feat(selfhost): checker step-2 milestone 2e — RS0028 invalid self parameter

Adds RS0028 (INVALID_SELF_PARAMETER): a parameter named `self` that is not the
first parameter of a qualified (dotted-name) method — mirrors check_params'
`self && (!is_qualified_method || index != 0)`.

CHECKER_TARGET_CODES now 11 codes; checker_parity_corpus byte-exact over 619
files, 0 mismatches, 0 run-failures.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: olwang

crates/rsscript/src/selfhost_parity.rs

@@ -503,7 +503,7 @@ fn parser_parity_corpus() {
 // ---------------------------------------------------------------------------
 
 /// Diagnostic codes the rss checker is expected to reproduce.
-const CHECKER_TARGET_CODES: &[&str] = &["RS0002","RS0003","RS0004","RS0005","RS0006","RS0010","RS0011","RS0012","RS0016","RS0017"];
+const CHECKER_TARGET_CODES: &[&str] = &["RS0002","RS0003","RS0004","RS0005","RS0006","RS0010","RS0011","RS0012","RS0016","RS0017","RS0028"];
 
 fn is_target_code(code: &str) -> bool {
     CHECKER_TARGET_CODES.contains(&code)

selfhost/check.rss

@@ -405,6 +405,56 @@ fn fn_has_share_param(toks: read List<Tok>, start: read Int) -> Bool {
     return hit
 }
 
+// RS0028: a parameter named `self` that is not the first parameter of a qualified
+// method (check_params: `self` && (!is_qualified_method || index != 0)). A method
+// is qualified iff its name is dotted (`Type.method`).
+fn fn_has_invalid_self(toks: read List<Tok>, start: read Int) -> Bool {
+    let ns = fn_name_start(toks: read toks, i: read start)
+    let qualified = at_symbol(toks: read toks, i: read (ns + 1), code: read SYM_DOT)
+    let sigEnd = function_signature_end(toks: read toks, start: read (ns - 1))
+    let open = find_param_open(toks: read toks, from: read ns, end: read sigEnd)
+    if open < 0 { return false }
+    let close = find_matching(toks: read toks, open: read open, openSym: read SYM_LPAREN, closeSym: read SYM_RPAREN)
+    if close < 0 { return false }
+    let mut k = open + 1
+    let mut depth = 0
+    let mut angle = 0
+    let mut segStart = open + 1
+    let mut idx = 0
+    let mut hit = false
+    while k < close {
+        let opener = at_symbol(toks: read toks, i: read k, code: read SYM_LPAREN) || at_symbol(toks: read toks, i: read k, code: read SYM_LBRACKET) || at_symbol(toks: read toks, i: read k, code: read SYM_LBRACE)
+        let closer = at_symbol(toks: read toks, i: read k, code: read SYM_RPAREN) || at_symbol(toks: read toks, i: read k, code: read SYM_RBRACKET) || at_symbol(toks: read toks, i: read k, code: read SYM_RBRACE)
+        if opener {
+            depth = depth + 1
+        } else if closer {
+            if depth > 0 { depth = depth - 1 }
+        } else if at_symbol(toks: read toks, i: read k, code: read SYM_LANGLE) {
+            angle = angle + 1
+        } else if at_symbol(toks: read toks, i: read k, code: read SYM_RANGLE) {
+            if angle > 0 { angle = angle - 1 }
+        } else if depth == 0 && angle == 0 && at_symbol(toks: read toks, i: read k, code: read SYM_COMMA) {
+            if seg_is_bad_self(toks: read toks, segStart: read segStart, segEnd: read k, idx: read idx, qualified: read qualified) { hit = true }
+            segStart = k + 1
+            idx = idx + 1
+        }
+        k = k + 1
+    }
+    if seg_is_bad_self(toks: read toks, segStart: read segStart, segEnd: read close, idx: read idx, qualified: read qualified) { hit = true }
+    return hit
+}
+
+// A param segment is an invalid `self` iff it is named `self` and either the
+// enclosing function is not a qualified method or this is not the first parameter.
+fn seg_is_bad_self(toks: read List<Tok>, segStart: read Int, segEnd: read Int, idx: read Int, qualified: read Bool) -> Bool {
+    if segStart >= segEnd { return false }
+    if is_ident_tok(toks: read toks, i: read segStart) == false { return false }
+    if tk_text(toks: read toks, i: read segStart) == "self" {
+        return qualified == false || idx != 0
+    }
+    return false
+}
+
 // `name : share` — ident, `:`, then `share` as the type's first token.
 fn seg_is_share(toks: read List<Tok>, segStart: read Int, segEnd: read Int) -> Bool {
     if (segStart + 2) >= segEnd { return false }
@@ -434,6 +484,7 @@ fn main() -> Unit {
     let mut shareParam = false
     let mut unknownEffect = false
     let mut removedRuntime = false
+    let mut invalidSelf = false
     let mut i = 0
     let mut running = true
     while running {
@@ -581,6 +632,7 @@ fn main() -> Unit {
                     if fn_has_share_param(toks: read toks, start: read i) { shareParam = true }
                     if effects_name_probe(toks: read toks, start: read i, mode: read 0) { unknownEffect = true }
                     if effects_name_probe(toks: read toks, start: read i, mode: read 1) { removedRuntime = true }
+                    if fn_has_invalid_self(toks: read toks, start: read i) { invalidSelf = true }
                     i = r
                 }
             } else {
@@ -613,6 +665,9 @@ fn main() -> Unit {
     if removedRuntime {
         Log.write(message: read "RS0012")
     }
+    if invalidSelf {
+        Log.write(message: read "RS0028")
+    }
     if hasUnknownFeat {
         Log.write(message: read "RS0016")
     }
@@ -622,7 +677,7 @@ fn main() -> Unit {
     if headerCount >= 2 {
         Log.write(message: read "RS0006")
     }
-    let anyDiag = found || hasUnknownFeat || hasDupFeat || headerCount >= 2 || missingReturn || untypedParam || hasProfile || shareParam || unknownEffect || removedRuntime
+    let anyDiag = found || hasUnknownFeat || hasDupFeat || headerCount >= 2 || missingReturn || untypedParam || hasProfile || shareParam || unknownEffect || removedRuntime || invalidSelf
     if anyDiag == false {
         Log.write(message: read "CLEAN")
     }