Reject duplicate call arguments

Author: olwang

src/checks/calls.rs

@@ -120,10 +120,27 @@ fn check_call_args(
         .map(|param| param.name.clone())
         .collect();
 
+    let mut seen_names = HashSet::new();
     for arg in args {
         let Some(name) = &arg.name else {
             continue;
         };
+        if !seen_names.insert(name.as_str()) {
+            analyzer.diagnostics.push(
+                Diagnostic::error(
+                    code::DUPLICATE_ARGUMENT,
+                    format!("call to `{call_name}` repeats argument `{name}`."),
+                    arg.span.clone(),
+                    "duplicate argument",
+                )
+                .with_cause("Each named parameter can be provided at most once.")
+                .with_fix(
+                    "remove_duplicate_argument",
+                    format!("Remove the extra `{name}: ...` argument."),
+                    "manual",
+                ),
+            );
+        }
         if !param_names.contains(name) {
             analyzer.diagnostics.push(
                 Diagnostic::error(

src/diagnostic.rs

@@ -10,6 +10,7 @@ pub mod code {
     pub const MISSING_DATA_EFFECT: &str = "RS0202";
     pub const UNKNOWN_ARGUMENT: &str = "RS0203";
     pub const MISSING_ARGUMENT: &str = "RS0204";
+    pub const DUPLICATE_ARGUMENT: &str = "RS0205";
     pub const MANAGED_TO_LOCAL: &str = "RS0301";
     pub const USE_AFTER_MANAGE: &str = "RS0401";
     pub const LOCAL_VALUE_RETAINED: &str = "RS0501";
@@ -231,6 +232,11 @@ static DIAGNOSTIC_EXPLANATIONS: &[DiagnosticExplanation] = &[
         title: "missing required argument",
         explanation: "Every parameter in the resolved callee signature must be supplied by name at the call site.",
     },
+    DiagnosticExplanation {
+        code: code::DUPLICATE_ARGUMENT,
+        title: "duplicate named argument",
+        explanation: "A call may bind each named parameter only once. Duplicate names make effect and ownership review ambiguous.",
+    },
     DiagnosticExplanation {
         code: code::MANAGED_TO_LOCAL,
         title: "managed-to-local conversion",

tests/fixtures/fail/builtin-duplicate-named-argument.rss

@@ -0,0 +1,11 @@
+// expect: RS0205
+mode: uses-local
+
+struct Image {
+    pixels: Buffer
+}
+
+fn bad_resize_duplicate_argument(path: read Path) -> Unit {
+    local image = Image.load(path: read path)
+    Image.resize(image: mut image, image: mut image, width: 800, height: 600)
+}