[AutoDiff] Improve Array.TangentVector precondition messages. (swiftlang#32154)

Improve count-related precondition messages.

Co-authored-by: Richard Wei <[email protected]>

Author: dan-zheng

stdlib/public/Differentiation/ArrayDifferentiation.swift

@@ -30,7 +30,7 @@ extension Array.DifferentiableView: Differentiable
 where Element: Differentiable {
   /// The viewed array.
   public var base: [Element] {
-    get { return _base }
+    get { _base }
     _modify { yield &_base }
   }
 
@@ -58,9 +58,10 @@ where Element: Differentiable {
 
   public mutating func move(along direction: TangentVector) {
     precondition(
-      base.count == direction.base.count,
-      "cannot move Array.DifferentiableView with count \(base.count) along "
-        + "direction with different count \(direction.base.count)")
+      base.count == direction.base.count, """
+        Count mismatch: \(base.count) ('self') and \(direction.base.count) \
+        ('direction')
+        """)
     for i in base.indices {
       base[i].move(along: direction.base[i])
     }
@@ -106,35 +107,31 @@ where Element: AdditiveArithmetic & Differentiable {
     lhs: Array.DifferentiableView,
     rhs: Array.DifferentiableView
   ) -> Array.DifferentiableView {
-    precondition(
-      lhs.base.count == 0 || rhs.base.count == 0
-        || lhs.base.count == rhs.base.count,
-      "cannot add Array.DifferentiableViews with different counts: "
-        + "\(lhs.base.count) and \(rhs.base.count)")
     if lhs.base.count == 0 {
       return rhs
     }
     if rhs.base.count == 0 {
       return lhs
     }
+    precondition(
+      lhs.base.count == rhs.base.count,
+      "Count mismatch: \(lhs.base.count) and \(rhs.base.count)")
     return Array.DifferentiableView(zip(lhs.base, rhs.base).map(+))
   }
 
   public static func - (
     lhs: Array.DifferentiableView,
     rhs: Array.DifferentiableView
   ) -> Array.DifferentiableView {
-    precondition(
-      lhs.base.count == 0 || rhs.base.count == 0
-        || lhs.base.count == rhs.base.count,
-      "cannot subtract Array.DifferentiableViews with different counts: "
-        + "\(lhs.base.count) and \(rhs.base.count)")
     if lhs.base.count == 0 {
       return rhs
     }
     if rhs.base.count == 0 {
       return lhs
     }
+    precondition(
+      lhs.base.count == rhs.base.count,
+      "Count mismatch: \(lhs.base.count) and \(rhs.base.count)")
     return Array.DifferentiableView(zip(lhs.base, rhs.base).map(-))
   }
 
@@ -202,10 +199,10 @@ extension Array where Element: Differentiable {
   ) {
     func pullback(_ v: TangentVector) -> (TangentVector, TangentVector) {
       precondition(
-        v.base.count == lhs.count + rhs.count,
-        "+ should receive gradient with count equal to sum of operand "
-          + "counts, but counts are: gradient \(v.base.count), "
-          + "lhs \(lhs.count), rhs \(rhs.count)")
+        v.base.count == lhs.count + rhs.count, """
+          Tangent vector with invalid count; expected to equal the sum of \
+          operand counts \(lhs.count) and \(rhs.count)
+          """)
       return (
         TangentVector([Element.TangentVector](v.base[0..<lhs.count])),
         TangentVector([Element.TangentVector](v.base[lhs.count...]))