Improve "missing @differentiable attribute" diagnostic message.

Change phrasing to remove "blame" from the non-matching witness.
Remove redundant DescriptiveDeclKind arguments.

Author: dan-zheng

include/swift/AST/DiagnosticsSema.def

@@ -3113,11 +3113,9 @@ ERROR(overriding_decl_missing_differentiable_attr,none,
 NOTE(protocol_witness_missing_differentiable_attr,none,
      "candidate is missing attribute '%0'", (StringRef))
 NOTE(protocol_witness_missing_differentiable_attr_other_file,none,
-     "%1 %2 must have explicit '%0' attribute to satisfy requirement %3 %4 (in "
-     "protocol %6) because it is declared in a different file than the "
-     "conformance of %5 to %6",
-     (StringRef, DescriptiveDeclKind, DeclName, DescriptiveDeclKind, DeclName,
-      Type, Type))
+     "candidate is missing explicit '%0' attribute to satisfy requirement %1 "
+     "(in protocol %3) because it is declared in a different file than the "
+     "conformance of %2 to %3", (StringRef, DeclName, Type, Type))
 
 // @derivative
 ERROR(derivative_attr_expected_result_tuple,none,

lib/Sema/TypeCheckProtocol.cpp

@@ -2461,11 +2461,8 @@ diagnoseMatch(ModuleDecl *module, NormalProtocolConformance *conformance,
       diags
           .diagnose(
               witness,
-              diag::
-                  protocol_witness_missing_differentiable_attr_other_file,
-              reqDiffAttrString, witness->getDescriptiveKind(),
-              witness->getName(), req->getDescriptiveKind(),
-              req->getName(), conformance->getType(),
+              diag::protocol_witness_missing_differentiable_attr_other_file,
+              reqDiffAttrString, req->getName(), conformance->getType(),
               conformance->getProtocol()->getDeclaredInterfaceType())
           .fixItInsert(match.Witness->getStartLoc(), reqDiffAttrString + ' ');
     }

test/AutoDiff/Sema/ImplicitDifferentiableAttributeCrossFile/Inputs/other_file.swift

@@ -32,20 +32,20 @@ protocol Protocol2: Differentiable {
 
 struct ConformingStruct: Differentiable {
   // Error for missing `@differentiable` attribute.
-  // expected-note @+1 {{instance method 'internalMethod1' must have explicit '@differentiable' attribute to satisfy requirement instance method 'internalMethod1' (in protocol 'Protocol1') because it is declared in a different file than the conformance of 'ConformingStruct' to 'Protocol1'}} {{3-3=@differentiable }}
+  // expected-note @+1 {{candidate is missing explicit '@differentiable' attribute to satisfy requirement 'internalMethod1' (in protocol 'Protocol1') because it is declared in a different file than the conformance of 'ConformingStruct' to 'Protocol1'}} {{3-3=@differentiable }}
   func internalMethod1(_ x: Float) -> Float {
     x
   }
 
   // Error for missing `@differentiable` superset attribute.
-  // expected-note @+2 {{instance method 'internalMethod2' must have explicit '@differentiable' attribute to satisfy requirement instance method 'internalMethod2' (in protocol 'Protocol1') because it is declared in a different file than the conformance of 'ConformingStruct' to 'Protocol1'}} {{3-3=@differentiable }}
+  // expected-note @+2 {{candidate is missing explicit '@differentiable' attribute to satisfy requirement 'internalMethod2' (in protocol 'Protocol1') because it is declared in a different file than the conformance of 'ConformingStruct' to 'Protocol1'}} {{3-3=@differentiable }}
   @differentiable(wrt: x)
   func internalMethod2(_ x: Float) -> Float {
     x
   }
 
   // Error for missing `@differentiable` subset attribute.
-  // expected-note @+2 {{instance method 'internalMethod3' must have explicit '@differentiable(wrt: x)' attribute to satisfy requirement instance method 'internalMethod3' (in protocol 'Protocol1') because it is declared in a different file than the conformance of 'ConformingStruct' to 'Protocol1'}} {{3-3=@differentiable(wrt: x) }}
+  // expected-note @+2 {{candidate is missing explicit '@differentiable(wrt: x)' attribute to satisfy requirement 'internalMethod3' (in protocol 'Protocol1') because it is declared in a different file than the conformance of 'ConformingStruct' to 'Protocol1'}} {{3-3=@differentiable(wrt: x) }}
   @differentiable(wrt: (self, x))
   func internalMethod3(_ x: Float) -> Float {
     x

test/AutoDiff/Sema/ImplicitDifferentiableAttributeCrossFile/Inputs/other_file_protocol_default_implementation_witness.swift

@@ -10,7 +10,7 @@ protocol P2: P1 {}
 
 extension P2 {
   @differentiable(wrt: (self, input))
-  // expected-note @+1 {{instance method 'callAsFunction' must have explicit '@differentiable(wrt: self)' attribute to satisfy requirement instance method 'callAsFunction' (in protocol 'P1') because it is declared in a different file than the conformance of 'ConformingStruct' to 'P1'}}
+  // expected-note @+1 {{candidate is missing explicit '@differentiable(wrt: self)' attribute to satisfy requirement 'callAsFunction' (in protocol 'P1') because it is declared in a different file than the conformance of 'ConformingStruct' to 'P1'}}
   public func callAsFunction(_ input: Float) -> Float {
     return input
   }