# Impl IndexMut for HashMap
**URL:** https://internals.rust-lang.org/t/impl-indexmut-for-hashmap/10963
**Category:** libs
**Created:** [September 18, 2019, 3:18pm UTC](https://internals.rust-lang.org/t/impl-indexmut-for-hashmap/10963 "2019-09-18T15:18:10Z")
**Posts on this page:** 3
**Page:** 2
### Author:  [@RustyYato](https://internals.rust-lang.org/u/RustyYato)
#### Post date: [September 22, 2019, 7:53am UTC](https://internals.rust-lang.org/t/impl-indexmut-for-hashmap/10963/21 "2019-09-22T07:53:12Z")
That is implementation defined. If you see my propsal above, we could always use `index_assign` when you have `foo[index] = value`, but give it the default impl to defer to `index_mut` to prevent a breaking change.
We can then change `HashMap` to update only on `index_assign` so that just accessing an element via `index_mut` doesn't change anything.
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### Author:  [@kornel](https://internals.rust-lang.org/u/kornel)
#### Post date: [September 22, 2019, 11:39am UTC](https://internals.rust-lang.org/t/impl-indexmut-for-hashmap/10963/22 "2019-09-22T11:39:42Z")
To me there isn't a dramatic difference between `foo[i] = v` and `foo.insert(i, v)`, so I don't really miss it. I don't even use `v = foo[i]`, because most of the time I don't know if they key is present, so I have to use `.get()` or `.entry()` anyway.
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### Author:  [@system](https://internals.rust-lang.org/u/system)
#### Post date: [December 21, 2019, 11:39am UTC](https://internals.rust-lang.org/t/impl-indexmut-for-hashmap/10963/23 "2019-12-21T11:39:45Z")
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