Constify DiagnosticState::determineBehavior

Split out the state mutation into a new `updateFor`
function that we call for diagnostic emission, allowing
`DiagnosticTransaction::hasErrors` to query the behavior without
mutating any state.

Author: hamishknight

include/swift/AST/DiagnosticEngine.h

@@ -995,7 +995,10 @@ namespace swift {
 
     /// Figure out the Behavior for the given diagnostic, taking current
     /// state such as fatality into account.
-    DiagnosticBehavior determineBehavior(const Diagnostic &diag);
+    DiagnosticBehavior determineBehavior(const Diagnostic &diag) const;
+
+    /// Updates the diagnostic state for a diagnostic to emit.
+    void updateFor(DiagnosticBehavior behavior);
 
     bool hadAnyError() const { return anyErrorOccurred; }
     bool hasFatalErrorOccurred() const { return fatalErrorOccurred; }
@@ -1023,7 +1026,7 @@ namespace swift {
 
     /// Returns a Boolean value indicating whether warnings belonging to the
     /// diagnostic group identified by `id` should be escalated to errors.
-    bool getWarningsAsErrorsForDiagGroupID(DiagGroupID id) {
+    bool getWarningsAsErrorsForDiagGroupID(DiagGroupID id) const {
       return warningsAsErrors[(unsigned)id];
     }
 

lib/AST/DiagnosticEngine.cpp

@@ -1263,7 +1263,8 @@ llvm::cl::opt<bool> AssertOnError("swift-diagnostics-assert-on-error",
 llvm::cl::opt<bool> AssertOnWarning("swift-diagnostics-assert-on-warning",
                                     llvm::cl::init(false));
 
-DiagnosticBehavior DiagnosticState::determineBehavior(const Diagnostic &diag) {
+DiagnosticBehavior
+DiagnosticState::determineBehavior(const Diagnostic &diag) const {
   // We determine how to handle a diagnostic based on the following rules
   //   1) Map the diagnostic to its "intended" behavior, applying the behavior
   //      limit for this particular emission
@@ -1310,21 +1311,23 @@ DiagnosticBehavior DiagnosticState::determineBehavior(const Diagnostic &diag) {
     if (suppressRemarks)
       lvl = DiagnosticBehavior::Ignore;
   }
+  return lvl;
+}
 
-  //   5) Update current state for use during the next diagnostic
-  if (lvl == DiagnosticBehavior::Fatal) {
+void DiagnosticState::updateFor(DiagnosticBehavior behavior) {
+  // Update current state for use during the next diagnostic
+  if (behavior == DiagnosticBehavior::Fatal) {
     fatalErrorOccurred = true;
     anyErrorOccurred = true;
-  } else if (lvl == DiagnosticBehavior::Error) {
+  } else if (behavior == DiagnosticBehavior::Error) {
     anyErrorOccurred = true;
   }
 
   ASSERT((!AssertOnError || !anyErrorOccurred) && "We emitted an error?!");
-  ASSERT((!AssertOnWarning || (lvl != DiagnosticBehavior::Warning)) &&
+  ASSERT((!AssertOnWarning || (behavior != DiagnosticBehavior::Warning)) &&
          "We emitted a warning?!");
 
-  previousBehavior = lvl;
-  return lvl;
+  previousBehavior = behavior;
 }
 
 void DiagnosticEngine::flushActiveDiagnostic() {
@@ -1369,6 +1372,8 @@ std::optional<DiagnosticInfo>
 DiagnosticEngine::diagnosticInfoForDiagnostic(const Diagnostic &diagnostic,
                                               bool includeDiagnosticName) {
   auto behavior = state.determineBehavior(diagnostic);
+  state.updateFor(behavior);
+
   if (behavior == DiagnosticBehavior::Ignore)
     return std::nullopt;