Build the hosted catalog from the platform header's hosted section

[rtfeldman]

A platform header's hosted section is the complete list of functions the host supplies, and it is what gives each one its linker symbol and its host dispatch slot. Three stages nevertheless built their own hosted catalog by scanning every hosted declaration in scope and then asserting that the scan agreed with the checked hosted binding table, so any platform whose section did not line up with its declarations aborted the compiler rather than reporting the section. A declaration the section omits aborted post-check lowering with platform hosted binding count 1 disagrees with hosted catalog size 2; a section naming one declaration twice aborted the same way with the counts reversed, and aborted the roc <app> run path and --specialize=no lowering through their own copies of that assertion. All three now build from the bindings, so the section decides how many dispatch slots exist and which declaration occupies each one — naming one declaration twice is two slots, which is what the header asked for and what checking already reports as a duplicate. Lowering without specialization consumes those bindings too, rather than re-deriving the section by matching declaration names against the header's text.

A declaration the section omits then has no catalog entry at all, which is right, since it has no symbol to call and no slot to occupy. A program that called one went on to abort with hosted procedure template was not output in the hosted catalog, so the checked-modules-to-LIR entrance now refuses a program that still carries such a declaration; checking already reports it, so the compile has failed by the time lowering starts and refusing there costs no diagnostic. Compile-time finalization lowers a module whose own checking has not finished, and such a module has no bindings yet, so that entrance reads nothing into their absence there.

That refusal is only sound if checking reports every declaration it covers, and one shape went unreported: a section entry names its target through an import, so no entry can reach a declaration written in the platform root itself, yet checking walked only the modules the root owns. A hosted declaration in main.roc therefore passed roc check cleanly and was stopped later with nothing to say. The root is now checked against the section alongside the modules it owns. Glue reads the same section to name each hosted function's symbol and aborted on the same input; it renders the platform's diagnostics already and now stops on them, because a platform checking rejected has no declared surface for glue to emit.

[greptile-apps]

Greptile Summary

The PR makes the checked platform header’s hosted bindings authoritative for hosted catalogs and prevents lowering or glue generation from consuming unbound hosted declarations.

• Builds CLI, boxy, and monotype hosted catalogs from checked bindings, preserving section order, symbols, and duplicate slots.
• Extends platform checking to report hosted declarations in the platform root.
• Adds a checked-to-LIR boundary error for hosted declarations omitted from the section and propagates it through compiler callers.
• Makes glue stop after rejected platform checking and adds regression coverage for omitted, duplicated, and root-level hosted declarations.

Confidence Score: 5/5

The PR appears safe to merge, with no concrete blocking or independently actionable non-blocking defect established in the changed behavior.

The hosted catalogs consistently derive their entries, symbols, and dispatch order from checked bindings, while invalid omitted or duplicate declarations are reported and stopped before unsupported lowering or glue output proceeds.

Important Files Changed

Filename	Overview
src/check/Check.zig	Refactors hosted-declaration collection and includes the platform root when checking declarations against the hosted section.
src/lir/checked_pipeline.zig	Adds the checked-to-LIR guard that rejects hosted procedures absent from the visible platform binding table.
src/postcheck/boxy/lower.zig	Rebuilds the boxy hosted catalog from checked bindings rather than reconciling a declaration scan with header text.
src/postcheck/monotype/lower.zig	Rebuilds the monotype hosted catalog in binding order, preserving each binding’s symbol and dispatch slot.
src/cli/main.zig	Applies checked bindings by replacing the scanned CLI hosted catalog and propagates the new lowering error.
src/glue/glue.zig	Stops glue generation after platform checking errors and handles unbound-hosted lowering failures without panicking.
src/eval/compile_time_finalization.zig	Preserves compile-time finalization behavior by lowering with the checking-finalization state and treating unexpected binding rejection as an invariant failure.
src/cli/test/parallel_cli_runner.zig	Adds regression coverage for omitted declarations, duplicate entries, platform-root declarations, run/no-specialize paths, and glue diagnostics.

Flowchart

%%{init: {'theme': 'neutral'}}%%
flowchart TD
    A[Platform hosted section] --> B[Checker resolves HostedBindingTable]
    B --> C{Checking diagnostics?}
    C -->|Errors| D[Stop build or glue]
    C -->|Valid| E[Checked-to-LIR boundary]
    E --> F{Every hosted procedure bound?}
    F -->|No| G[HostedFunctionNotBound]
    F -->|Yes| H[Build catalog in binding order]
    H --> I[Assign linker symbols and dispatch slots]
    I --> J[Boxy, monotype, or CLI lowering]

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