VNV type stability improvements and tests

[mhauru]

More aggressively tighten element types after each setindex_internal!!. This fixes performance for some models, most notably the Loop univariate 10k that showed substantial disadvantage compared to Metadata in still in #1098.

Also, add tests for tighten_ and loosen_types!!. These become more important since we are calling those two functions all the time now, and they must be compile-time no-ops for this to not cause a large overhead. Tests check this the best they can (I've checked more thoroughly manually with code_typed).

Also fix a type instability in loosen_types!! caught by the new tests.

Benchmarking now, depending on results will either mark this ready or add more performance improvements to this if needed.

[github-actions]

Benchmark Report for Commit 6972713

Computer Information

Julia Version 1.11.7
Commit f2b3dbda30a (2025-09-08 12:10 UTC)
Build Info:
  Official https://julialang.org/ release
Platform Info:
  OS: Linux (x86_64-linux-gnu)
  CPU: 4 × AMD EPYC 7763 64-Core Processor
  WORD_SIZE: 64
  LLVM: libLLVM-16.0.6 (ORCJIT, znver3)
Threads: 1 default, 0 interactive, 1 GC (on 4 virtual cores)

Benchmark Results

┌───────────────────────┬───────┬─────────────┬───────────────────┬────────┬────────────────┬─────────────────┐
│                 Model │   Dim │  AD Backend │           VarInfo │ Linked │ t(eval)/t(ref) │ t(grad)/t(eval) │
├───────────────────────┼───────┼─────────────┼───────────────────┼────────┼────────────────┼─────────────────┤
│ Simple assume observe │     1 │ forwarddiff │             typed │  false │            6.8 │             1.7 │
│           Smorgasbord │   201 │ forwarddiff │             typed │  false │          757.0 │            43.2 │
│           Smorgasbord │   201 │ forwarddiff │ simple_namedtuple │   true │          436.9 │            53.4 │
│           Smorgasbord │   201 │ forwarddiff │           untyped │   true │          809.8 │            37.2 │
│           Smorgasbord │   201 │ forwarddiff │       simple_dict │   true │         7595.7 │            26.4 │
│           Smorgasbord │   201 │ forwarddiff │      typed_vector │   true │          847.9 │            39.1 │
│           Smorgasbord │   201 │ forwarddiff │    untyped_vector │   true │          780.8 │            39.8 │
│           Smorgasbord │   201 │ reversediff │             typed │   true │          938.2 │            45.6 │
│           Smorgasbord │   201 │    mooncake │             typed │   true │          768.4 │             5.7 │
│           Smorgasbord │   201 │      enzyme │             typed │   true │          941.3 │             3.8 │
│    Loop univariate 1k │  1000 │    mooncake │             typed │   true │         4154.5 │             5.8 │
│       Multivariate 1k │  1000 │    mooncake │             typed │   true │         1036.3 │             9.0 │
│   Loop univariate 10k │ 10000 │    mooncake │             typed │   true │        45345.7 │             5.5 │
│      Multivariate 10k │ 10000 │    mooncake │             typed │   true │         8974.2 │            10.0 │
│               Dynamic │    10 │    mooncake │             typed │   true │          123.1 │            11.5 │
│              Submodel │     1 │    mooncake │             typed │   true │            8.9 │             6.5 │
│                   LDA │    12 │ reversediff │             typed │   true │         1038.0 │             2.0 │
└───────────────────────┴───────┴─────────────┴───────────────────┴────────┴────────────────┴─────────────────┘

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 81.20%. Comparing base (80cf12d) to head (6972713).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1102      +/-   ##
==========================================
+ Coverage   81.17%   81.20%   +0.02%     
==========================================
  Files          40       40              
  Lines        3793     3798       +5     
==========================================
+ Hits         3079     3084       +5     
  Misses        714      714              

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[github-actions]

DynamicPPL.jl documentation for PR #1102 is available at:
https://TuringLang.github.io/DynamicPPL.jl/previews/PR1102/

[mhauru]

Current benchmarks:

