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Lint warnings

mx includes a built-in static analysis linter that catches common Modula-2 pitfalls at compile time. Warnings appear in VS Code as yellow squiggles (via the LSP) and on stderr during mx build. Warnings never block compilation.

Warning codes

Code Category Description
W01 Unsigned arithmetic Comparison of unsigned type against 0 (CARDINAL >= 0 is always true, CARDINAL < 0 is always false)
W02 Unsigned arithmetic WHILE c >= 0 DO DEC(c) END with unsigned variable — infinite loop since unsigned can never be negative
W03 Short-circuit Pointer dereference or array index on RHS of AND/OR where short-circuit evaluation is required for safety (e.g. (p # NIL) AND (p^.x = 1))
W04 Aliasing Same variable passed to multiple VAR parameters in one call — modifications through one alias may interfere with the other
W05 Subrange INC or DEC on a bounded subrange type ([0..15]) may overflow/underflow the declared bounds
W06 Portability FROM SYSTEM IMPORT — use of SYSTEM module reduces portability across compilers and platforms
W07 Exhaustiveness CASE on enumeration or subrange without ELSE does not cover all possible values
W08 Signedness Mixed signed (INTEGER) and unsigned (CARDINAL) operands in arithmetic — implicit conversion may produce unexpected results
W09 Unsigned arithmetic Subtraction in FOR upper bound with unsigned loop variable (FOR i := 0 TO n - 1) — underflows to MAX(CARDINAL) when n = 0
W10 Initialization Variable may be used before being assigned a value (path-sensitive: considers all branches)
W11 Nil safety Pointer may be NIL when dereferenced — uninitialized, assigned NIL, or set to NIL after DISPOSE

Analysis tiers

Warnings are checked at two levels:

Tier 1 (AST-level) — W01–W09. Pattern matching on the typed syntax tree. Runs instantly during editing in VS Code.

Tier 2 (CFG dataflow) — W10–W11. Forward dataflow analysis over the control flow graph. Builds HIR and CFG, then runs a worklist solver to fixpoint. Also runs in VS Code via the LSP.

Suppressing warnings

Line-level suppression

Add (*!Wxx*) as a comment on the line to suppress that warning:

FROM SYSTEM IMPORT ADDRESS, ADR; (*!W06*)

File-level suppression

Place (*!Wxx*) before the MODULE keyword to suppress file-wide:

(*!W06*)
(*!W08*)
MODULE LowLevel;
FROM SYSTEM IMPORT ADDRESS, ADR, TSIZE;
(* All W06 and W08 warnings suppressed in this file *)

Suppression scope

  • Each (*!Wxx*) pragma suppresses exactly one warning code
  • Multiple codes can be suppressed with multiple pragmas
  • Suppression does not affect other warning codes
  • File-level suppression applies to the entire file including all procedures

Examples

W09: CARDINAL underflow in FOR loop

(* WARNING: if count = 0, "count - 1" underflows to MAX(CARDINAL) *)
FOR i := 0 TO count - 1 DO
  Process(items[i])
END;

(* FIX: guard with IF *)
IF count > 0 THEN
  FOR i := 0 TO count - 1 DO
    Process(items[i])
  END
END;

W03: Short-circuit safety

(* WARNING: if p = NIL, p^.value crashes — AND does not guarantee
   short-circuit evaluation in standard Modula-2 *)
IF (p # NIL) AND (p^.value > 0) THEN ...

(* FIX: use & (AND THEN) which guarantees short-circuit *)
IF (p # NIL) & (p^.value > 0) THEN ...

W10: Uninitialized variable

(* WARNING: x used on ELSE path without assignment *)
PROCEDURE Foo(flag: BOOLEAN): INTEGER;
VAR x: INTEGER;
BEGIN
  IF flag THEN x := 10 END;
  RETURN x    (* W10: x may be uninitialized when flag is FALSE *)
END Foo;

W11: Pointer NIL dereference

(* WARNING: p is NIL after DISPOSE *)
NEW(p);
p^.val := 42;
DISPOSE(p);
RETURN p^.val   (* W11: p may be NIL *)

Architecture

The lint system is implemented across three layers:

  • src/sema.rs — Tier 1 checks: AST pattern matching during semantic analysis
  • src/cfg/dataflow.rs — Generic forward dataflow framework (worklist solver over CFG)
  • src/cfg/lint.rs — Tier 2 checks: DefinitelyAssigned (W10) and NilSafety (W11) analyses
  • src/analyze.rs — Integration: runs both tiers, applies suppression, feeds LSP diagnostics
  • src/errors.rs — ErrorKind::Warning with optional code, never blocks compilation