Deprecate ForceEnc: remove implicit debugger-attach EnC enablement, migrate to DOTNET_MODIFIABLE_ASSEMBLIES

[Copilot]

ForceEnc was an undocumented, broken mechanism where attaching a debugger would implicitly set MetadataUpdater.IsSupported = true — even though MetadataUpdater.ApplyUpdate throws NotSupportedException when a debugger is attached. The correct path is DOTNET_MODIFIABLE_ASSEMBLIES=debug.

Changes

• src/coreclr/vm/assemblynative.cpp — Simplified AssemblyNative_IsApplyUpdateSupported() to require explicit opt-in:

  // Before: implicit ForceEnc via debugger attachment
  result = (g_pConfig->ModifiableAssemblies() != MODIFIABLE_ASSM_NONE) &&
           (CORDebuggerAttached() || g_pConfig->ModifiableAssemblies() == MODIFIABLE_ASSM_DEBUG);
  
  // After: explicit opt-in only
  result = g_pConfig->ModifiableAssemblies() == MODIFIABLE_ASSM_DEBUG;

  The CORDebuggerAttached() path was doubly broken: ApplyUpdate throws when a debugger is attached, and since IsSupported is a cached static property, the check was also unreliable depending on when the MetadataUpdater type was first loaded relative to debugger attachment.

• src/mono/mono/component/hot_reload.c — Fixed error messages to use lowercase 'debug' (matching the actual accepted env var value) instead of 'Debug'.

[dotnet-policy-service]

Tagging subscribers to this area: @steveisok, @tommcdon, @dotnet/dotnet-diag
See info in area-owners.md if you want to be subscribed.

[Copilot]

Pull request overview

This PR tightens metadata update / Hot Reload enablement to require explicit opt-in via DOTNET_MODIFIABLE_ASSEMBLIES=debug, and aligns Mono’s user-facing error text with the canonical value.

Changes:

• CoreCLR: AssemblyNative_IsApplyUpdateSupported() now returns true only when g_pConfig->ModifiableAssemblies() == MODIFIABLE_ASSM_DEBUG (removing the implicit “debugger attached” enablement).
• Mono: error messages now refer to 'debug' instead of 'Debug' when explaining the required DOTNET_MODIFIABLE_ASSEMBLIES value.

Show a summary per file

File	Description
src/coreclr/vm/assemblynative.cpp	Removes debugger-attach-based enablement for IsApplyUpdateSupported, requiring explicit MODIFIABLE_ASSM_DEBUG.
src/mono/mono/component/hot_reload.c	Adjusts Hot Reload error messages to use lowercase 'debug' to match the documented/canonical env var value.

Copilot's findings

• Files reviewed: 2/2 changed files
• Comments generated: 1