┌───────────────────────┬───────┬─────────────┬────────────────┬────────┬────────────────┬─────────────────┐
│                 Model │   Dim │  AD Backend │        VarInfo │ Linked │ t(eval)/t(ref) │ t(grad)/t(eval) │
├───────────────────────┼───────┼─────────────┼────────────────┼────────┼────────────────┼─────────────────┤
│ Simple assume observe │     1 │    mooncake │          typed │  false │            4.5 │             5.8 │
│ Simple assume observe │     1 │    mooncake │   typed_vector │  false │            8.1 │             6.6 │
│ Simple assume observe │     1 │    mooncake │        untyped │  false │           36.0 │             1.6 │
│ Simple assume observe │     1 │    mooncake │ untyped_vector │  false │            6.3 │             7.3 │
│           Smorgasbord │   201 │ reversediff │          typed │  false │          370.5 │            50.8 │
│           Smorgasbord │   201 │ reversediff │   typed_vector │  false │          407.4 │            46.5 │
│           Smorgasbord │   201 │ reversediff │        untyped │  false │         4072.0 │             4.6 │
│           Smorgasbord │   201 │ reversediff │ untyped_vector │  false │          336.3 │            56.4 │
│    Loop univariate 1k │  1000 │    mooncake │          typed │   true │         1742.8 │             4.7 │
│    Loop univariate 1k │  1000 │    mooncake │   typed_vector │   true │         1822.0 │             4.7 │
│    Loop univariate 1k │  1000 │    mooncake │        untyped │   true │         1779.7 │            18.0 │
│    Loop univariate 1k │  1000 │    mooncake │ untyped_vector │   true │         1818.4 │             4.6 │
│       Multivariate 1k │  1000 │    mooncake │          typed │   true │          425.4 │             8.4 │
│       Multivariate 1k │  1000 │    mooncake │   typed_vector │   true │          437.9 │             8.6 │
│       Multivariate 1k │  1000 │    mooncake │        untyped │   true │         1625.9 │             2.3 │
│       Multivariate 1k │  1000 │    mooncake │ untyped_vector │   true │          419.1 │             8.2 │
│   Loop univariate 10k │ 10000 │    mooncake │          typed │   true │        17784.3 │             4.9 │
│   Loop univariate 10k │ 10000 │    mooncake │   typed_vector │   true │        19867.9 │             4.7 │
│   Loop univariate 10k │ 10000 │    mooncake │        untyped │   true │        19939.9 │            18.1 │
│   Loop univariate 10k │ 10000 │    mooncake │ untyped_vector │   true │        18634.1 │             4.7 │
│      Multivariate 10k │ 10000 │    mooncake │          typed │   true │         3693.4 │             9.4 │
│      Multivariate 10k │ 10000 │    mooncake │   typed_vector │   true │         3759.9 │             9.2 │
│      Multivariate 10k │ 10000 │    mooncake │        untyped │   true │        14833.3 │             2.4 │
│      Multivariate 10k │ 10000 │    mooncake │ untyped_vector │   true │         3545.0 │             9.5 │
│               Dynamic │    10 │    mooncake │          typed │   true │           71.1 │             6.2 │
│               Dynamic │    10 │    mooncake │   typed_vector │   true │           97.1 │             5.7 │
│               Dynamic │    10 │    mooncake │ untyped_vector │   true │           78.2 │             6.8 │
│              Submodel │     1 │    mooncake │          typed │   true │            5.4 │             5.2 │
│              Submodel │     1 │    mooncake │   typed_vector │   true │            9.9 │             5.7 │
│              Submodel │     1 │    mooncake │        untyped │   true │            4.5 │            10.8 │
│              Submodel │     1 │    mooncake │ untyped_vector │   true │            8.1 │             5.7 │
│                   LDA │    12 │ reversediff │          typed │   true │          468.5 │             2.0 │
│                   LDA │    12 │ reversediff │   typed_vector │   true │          497.3 │             1.9 │
└───────────────────────┴───────┴─────────────┴────────────────┴────────┴────────────────┴─────────────────┘

Still not happy with that overhead, will try to understand it better and fix.