`cannot assign to data in an index of HashMap` fixit generates invalid code

[narpfel]

The autofix for map[&key] = value; generates invalid code:

use std::collections::HashMap;

fn main() {
    let mut map = HashMap::<usize, usize>::new();
    map[&42] = 27;
}

==>

use std::collections::HashMap;

fn main() {
    let mut map = HashMap::<usize, usize>::new();
    let val = map.insert(get_mut(entry(&42, p(|val| { *val.or_insert(27); }));
}

rust-analyzer version: rust-analyzer 1 (e402c49 2023-12-01) (installed from the Arch Linux repos)

rustc version:

$ rustc -vV
rustc 1.76.0-nightly (06e02d5b2 2023-12-09)
binary: rustc
commit-hash: 06e02d5b259c1e88cbf0c74366d9e0a4c7cfd6d9
commit-date: 2023-12-09
host: x86_64-unknown-linux-gnu
release: 1.76.0-nightly
LLVM version: 17.0.5

(installed via rustup)

relevant settings: Editor used is nvim version NVIM v0.10.0-dev-1807+g096211a87b (current git head) using current git head of nvim-lspconfig. The command used to trigger the refactor is :lua vim.lsp.buf.code_action() and then selecting “to modify a HashMap [...]” while in normal mode with the cursor above the line map[&42] = 27;.

[flodiebold]

The error comes from rustc, which apparently provides all the changes for all the different options at the same time marked as machine-applicable. So I'd say this is a rustc bug.

[lnicola]

Triage: rustc now says

error[E0594]: cannot assign to data in an index of `HashMap<usize, usize>`
  --> src/main.rs:24:5
   |
24 |     map[&42] = 27;
   |     ^^^^^^^^^^^^^ cannot assign
   |
   = help: trait `IndexMut` is required to modify indexed content, but it is not implemented for `HashMap<usize, usize>`
help: to modify a `HashMap<usize, usize>`, use `.get_mut()`, `.insert()` or the entry API
   |
24 |     map.insert(&42, 27);
   |        ~~~~~~~~   ~   +
24 |     map.get_mut(&42).map(|val| { *val = 27; });
   |        ~~~~~~~~~   ~~~~~~~~~~~~~~~~~~     ++++
24 |     let val = map.entry(&42).or_insert(27);
   |     +++++++++    ~~~~~~~   ~~~~~~~~~~~~  +

which is marked as MachineApplicable, but we don't do anything when the user selects the fixit.