rust - "parameter `'a` is never used" error when 'a is used in type parameter bound - 


use std::iter::iterator;  trait listterm<'a> {    type iter:iterator<item=&'a u32>;    fn iter(&'a self)-> self::iter; }  enum termvalue<'a, lt> lt:listterm<'a>+sized+'a {    str(lt) }  src/lib.rs:8:16: 8:18 error: parameter `'a` never used [e0392] src/lib.rs:8 enum termvalue<'a, lt> lt:listterm<'a>+'a {                             ^~

'a being used. bug, or parametric enums not finished?.. rustc --explain e0392 recommends use of phantomdata<&'a _>, don't think there's opportunity in use case, may able see.

'a being used.

not far compiler concerned. cares of generic parameters used somewhere in body of struct or enum. constraints do not count.

what might want use higher-ranked lifetime bound:

enum termvalue<lt>     for<'a> lt: 'a + listterm<'a> + sized {    str(lt) }

in other situations, might want use phantomdata indicate want type act as though uses parameter:

use std::marker::phantomdata;  struct thing<'a> {     // causes type function *as though* has `&'a ()` field,     // despite not *actually* having one.     _marker: phantomdata<&'a ()>, }

and clear: can use phantomdata in enum: put in 1 of variants.


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