust will also accept TryInto<i32> for u16. This also can't fail - and it's going to run the identical code, but TryInto has an associated Error type, this must be filled out, it's filled out as Infallible. The compiler can see that Infallible is empty, therefore where somebody wrote error handling code for their TryInto<i32> if the actual type was u16 that Error type will be Infallible, therefore the code using it is dead.
Now, compare converting signed 16-bit integers to unsigned. This can clearly fail, -10 is a perfectly good signed 16-bit integer, but it's out of range for unsigned. So From<i16> for u16 does not exist. But TryInto<u16> for i16 does exist - but this type that really does have an error type, this conversion can and does fail with a "TryFromIntError" type apparently, which I expect has some diagnostics inside it.
Not quite. From can't fail, but TryFrom for example could fail.
Lets try a couple very concrete examples, From<u16> for i32 exists. Turning any 16-bit unsigned integer into a 32-bit signed integer works easily. As a result of the "chain" I mentioned, Rust will also accept TryInto<i32> for u16. This also can't fail - and it's going to run the identical code, but TryInto has an associated Error type, this must be filled out, it's filled out as Infallible. The compiler can see that Infallible is empty, therefore where somebody wrote error handling code for their TryInto<i32> if the actual type was u16 that Error type will be Infallible, therefore the code using it is dead.
Now, compare converting signed 16-bit integers to unsigned. This can clearly fail, -10 is a perfectly good signed 16-bit integer, but it's out of range for unsigned. So From<i16> for u16 does not exist. But TryInto<u16> for i16 does exist - but this type that really does have an error type, this conversion can and does fail with a "TryFromIntError" type apparently, which I expect has some diagnostics inside it.