Ban @abi on multi-var PBDs

Per a comment in the review thread that the comma in a multi-variable pattern binding makes it look like the `@abi` attribute has several arguments.

Author: beccadax

include/swift/AST/DiagnosticsSema.def

@@ -8434,14 +8434,10 @@ ERROR(attr_abi_mismatched_async,none,
       "cannot give %0 the ABI of %select{a non-async|an async}1 %kindonly0",
       (Decl *, /*abiIsAsync=*/bool))
 
-ERROR(attr_abi_mismatched_pbd_size,none,
-      "cannot give pattern binding the ABI of a binding with "
-      "%select{more|fewer}0 patterns",
-      (/*abiHasExtra=*/bool))
-
-ERROR(attr_abi_mismatched_var,none,
-      "no match for %select{%kind0 in the ABI|ABI %kind0}1",
-      (Decl *, /*isABI=*/bool))
+ERROR(attr_abi_multiple_vars,none,
+      "'abi' attribute can only be applied to a single %0; declare each "
+      "%0 separately",
+      (DescriptiveDeclKind))
 
 ERROR(attr_abi_incompatible_with_silgen_name,none,
       "cannot use '@_silgen_name' and '@abi' on the same %0 because they serve "

lib/Sema/TypeCheckAttrABI.cpp

@@ -1103,60 +1103,29 @@ class ABIDeclChecker : public ASTComparisonVisitor<ABIDeclChecker> {
 };
 
 void checkABIAttrPBD(PatternBindingDecl *APBD, VarDecl *VD) {
-  auto &diags = VD->getASTContext().Diags;
   auto PBD = VD->getParentPatternBinding();
 
-  // To make sure we only diagnose this stuff once, check that VD is the first
-  // anchoring variable in the PBD.
-  bool isFirstAnchor = false;
+  Decl *anchorVD = nullptr;
   for (auto i : range(PBD->getNumPatternEntries())) {
-    auto anchorVD = PBD->getAnchoringVarDecl(i);
-    if (anchorVD) {
-      isFirstAnchor = (anchorVD == VD);
+    anchorVD = PBD->getAnchoringVarDecl(i);
+    if (anchorVD)
       break;
-    }
   }
 
-  if (!isFirstAnchor)
+  // To make sure we only diagnose this stuff once, check that VD is the
+  // first anchoring variable in the PBD.
+  if (anchorVD != VD)
     return;
 
-  // Check that the PBDs have the same number of patterns.
-  if (PBD->getNumPatternEntries() < APBD->getNumPatternEntries()) {
-    diags.diagnose(APBD->getPattern(PBD->getNumPatternEntries())->getLoc(),
-                   diag::attr_abi_mismatched_pbd_size, /*abiHasExtra=*/false);
-    return;
-  }
-  if (PBD->getNumPatternEntries() > APBD->getNumPatternEntries()) {
-    diags.diagnose(PBD->getPattern(APBD->getNumPatternEntries())->getLoc(),
-                   diag::attr_abi_mismatched_pbd_size, /*abiHasExtra=*/true);
+  // In the final approved feature, we only permit single-variable patterns.
+  // (However, the rest of the compiler tolerates them.)
+  if (!PBD->getSingleVar() || !APBD->getSingleVar()) {
+    PBD->diagnose(diag::attr_abi_multiple_vars,
+                  anchorVD ? anchorVD->getDescriptiveKind()
+                           : PBD->getDescriptiveKind());
     return;
   }
 
-  // Check that each pattern has the same number of variables.
-  bool didDiagnose = false;
-  for (auto i : range(PBD->getNumPatternEntries())) {
-    SmallVector<VarDecl *, 8> VDs;
-    SmallVector<VarDecl *, 8> AVDs;
-
-    PBD->getPattern(i)->collectVariables(VDs);
-    APBD->getPattern(i)->collectVariables(AVDs);
-
-    if (VDs.size() < AVDs.size()) {
-      for (auto AVD : drop_begin(AVDs, VDs.size())) {
-        AVD->diagnose(diag::attr_abi_mismatched_var, AVD, /*isABI=*/true);
-        didDiagnose = true;
-      }
-    }
-    else if (VDs.size() > AVDs.size()) {
-      for (auto VD : drop_begin(VDs, AVDs.size())) {
-        VD->diagnose(diag::attr_abi_mismatched_var, VD, /*isABI=*/false);
-        didDiagnose = true;
-      }
-    }
-  }
-  if (didDiagnose)
-    return;
-
   // Check the ABI PBD--this is what checks the underlying vars.
   TypeChecker::typeCheckDecl(APBD);
 }

test/attr/attr_abi.swift

@@ -279,17 +279,20 @@ var async11Var: Int { get async { fatalError() } }
 // PBD shape checking
 //
 
-@abi(var x1, y1: Int) // expected-error {{cannot give pattern binding the ABI of a binding with more patterns}}
-var x1: Int = 0
+@abi(var x1, y1: Int)
+var x1: Int = 0 // expected-error {{'abi' attribute can only be applied to a single var; declare each var separately}}
 
 @abi(var x2: Int)
-var x2 = 0, y2: Int = 0 // expected-error {{cannot give pattern binding the ABI of a binding with fewer patterns}}
+var x2 = 0, y2: Int = 0 // expected-error {{'abi' attribute can only be applied to a single var; declare each var separately}}
 
-@abi(var (x3, y3): (Int, Int), (a3, b3): (Int, Int)) // expected-error {{no match for ABI var 'b3'}}
-var (x3, y3): (Int, Int) = (0, 0), a3: Int = 0
+@abi(var (x3, y3): (Int, Int), (a3, b3): (Int, Int))
+var (x3, y3): (Int, Int) = (0, 0), a3: Int = 0 // expected-error {{'abi' attribute can only be applied to a single var; declare each var separately}}
 
 @abi(var (x4, y4): (Int, Int), a4: Int)
-var (x4, y4): (Int, Int) = (0, 0), (a4, b4): (Int, Int) = (0, 0) // expected-error {{no match for var 'b4' in the ABI}}
+var (x4, y4): (Int, Int) = (0, 0), (a4, b4): (Int, Int) = (0, 0) // expected-error {{'abi' attribute can only be applied to a single var; declare each var separately}}
+
+@abi(var x5: Int)
+var x5: Int = 0
 
 //
 // Redeclaration diagnostics