Fix 'assign to data in an index of' collection suggestions

[GTimothy]

fixes #150001
fixes rust-lang/rust-analyzer#16076
fixes #134917

The issues are threefold and linked:

1. Assigning data to a collection suggests 3 solutions all marked as MachineApplicable
2. The suggestions are slightly wrong with regards to their borrowing needs.
3. The suggestions are not guaranteed to produce code that is valid, and suggestion number two is equivalent to an update, not an insertion.

This PR:

• splits the large triple suggestion into three
• sets them to MaybeIncorrect
• automatically determines the required borrowing to use.

I think this solution may not be very elegant, expecially the key typechecking / borrowing part, but it works. I am however very open to any improvement/change :)

[rustbot]

r? @JonathanBrouwer

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[GTimothy]

oops, meant to title "Fix 'assign to data in an index of' map suggestions"
Should probably rename the commit too.

[rust-log-analyzer]

The job tidy failed! Check out the build log: (web) (plain enhanced) (plain)

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checking C++ file formatting
spellchecking files
building external tool typos from package typos-cli@1.38.1
finished building tool typos
error: `exessive` should be `excessive`
    ╭▸ compiler/rustc_borrowck/src/diagnostics/mutability_errors.rs:664:39
    │
664 │                         // remove the exessive referencing if necessary, but get_mut requires a ref
    ╰╴                                      ━━━━━━━━
rerun tidy with `--extra-checks=spellcheck --bless` to fix typos
tidy [extra_checks]: checks with external tool 'typos' failed
tidy [extra_checks]: FAIL
tidy: The following check failed: extra_checks
Command `/checkout/obj/build/x86_64-unknown-linux-gnu/stage1-tools-bin/rust-tidy --root-path=/checkout --cargo-path=/checkout/obj/build/x86_64-unknown-linux-gnu/stage0/bin/cargo --output-dir=/checkout/obj/build --concurrency=4 --npm-path=/node/bin/yarn --extra-checks=py,cpp,js,spellcheck` failed with exit code 1
Created at: src/bootstrap/src/core/build_steps/tool.rs:1612:23
Executed at: src/bootstrap/src/core/build_steps/test.rs:1365:29

Command has failed. Rerun with -v to see more details.
Bootstrap failed while executing `test src/tools/tidy tidyselftest --extra-checks=py,cpp,js,spellcheck`
Build completed unsuccessfully in 0:02:40
  local time: Fri Feb  6 12:25:01 UTC 2026
  network time: Fri, 06 Feb 2026 12:25:01 GMT
##[error]Process completed with exit code 1.