Change const error message to use 'literal'

[archer884]

The verbiage for E0015 ("calls in constants are limited to...") gives "struct and enum constructors" as a suggestion. The word "constructor," while a good descriptor for a particular concept in Rust, also describes a special class of user-defined functions in other languages. This commit substitutes the word "literal" in place of the word "constructor" in an effort to allay potential confusion.

Related issue here.

[rust-highfive]

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[petrochenkov]

This error is reported for calls specifically, e.g. syntax EXPR(ARGS), not braced form PATH { FIELDS }, so "struct and enum constructors" refer to function-like constructors of tuple structs and variants:TupleVariant(Fields)/TupleStruct(Fields), so I wouldn't use the word "literals" here.

If we want to avoid the word "constructor" then we can say

"calls in {}s are limited to tuple structs and tuple variants"
"calls in {}s are limited to constant functions, tuple structs and tuple variants"

or

"function calls in {}s are limited to tuple structs and tuple variants"
"function calls in {}s are limited to constant functions, tuple structs and tuple variants"

[archer884]

@petrochenkov, I'm not sure I understand. Are you saying that PATH { FIELDS } doesn't work, or shouldn't work, or that we just should not mention it here, or...? (In my particular case, the solution was to write exactly that.)

[petrochenkov]

we just should not mention it here

This.
What this specific error wants to convey is "you wrote function call f(args). normally you can call function f, but in constant context you can't call it because it's not constant enough".

I mean, maybe there's a chance that you didn't actually want to call anything, but that's guessing. Maybe an extra suggestion attached to the primary error mentioning Struct { ... } would be better in this case.

[petrochenkov]

r? @estebank

[archer884]

Maybe an extra suggestion attached to the primary error mentioning Struct { ... } would be better in this case.

That seems to me to be the clearest option, although I'm probably not the best to make that judgment. :)

[estebank]

@archer884 I would go with @petrochenkov's suggestion.

Also, I'm surprised that there isn't an existing test for this output that caught your change. Could you add one?

[archer884]

Honestly, all this other stuff is just beyond what I have knowledge/time for.

[estebank]

@archer884 sad to hear that. If you ever get the time and wish to contribute again and get familiar with the codebase/rust, we have issues with mentoring instructions for the impl period. Also, feel free to reach out in gitter and IRC for help.