fix(diagnostics): print each identical diagnostic once

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

1 files changed, 14 insertions, 3 deletions

diff --git a/include/cps/Diagnostics.h b/include/cps/Diagnostics.h
index 4bcb877..9c9ea37 100644
--- a/include/cps/Diagnostics.h
+++ b/include/cps/Diagnostics.h
@@ -1,6 +1,7 @@
 #pragma once
 #include <cstdarg>
 #include <cstdio>
+#include <set>
 #include <string>
 
 namespace cps {
@@ -30,14 +31,24 @@ inline std::string diagnosticPrefix(Severity S, const std::string &Locus = "cpsc
     return (Locus.empty() ? "" : Span("01", Locus + ":") + " ") + Span(Color, Label);
 }
 
+// Every diagnostic line printed so far, to suppress exact repeats.
+inline std::set<std::string> PrintedDiagnostics;
+
 // Prints "<path>:<line>: <label>: <message>\n" (just "<path>:" when the line is
 // unknown, "cpsc:" when no source is set).
 inline void vsourceDiagnostic(Severity S, int Line, const char *Format, va_list Args) {
     std::string Locus = DiagnosticSource.empty() ? "cpsc" : DiagnosticSource;
     if (!DiagnosticSource.empty() && Line > 0) Locus += ":" + std::to_string(Line);
-    std::fputs(diagnosticPrefix(S, Locus).c_str(), stderr);
-    std::vfprintf(stderr, Format, Args);
-    std::fputc('\n', stderr);
+    va_list Copy;
+    va_copy(Copy, Args);
+    int Length = std::vsnprintf(nullptr, 0, Format, Copy);
+    va_end(Copy);
+    std::string Message(Length > 0 ? Length : 0, '\0');
+    if (Length > 0) std::vsnprintf(Message.data(), Message.size() + 1, Format, Args);
+    std::string Text = diagnosticPrefix(S, Locus) + Message + '\n';
+    // The same node can be diagnosed more than once; print each line once.
+    if (!PrintedDiagnostics.insert(Text).second) return;
+    std::fputs(Text.c_str(), stderr);
 }
 
 __attribute__((format(printf, 2, 3)))