fix: size dict elements buffer to HashMap capacity, not len

[TomerStarkware]

Summary

When materializing a Felt252Dict value (Value::Felt252Dict → native), the elements buffer was arena-allocated with room for map.len() slots. The runtime's dict_get only grows the elements buffer once the backing HashMap is full, so a runtime insert of a new key could write one slot past the end of the buffer and corrupt the arena.

This reorders the allocation so the FeltDict struct is created and reserved first, then sizes the elements buffer to the HashMap's actual capacity() (which reserve usually over-allocates). That guarantees there is always room for the runtime to fill every slot the HashMap can hold.

Changes

• src/values.rs: allocate/register the FeltDict before the elements buffer, and size the buffer to dict.mappings.capacity() instead of map.len().

🤖 Generated with Claude Code

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This change is 

[orizi]

@orizi reviewed 1 file and all commit messages, and made 1 comment.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on TomerStarkware).

[github-actions]

Benchmarking results

Benchmark for program dict_insert

Open benchmarks

Command	Mean [s]	Min [s]	Max [s]	Relative
Cairo-vm (Rust, Cairo 1)	10.432 ± 0.045	10.368	10.496	5.74 ± 0.04
cairo-native (embedded AOT)	1.818 ± 0.011	1.806	1.838	1.00
cairo-native (embedded JIT using LLVM's ORC Engine)	1.828 ± 0.015	1.801	1.848	1.01 ± 0.01

Benchmark for program dict_snapshot

Open benchmarks

Command	Mean [ms]	Min [ms]	Max [ms]	Relative
Cairo-vm (Rust, Cairo 1)	517.0 ± 3.8	510.3	523.8	1.00
cairo-native (embedded AOT)	1629.6 ± 12.2	1614.2	1654.5	3.15 ± 0.03
cairo-native (embedded JIT using LLVM's ORC Engine)	1666.1 ± 12.0	1649.7	1689.0	3.22 ± 0.03

Benchmark for program factorial_2M

Open benchmarks

Command	Mean [s]	Min [s]	Max [s]	Relative
Cairo-vm (Rust, Cairo 1)	4.639 ± 0.014	4.620	4.662	2.21 ± 0.01
cairo-native (embedded AOT)	2.097 ± 0.007	2.089	2.109	1.00
cairo-native (embedded JIT using LLVM's ORC Engine)	2.108 ± 0.012	2.082	2.127	1.01 ± 0.01

Benchmark for program fib_2M

Open benchmarks

Command	Mean [s]	Min [s]	Max [s]	Relative
Cairo-vm (Rust, Cairo 1)	4.560 ± 0.043	4.524	4.670	2.79 ± 0.03
cairo-native (embedded AOT)	1.635 ± 0.012	1.614	1.652	1.00
cairo-native (embedded JIT using LLVM's ORC Engine)	1.669 ± 0.007	1.658	1.682	1.02 ± 0.01

Benchmark for program linear_search

Open benchmarks

Command	Mean [ms]	Min [ms]	Max [ms]	Relative
Cairo-vm (Rust, Cairo 1)	560.8 ± 4.0	555.4	565.0	1.00
cairo-native (embedded AOT)	1649.8 ± 9.3	1636.1	1663.7	2.94 ± 0.03
cairo-native (embedded JIT using LLVM's ORC Engine)	1692.6 ± 6.6	1684.8	1708.9	3.02 ± 0.02

Benchmark for program logistic_map

Open benchmarks

Command	Mean [ms]	Min [ms]	Max [ms]	Relative
Cairo-vm (Rust, Cairo 1)	474.9 ± 3.7	470.7	482.0	1.00
cairo-native (embedded AOT)	1801.4 ± 10.7	1785.6	1815.3	3.79 ± 0.04
cairo-native (embedded JIT using LLVM's ORC Engine)	1911.0 ± 13.5	1895.2	1938.2	4.02 ± 0.04

[github-actions]

Benchmark results Main vs HEAD.

Base

Command	Mean [s]	Min [s]	Max [s]	Relative
base dict_insert.cairo (JIT)	2.046 ± 0.037	2.005	2.137	1.01 ± 0.02
base dict_insert.cairo (AOT)	2.020 ± 0.027	1.969	2.067	1.00

Head

Command	Mean [s]	Min [s]	Max [s]	Relative
head dict_insert.cairo (JIT)	1.842 ± 0.009	1.824	1.854	1.00 ± 0.01
head dict_insert.cairo (AOT)	1.838 ± 0.012	1.811	1.855	1.00

Base

Command	Mean [s]	Min [s]	Max [s]	Relative
base dict_snapshot.cairo (JIT)	1.821 ± 0.028	1.783	1.859	1.03 ± 0.02
base dict_snapshot.cairo (AOT)	1.766 ± 0.017	1.741	1.792	1.00

Head

Command	Mean [s]	Min [s]	Max [s]	Relative
head dict_snapshot.cairo (JIT)	1.714 ± 0.015	1.687	1.744	1.04 ± 0.02
head dict_snapshot.cairo (AOT)	1.650 ± 0.021	1.627	1.688	1.00

Base

Command	Mean [s]	Min [s]	Max [s]	Relative
base factorial_2M.cairo (JIT)	2.279 ± 0.016	2.262	2.321	1.00 ± 0.01
base factorial_2M.cairo (AOT)	2.273 ± 0.018	2.246	2.302	1.00

Head

Command	Mean [s]	Min [s]	Max [s]	Relative
head factorial_2M.cairo (JIT)	2.124 ± 0.011	2.107	2.137	1.01 ± 0.01
head factorial_2M.cairo (AOT)	2.111 ± 0.016	2.093	2.133	1.00

Base

Command	Mean [s]	Min [s]	Max [s]	Relative
base fib_2M.cairo (JIT)	1.813 ± 0.026	1.765	1.854	1.00
base fib_2M.cairo (AOT)	1.819 ± 0.023	1.790	1.859	1.00 ± 0.02

Head

Command	Mean [s]	Min [s]	Max [s]	Relative
head fib_2M.cairo (JIT)	1.668 ± 0.007	1.659	1.684	1.01 ± 0.01
head fib_2M.cairo (AOT)	1.655 ± 0.008	1.645	1.670	1.00

Base

Command	Mean [s]	Min [s]	Max [s]	Relative
base linear_search.cairo (JIT)	1.884 ± 0.031	1.825	1.920	1.03 ± 0.02
base linear_search.cairo (AOT)	1.835 ± 0.024	1.804	1.869	1.00

Head

Command	Mean [s]	Min [s]	Max [s]	Relative
head linear_search.cairo (JIT)	1.707 ± 0.021	1.674	1.735	1.02 ± 0.02
head linear_search.cairo (AOT)	1.682 ± 0.017	1.655	1.709	1.00

Base

Command	Mean [s]	Min [s]	Max [s]	Relative
base logistic_map.cairo (JIT)	2.114 ± 0.018	2.089	2.147	1.06 ± 0.02
base logistic_map.cairo (AOT)	2.001 ± 0.023	1.973	2.040	1.00

Head

Command	Mean [s]	Min [s]	Max [s]	Relative
head logistic_map.cairo (JIT)	1.939 ± 0.015	1.919	1.970	1.07 ± 0.01
head logistic_map.cairo (AOT)	1.809 ± 0.011	1.791	1.825	1.00