TODO

Open tasks collected from in-code comments. Resolve by implementing or opening a GitHub issue.

Medium: Type-checking warnings and errors

These can be implemented incrementally once the basic type system is stable.

Logical-operator operand types (ndc_analyser/src/analyser.rs ~line 58) and / or operands are not checked to be Bool. Should emit a warning or error when the operand type is known and incompatible.
Mismatched if branch types (ndc_analyser/src/analyser.rs ~line 178) When true-branch and false-branch types differ (and neither is Any), a warning could be emitted.
Missing semicolon warning in if (ndc_analyser/src/analyser.rs ~line 174) When the true-branch of an if produces a non-unit value but no else is present, a warning for the missing semicolon would be helpful.
never type for variable declarations (ndc_analyser/src/analyser.rs ~line 66) let x = … currently resolves to unit. Introducing a never / ! type might be more accurate, once the type lattice is fleshed out.

Conflicting binding on re-declaration (ndc_analyser/src/analyser.rs ~line 141) When a function name is declared a second time in the same scope, the analyser silently creates a new binding instead of either updating the old one or raising an error. The right policy needs to be decided and implemented.
debug_assert → unreachable! in find_function_candidates (ndc_analyser/src/scope.rs ~line 99) A variadic function match should be impossible at this call-site. The debug_assert!(false, …) should be replaced with unreachable! once we are confident the invariant holds.

Bitwise NOT on non-integer numbers (ndc_core/src/num.rs ~line 181) Currently !float and !rational return NaN (matching Noulith behaviour). Decide whether this is intentional for this language or whether it should be an error.
bigint → int rounding in floor/ceil/round (ndc_core/src/num.rs ~line 584) After rounding a Rational, the result is converted to BigInt rather than trying to fit it back into a machine i64. Should attempt the smaller representation first.
Division performance (ndc_core/src/num.rs ~line 323) Div always promotes both operands to Rational. In the common Int / Int case this is unnecessary. A fast path for integer operands would avoid the allocation.

Unicode escape sequences (ndc_lexer/src/string.rs ~line 72) String literals do not support \uXXXX escape sequences. Add support.
_ separator after decimal point (ndc_lexer/src/number.rs ~line 130) 1_000.0 is valid, but 1.0_0 is probably not intended. Consider rejecting _ after ..
Number suffix error interception (ndc_lexer/src/number.rs ~line 48) The suffix-error checks inside lex_number may be redundant since no numeric suffixes are supported. Consider moving the check to after the lexer returns so it applies uniformly.
validator_for_radix performance (ndc_lexer/src/number.rs ~line 231) The string-slice approach for validating digits by radix is O(radix) per character. A lookup table or char::to_digit would be faster.
consume() internal error handling (ndc_lexer/src/lib.rs ~line 202) consume() panics with expect on underflow. Document the invariant or add a proper internal error type.

Better error for boolean-returning if without semicolon (ndc_parser/src/parser.rs ~line 738) The pattern if x == y { true } else { false } triggers a generic parse error. A targeted diagnostic would be more helpful.
"Expected expression" error quality (ndc_parser/src/parser.rs ~line 1001) The fallback "Expected an expression but got '…'" message may not always accurately describe the failure. Audit and improve.

Error rendering in block-scope test (tests/programs/004_basic/005_block_scope_destroys_local_variables.ndc line 6) The error is reported correctly but rendered weirdly in the test output. Investigate why and fix the display.