Fix compiler output for sub-variables of immutable data

[penelopeysm]

Closes #1176. The result of this PR on the model macro

using DynamicPPL, Distributions
@macroexpand @model function test(data)
    data.x ~ Normal()
end

is shown in the following diff. Essentially the result is to avoid setting to data.x directly:

 quote
     function test(__model__::Model, __varinfo__::AbstractVarInfo, data::Any; )
         #= REPL[4]:1 =#
         begin
             var"##dist#246" = Normal()
             var"##vn#243" = (DynamicPPL.resolve_varnames)((VarName){:data}((Accessors.opticcompose)((Accessors.PropertyLens){:x}())), var"##dist#246")
             var"##isassumption#244" = begin
                     if (DynamicPPL.contextual_isassumption)(__model__.context, (prefix)(__model__.context, var"##vn#243"))
                         if !((DynamicPPL.inargnames)(var"##vn#243", __model__)) || (DynamicPPL.inmissings)(var"##vn#243", __model__)
                             true
                         else
                             data.x === missing
                         end
                     else
                         false
                     end
                 end
             begin
                 #= /Users/pyong/ppl/dppl/src/compiler.jl:599 =#
                 var"##retval#248" = if (DynamicPPL.contextual_isfixed)(__model__.context, (prefix)(__model__.context, var"##vn#243"))
-                        data.x = (DynamicPPL.getfixed_nested)(__model__.context, (prefix)(__model__.context, var"##vn#243"))
+                        data = (Accessors.set)(data, (BangBang.AccessorsImpl.prefermutation)((Accessors.opticcompose)((Accessors.PropertyLens){:x}())), (DynamicPPL.getfixed_nested)(__model__.context, (prefix)(__model__.context, var"##vn#243")))
                     elseif var"##isassumption#244"
                         begin
                             (var"##value#247", __varinfo__) = (tilde_assume!!)(__model__.context, (DynamicPPL.unwrap_right_vn)((DynamicPPL.check_tilde_rhs)(var"##dist#246"), var"##vn#243")..., __varinfo__)
                             data = (Accessors.set)(data, (BangBang.AccessorsImpl.prefermutation)((Accessors.opticcompose)((Accessors.PropertyLens){:x}())), var"##value#247")
                             var"##value#247"
                         end
                     else
-                        data.x = if (DynamicPPL.inargnames)(var"##vn#243", __model__)
+                        var"##supplied_val#236" = if (DynamicPPL.inargnames)(var"##vn#243", __model__)
                                 data.x
                             else
                                 (DynamicPPL.getconditioned_nested)(__model__.context, (prefix)(__model__.context, var"##vn#243"))
                             end
-                        (var"##value#245", __varinfo__) = (tilde_observe!!)(__model__.context, (DynamicPPL.check_tilde_rhs)(var"##dist#246"), data.x, var"##vn#243", __varinfo__)
+                        (var"##value#245", __varinfo__) = (tilde_observe!!)(__model__.context, (DynamicPPL.check_tilde_rhs)(var"##dist#246"), var"##supplied_val#236", var"##vn#243", __varinfo__)
+                        data = (Accessors.set)(data, (BangBang.AccessorsImpl.prefermutation)((Accessors.opticcompose)((Accessors.PropertyLens){:x}())), var"##value#245")
                         var"##value#245"
                     end
                 #= /Users/pyong/ppl/dppl/src/compiler.jl:600 =#
                 return (var"##retval#248", __varinfo__)
             end
         end
     end
     begin
         $(Expr(:meta, :doc))
         function test(data::Any; )
             #= REPL[4]:1 =#
             return (Model){false}(test, NamedTuple{(:data,)}((data,)); )
         end
     end
 end

[github-actions]

Benchmark Report

• this PR's head: 870faab7812138b2e15106fdb2451bb0549b0f5a
• base branch: 6266f644ce8caaa3b98fc65c1eb960f4f77243b1

