Convert FieldTypeTp to FieldType explicitly

[iosmanthus]

Signed-off-by: iosmanthus myosmanthustree@gmail.com

What have you changed? (mandatory)

Improve #4649, use FieldType::From(FieldTypeTp::xxx) to convert FieldTypeTp to FieldType which is more explicit than FieldTypeTp.into.

• Improvement (a change which is an improvement to an existing feature)

How has this PR been tested? (mandatory)

No test needed if the code passed compiling.

[iosmanthus]

@breeswish @AndreMouche @rleungx PTAL 😃

[breezewish]

Looks like this makes the code more complex than before 😂 It is intended to only use FieldTypeTp where FieldType is needed, which can simplify the code because in the most of the time we only want a FieldType which FieldTypeTp is set to our given value.

[ngaut]

It's more readable. LGTM

[breezewish]

@ngaut

Places where it is used, for example,

let schema = &[……];
let ft = ……;
let field_types = &[……];

Already clearly indicates that it is a FieldType. These Froms are just speaking for nothing.

[breezewish]

I'm going to close this PR, since I think it adds unnecessary semantics. The existing code base, for example, variable names, contexts, are already verbose enough about what exactly the type it is.

However, feel free to open a new PR, changing from impl Into to impl From, which is a more general implementation and should be preferred.