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Add default value for properties (takes a function instead of an expression) #881

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merged 4 commits into from
Jan 19, 2020
Merged

Add default value for properties (takes a function instead of an expression) #881

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6a594be
Add `default` value for properties
AlephAlpha Jan 17, 2020
2eff9ea
Default values specified by paths instead of exprs
AlephAlpha Jan 18, 2020
854eb01
Fix misleading error message
AlephAlpha Jan 19, 2020
891d70a
Remove confusing Option
AlephAlpha Jan 19, 2020
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10 changes: 6 additions & 4 deletions crates/macro/src/derive_props/field.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -90,12 +90,14 @@ impl PropField {
// Hacks to avoid misleading error message.
quote_spanned! {span=>
#name: {
let none: Option<#ty> = None;
match true {
false => none,
true => Some(#default())
#[allow(unreachable_code)]
false => {
let __unreachable: #ty = ::std::unreachable!();
__unreachable
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Do you need an assign here? I think you can just do false => ::std::unreachable!() right?

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The false arm must return the correct type, otherwise Rust would think that it has the same type as #default(), and might show that misleading message: "help: try using a conversion method: "foo".to_string()".

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Hmm your explanation makes sense but I had it working with unimplemented like that. This is fine though!

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Sorry, I was wrong about that. I did not test it before leaving the comment. It seems that false => ::std::unreachable!() does work.

},
true => #default()
}
.unwrap()
},
}
}
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6 changes: 3 additions & 3 deletions tests/derive_props/fail.stderr
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Expand Up @@ -78,7 +78,7 @@ error[E0308]: match arms have incompatible types
| |
| expected struct `std::string::String`, found i32
| `match` arms have incompatible types
| this is found to be of type `std::option::Option<std::string::String>`
| this is found to be of type `std::string::String`
|
= note: expected type `std::option::Option<std::string::String>`
found type `std::option::Option<i32>`
= note: expected type `std::string::String`
found type `i32`