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

┌───────────────────────┬───────┬─────────────┬───────────────────┬────────┬────────────────────────────────┬────────────────────────────┬─────────────────────────────────┐
│                       │       │             │                   │        │        t(eval) / t(ref)        │     t(grad) / t(eval)      │        t(grad) / t(ref)         │
│                       │       │             │                   │        │ ─────────┬───────────┬──────── │ ───────┬─────────┬──────── │ ──────────┬───────────┬──────── │
│                 Model │   Dim │  AD Backend │           VarInfo │ Linked │     base │   this PR │ speedup │   base │ this PR │ speedup │      base │   this PR │ speedup │
├───────────────────────┼───────┼─────────────┼───────────────────┼────────┼──────────┼───────────┼─────────┼────────┼─────────┼─────────┼───────────┼───────────┼─────────┤
│               Dynamic │    10 │    mooncake │             typed │   true │   342.88 │    363.01 │    0.94 │  10.03 │    9.81 │    1.02 │   3439.50 │   3562.79 │    0.97 │
│                   LDA │    12 │ reversediff │             typed │   true │  2435.87 │   2669.78 │    0.91 │   5.00 │    4.93 │    1.01 │  12182.59 │  13168.83 │    0.93 │
│   Loop univariate 10k │ 10000 │    mooncake │             typed │   true │ 94579.65 │ 108031.05 │    0.88 │   3.97 │    3.71 │    1.07 │ 375424.78 │ 400551.29 │    0.94 │
├───────────────────────┼───────┼─────────────┼───────────────────┼────────┼──────────┼───────────┼─────────┼────────┼─────────┼─────────┼───────────┼───────────┼─────────┤
│    Loop univariate 1k │  1000 │    mooncake │             typed │   true │  7216.54 │   7909.86 │    0.91 │   4.82 │    4.71 │    1.02 │  34801.94 │  37289.60 │    0.93 │
│      Multivariate 10k │ 10000 │    mooncake │             typed │   true │ 29365.21 │  78263.05 │    0.38 │  10.25 │    5.92 │    1.73 │ 300926.63 │ 462928.28 │    0.65 │
│       Multivariate 1k │  1000 │    mooncake │             typed │   true │  3260.69 │   3546.10 │    0.92 │   9.38 │    9.40 │    1.00 │  30569.56 │  33341.79 │    0.92 │
├───────────────────────┼───────┼─────────────┼───────────────────┼────────┼──────────┼───────────┼─────────┼────────┼─────────┼─────────┼───────────┼───────────┼─────────┤
│ Simple assume observe │     1 │ forwarddiff │             typed │  false │     5.08 │      2.69 │    1.89 │   1.92 │    3.99 │    0.48 │      9.78 │     10.73 │    0.91 │
│           Smorgasbord │   201 │ forwarddiff │             typed │  false │  1140.24 │   1232.75 │    0.92 │  61.47 │   60.65 │    1.01 │  70085.52 │  74769.29 │    0.94 │
│           Smorgasbord │   201 │ forwarddiff │       simple_dict │   true │      err │       err │     err │    err │     err │     err │       err │       err │     err │
├───────────────────────┼───────┼─────────────┼───────────────────┼────────┼──────────┼───────────┼─────────┼────────┼─────────┼─────────┼───────────┼───────────┼─────────┤
│           Smorgasbord │   201 │ forwarddiff │ simple_namedtuple │   true │      err │       err │     err │    err │     err │     err │       err │       err │     err │
│           Smorgasbord │   201 │      enzyme │             typed │   true │  1540.22 │   1731.06 │    0.89 │   5.92 │    6.25 │    0.95 │   9121.03 │  10815.76 │    0.84 │
│           Smorgasbord │   201 │    mooncake │             typed │   true │  1564.79 │   2139.89 │    0.73 │   5.19 │    4.62 │    1.12 │   8124.62 │   9882.50 │    0.82 │
├───────────────────────┼───────┼─────────────┼───────────────────┼────────┼──────────┼───────────┼─────────┼────────┼─────────┼─────────┼───────────┼───────────┼─────────┤
│           Smorgasbord │   201 │ reversediff │             typed │   true │  1543.83 │   1729.48 │    0.89 │  90.78 │   87.11 │    1.04 │ 140156.44 │ 150658.78 │    0.93 │
│           Smorgasbord │   201 │ forwarddiff │      typed_vector │   true │  1551.06 │   1691.52 │    0.92 │  54.54 │   58.24 │    0.94 │  84601.86 │  98520.14 │    0.86 │
│           Smorgasbord │   201 │ forwarddiff │           untyped │   true │  1561.18 │   1691.88 │    0.92 │  54.90 │  118.58 │    0.46 │  85714.12 │ 200616.42 │    0.43 │
├───────────────────────┼───────┼─────────────┼───────────────────┼────────┼──────────┼───────────┼─────────┼────────┼─────────┼─────────┼───────────┼───────────┼─────────┤
│           Smorgasbord │   201 │ forwarddiff │    untyped_vector │   true │  1543.83 │   1679.58 │    0.92 │ 127.05 │   55.12 │    2.31 │ 196141.08 │  92575.17 │    2.12 │
│              Submodel │     1 │    mooncake │             typed │   true │     6.92 │      7.28 │    0.95 │   4.94 │    5.24 │    0.94 │     34.23 │     38.19 │    0.90 │
└───────────────────────┴───────┴─────────────┴───────────────────┴────────┴──────────┴───────────┴─────────┴────────┴─────────┴─────────┴───────────┴───────────┴─────────┘

[github-actions]

DynamicPPL.jl documentation for PR #1177 is available at:
https://TuringLang.github.io/DynamicPPL.jl/previews/PR1177/

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 78.96%. Comparing base (6266f64) to head (870faab).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1177      +/-   ##
==========================================
+ Coverage   78.95%   78.96%   +0.01%     
==========================================
  Files          41       41              
  Lines        3896     3899       +3     
==========================================
+ Hits         3076     3079       +3     
  Misses        820      820              

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