Reject cast-style implicit conversions

Author: olwang

src/checks/forbidden.rs

@@ -4,6 +4,56 @@ use crate::lexer::TokenKind;
 
 pub(crate) fn check(analyzer: &mut Analyzer<'_>) {
     check_operator_overload_attempts(analyzer);
+    check_implicit_conversion_attempts(analyzer);
+}
+
+fn check_implicit_conversion_attempts(analyzer: &mut Analyzer<'_>) {
+    for index in 0..analyzer.tokens.len() {
+        if !analyzer.tokens[index].is_ident_text("as") || as_belongs_to_with(analyzer, index) {
+            continue;
+        }
+        analyzer.diagnostics.push(
+            Diagnostic::error(
+                code::IMPLICIT_CONVERSION_ATTEMPT,
+                "cast-style conversions are not part of RSScript.",
+                analyzer.tokens[index].span.clone(),
+                "implicit conversion attempt",
+            )
+            .with_cause("RSScript requires conversions to be explicit named APIs so review tools can see them.")
+            .with_fix(
+                "use_named_conversion",
+                "Use a named conversion such as `Target.from(value: read source)`.",
+                "manual",
+            ),
+        );
+    }
+}
+
+fn as_belongs_to_with(analyzer: &Analyzer<'_>, as_index: usize) -> bool {
+    for token in analyzer.tokens[..as_index].iter().rev() {
+        if token.is_ident_text("with") {
+            return true;
+        }
+        if token.symbol("{")
+            || token.symbol("}")
+            || token.is_ident_text("let")
+            || token.is_ident_text("local")
+            || token.is_ident_text("return")
+            || token.is_ident_text("fn")
+            || token.is_ident_text("class")
+            || token.is_ident_text("struct")
+            || token.is_ident_text("resource")
+            || token.is_ident_text("if")
+            || token.is_ident_text("else")
+            || token.is_ident_text("loop")
+            || token.is_ident_text("while")
+            || token.is_ident_text("break")
+            || token.is_ident_text("continue")
+        {
+            return false;
+        }
+    }
+    false
 }
 
 fn check_operator_overload_attempts(analyzer: &mut Analyzer<'_>) {

src/diagnostic.rs

@@ -31,6 +31,7 @@ pub mod code {
     pub const LOCAL_CAPTURED_BY_MANAGED_CLOSURE: &str = "RS0801";
     pub const TAKE_HANDLE_FIELD: &str = "RS0901";
     pub const OPERATOR_OVERLOAD_ATTEMPT: &str = "RS1001";
+    pub const IMPLICIT_CONVERSION_ATTEMPT: &str = "RS1002";
 
     pub const REVIEW_MODE_CHANGED: &str = "RSR001";
     pub const REVIEW_FUNCTION_REMOVED: &str = "RSR002";
@@ -347,6 +348,11 @@ static DIAGNOSTIC_EXPLANATIONS: &[DiagnosticExplanation] = &[
         title: "operator overload attempt",
         explanation: "The MVP language surface rejects likely user-defined operator overloads to keep review semantics explicit.",
     },
+    DiagnosticExplanation {
+        code: code::IMPLICIT_CONVERSION_ATTEMPT,
+        title: "implicit conversion attempt",
+        explanation: "RSScript rejects cast-style conversion syntax. Conversions must be visible named APIs such as `Type.from(value: read x)`.",
+    },
 ];
 
 pub fn format_diagnostics_json(diagnostics: &[Diagnostic]) -> String {

tests/fixtures/fail/implicit-conversion-as.rss

@@ -0,0 +1,7 @@
+// expect: RS1002
+mode: managed
+
+fn widen(value: Int) -> Int64 {
+    let widened = value as Int64
+    return widened
+}

tests/fixtures/pass/multiline-with-as-resource.rss

@@ -0,0 +1,13 @@
+mode: managed
+
+resource File {
+    fd: Int
+}
+
+fn read_file(path: read Path) -> Result<fresh Bytes, IOError> {
+    with File.open_read(
+        path: read path
+    ) as file {
+        return File.read_all(file: mut file)
+    }
+}