Add --trim-mode (compile-time) preference #31 (fixed)

[vishal2005025]

Title: Add compile-time _trim_enabled preference during --trim precompilation

Summary

When building with --trim, some packages need to know at precompile time that trimming is enabled so they can register / specialize code accordingly (see SciML/LinearSolve.jl#778). This PR ensures a compile-time preference _trim_enabled is visible to packages during the Pkg.instantiate(); Pkg.precompile() step invoked by JuliaC when trimming is requested.

What I changed

• Implemented temporary depot-based LocalPreferences injection for precompilation in:
  • src/compiling.jl

Behavior

• If an ImageRecipe has trimming enabled (i.e. recipe.trim_mode !== nothing && recipe.trim_mode != "no"), the precompile subprocess will:
  1. Create a temporary depot directory.
  2. Write config/LocalPreferences.toml containing:

     [Preferences]
     _trim_enabled = true
     

  3. Invoke Pkg.instantiate(); Pkg.precompile() with JULIA_DEPOT_PATH pointed to the temporary depot so Preferences.load_preference(Preferences, "_trim_enabled") will return true for packages during precompilation.
  4. Clean up the temporary depot after the precompile subprocess finishes.

Why this approach

• The repo discussion suggested using a LocalPreferences.toml hack so packages can call:
  Preferences.load_preference(Preferences, "_trim_enabled")
  and see a compile-time flag during precompilation.
• Using a temporary depot avoids touching the user's global depot or permanently modifying configuration.
• This design is minimal, low-risk, and limited only to the precompile subprocess invoked by JuliaC.

Files edited

• src/compiling.jl — add logic to create temporary depot, write LocalPreferences.toml, set JULIA_DEPOT_PATH for the precompile subprocess, and cleanup afterward.

Testing & verification

• Ran the package test suite: julia --project -e "using Pkg; Pkg.test()" — all tests passed locally on Windows.
• The existing test suite includes trimming-related tests (CLI and programmatic flows) and completed successfully.
• Suggested additional test (optional): add a small integration test that asserts a package reading the preference during precompile returns true, e.g. a tiny test project that calls Preferences.load_preference(Preferences, "_trim_enabled") in a top-level file and checks the behavior during the Pkg.precompile() invoked by the compile pipeline.

Caveats & notes

• This relies on packages using Preferences.jl and reading LocalPreferences.toml via Preferences.load_preference. Packages that obtain preferences from other sources will not be affected.
• The temporary depot is only used during Pkg.instantiate(); Pkg.precompile() subprocess and is removed afterwards; user's depot is not modified.
• The change affects only the precompile subprocess for trimming-enabled runs. Runtime behavior is unchanged.
• If you want to expose the preference differently (e.g., via an exported Base function or environment variable), we can iterate — this approach follows the repo discussion and avoids invalidating the global pkgimage cache.

How to try locally

Run the existing test-suite (this was used to validate the change):

julia --project -e "using Pkg; Pkg.test()"

[vishal2005025]

please review!