"This for-loop is missing ... in its where clause" (E0038, E0039)

[mndrix]

As of moon 0.1.20260110 (0e584ac 2026-01-10), I see the warnings missing_reason and missing_invariant for many for loops. The --explain output says "No longer emitted". The relevant for loops have no where clause at all, so I assume these warnings are mistaken; or is it the intention that every for loop have a where clause with invariants? If the latter, maybe the error message should just say "This for-loop is missing a where clause"

Environment

Operating System: Linux

moon 0.1.20260110 (0e584ac 2026-01-10) ~/conf/vendor/moonbit/build/bin/moon
moonc v0.7.1+adb125543 (2026-01-10) ~/conf/vendor/moonbit/build/bin/moonc
moonrun 0.1.20260110 (0e584ac 2026-01-10) ~/conf/vendor/moonbit/build/bin/moonrun
moon-pilot unknown-df92511 (2026-01-10) ~/conf/vendor/moonbit/build/bin/moon-pilot

Feature flags enabled: rupes_recta
-> You're currently using the experimental build graph generator "Rupes Recta". If you encounter a problem, please verify whether it also reproduces with the legacy build (by setting NEW_MOON=0).

[mndrix]

I forgot the mention that the warnings appear with NEW_MOON=0 as well

[Yu-zh]

@mndrix Did you enable all warnings in your config? It's a new feature and those warnings should be disabled by default.

[mndrix]

Yes, I originally had "warn-list":"+A" in moon.mod.json. It simply surprised me that a for loop without invariants would be considered a warning at all, since that seems like the common case.

Perhaps I've misunderstood the intention of MoonBit's warning system. Most compilers use warnings to mean "this is bad practice and you should probably avoid it". Is it more correct to understand MoonBit's warnings as, "here is the result of a particular static analysis that you requested"? Or is it maybe that MoonBit considers a for loop without invariants to be bad practice? (which would be fine)

[Yu-zh]

Most compilers use warnings to mean "this is bad practice and you should probably avoid it".

If that's the case, the warnings is enabled by default

"here is the result of a particular static analysis that you requested"

This is for the warnings that are turned off by default.

Let me know if you think any of the warnings has been misplaced.

[mndrix]

Is there a list of which warnings are enabled by default? I know that moonc build-package -warn-help lists all the warning categories, and I searched the MoonBit Language Manual for a list of defaults, but haven't found one yet.

Let me know if you think any of the warnings has been misplaced.

Other than missing_reasoning and missing_invariant, I think all existing warnings could be enabled by default. Building with +A previously and now with +A-missing_invariant-missing_reasoning has been excellent for me.

[Yu-zh]

You can find the following information with moonc build-package --help, and we will provide a moon subcommand for it soon:

moonc build-package [options] <input files>
  -i dependent .mi files
  -g save debugging information
  -O0  turn off optimization
  -no-dedup turn off dedup-wasm
  -source-map emit sourcemap
  -source-map-url set url of source map
  -dsource debug source
  -o output file (suffix with .core by convention)
  -target {wasm-gc|wasm|js|native|llvm}set compilation target. available targets: wasm, wasm-gc, js, native
  -pkg package name
  -all-pkgs package description file
  -pkg-sources Tell compiler where the package sources are. format: <package name>:<package source directory>
  -workspace-path path to the project root directory in file system
  -std-path path containing standard library .mi files
  -enable-coverage  Enable code coverage tracking
  -coverage-package-override Override package name for coverage tracking. use "@self" for current package.
  -is-main main package
  -blackbox-test check blackbox tests
  -whitebox-test check whitebox tests
  -error-format {human|json} set format of diagnostics. available formats: human, json
  -patch-file patch file
  -no-mi don't generate .mi file
  -test-mode enable test mode
  -check-mi check whether the package implements .mi file
  -impl-virtual implement virtual package (the virtual package must be specified by -check-mi option)
  -cfg set a configuration flag. The value format: <flag>=<value>
  -llvm-opt set llvm optimization level. available levels: 0, 1, 2, 3, s, z. (LLVM backend only)
  -llvm-target set llvm target triple. unset means current target. (LLVM backend only)
  -S emit text format of the result instead of binary (LLVM backend only)
  -emit-llvm-ir emit LLVM IR or bitcode instead of machine code (LLVM backend only)
  -include-doctests include doctests in the compilation, for black box tests
  -doctest-only additionally include only doctests of the provided files for compiling, requires `-include-doctests`
  -warn-help  show description of warning numbers
  -w  <list>  Enable or disable warnings according to <list>:
        +<spec>   enable warnings in <spec>
        -<spec>   disable warnings in <spec>
        @<spec>   enable warnings in <spec> and treat them as errors
        <spec> can be:
        <num>             a single warning number
        <num1>..<num2>    a range of consecutive warning numbers
        default setting is +a-31-32@11@15@44-25-38-39-65-66-69-70-alert_unsafe
  -alert  <list>  Enable or disable warnings according to <list>:
        +<id>   enable alert
        -<id>   disable alert
        @<id>   enable alert and treat it as error

  -help  Display this list of options
  --help  Display this list of options

So here the default setting is: +a-31-32@11@15@44-25-38-39-65-66-69-70-alert_unsafe