Improve Should-BeString to show string diff with arrow marker

src/functions/assert/String/Should-BeString.ps1

@@ -96,8 +96,75 @@ function Should-BeString {
 
     $stringsAreEqual = Test-StringEqual -Expected $Expected -Actual $Actual -CaseSensitive:$CaseSensitive -IgnoreWhitespace:$IgnoreWhiteSpace -TrimWhitespace:$TrimWhitespace
     if (-not ($stringsAreEqual)) {
-        $Message = Get-AssertionMessage -Expected $Expected -Actual $Actual -Because $Because -DefaultMessage "Expected <expectedType> <expected>, but got <actualType> <actual>."
+        if ($Actual -is [string]) {
+            $Message = Get-StringDifferenceMessage -Expected $Expected -Actual $Actual -CaseSensitive:$CaseSensitive -Because $Because
+        }
+        else {
+            $Message = Get-AssertionMessage -Expected $Expected -Actual $Actual -Because $Because -DefaultMessage "Expected <expectedType> <expected>, but got <actualType> <actual>."
+        }
         Invoke-AssertionFailed -Message $Message -CallerCmdlet $PSCmdlet
     }
     Set-AssertionPassResult
 }
+
+function Get-StringDifferenceMessage {
+    param (
+        [Parameter(Mandatory)]
+        [AllowEmptyString()]
+        [string] $Expected,
+        [Parameter(Mandatory)]
+        [AllowEmptyString()]
+        [string] $Actual,
+        [switch] $CaseSensitive,
+        [string] $Because
+    )
+
+    $maxLength = [Math]::Max($Expected.Length, $Actual.Length)
+
+    $differenceIndex = $null
+    for ($i = 0; $i -lt $maxLength -and ($null -eq $differenceIndex); ++$i) {
+        if ($CaseSensitive) {
+            if ($Expected[$i] -cne $Actual[$i]) { $differenceIndex = $i }
+        }
+        else {
+            if ($Expected[$i] -ne $Actual[$i]) { $differenceIndex = $i }
+        }
+    }
+
+    $because = if ($Because) { " because $Because," } else { "" }
+
+    $lines = @(
+        "Expected strings to be the same,$because but they were different."
+    )
+
+    if ($Expected.Length -ne $Actual.Length) {
+        $lines += "Expected length: $($Expected.Length)"
+        $lines += "Actual length:   $($Actual.Length)"
+    }
+    else {
+        $lines += "String lengths are both $($Expected.Length)."
+    }
+    $lines += "Strings differ at index $differenceIndex."
+
+    $expectedExpanded = Expand-SpecialCharacters -InputObject $Expected
+    $actualExpanded = Expand-SpecialCharacters -InputObject $Actual
+
+    # Recompute difference index on expanded strings
+    $maxLength = [Math]::Max($expectedExpanded.Length, $actualExpanded.Length)
+    $expandedDiffIndex = $null
+    for ($i = 0; $i -lt $maxLength -and ($null -eq $expandedDiffIndex); ++$i) {
+        if ($CaseSensitive) {
+            if ($expectedExpanded[$i] -cne $actualExpanded[$i]) { $expandedDiffIndex = $i }
+        }
+        else {
+            if ($expectedExpanded[$i] -ne $actualExpanded[$i]) { $expandedDiffIndex = $i }
+        }
+    }
+
+    $prefix = "Expected: '"
+    $lines += "$prefix$expectedExpanded'"
+    $lines += "But was:  '$actualExpanded'"
+    $lines += (' ' * ($prefix.Length - 1)) + ('-' * $expandedDiffIndex) + '^'
+
+    $lines -join "`n"
+}

tst/functions/assert/String/Should-BeString.Tests.ps1

@@ -118,6 +118,51 @@ Describe "Should-BeString" {
 
     It "Throws with default message when test fails" {
         $err = { Should-BeString -Expected "abc" -Actual "bde" } | Verify-AssertionFailed
-        $err.Exception.Message | Verify-Equal "Expected [string] 'abc', but got [string] 'bde'."
+        $err.Exception.Message | Verify-Equal (@'
+Expected strings to be the same, but they were different.
+String lengths are both 3.
+Strings differ at index 0.
+Expected: 'abc'
+But was:  'bde'
+          ^
+'@ -replace "`r`n", "`n")
+    }
+
+    It "Shows arrow at correct position for case-sensitive difference" {
+        $err = { "hello world" | Should-BeString "Hello World" -CaseSensitive } | Verify-AssertionFailed
+        $err.Exception.Message | Verify-Equal (@'
+Expected strings to be the same, but they were different.
+String lengths are both 11.
+Strings differ at index 0.
+Expected: 'Hello World'
+But was:  'hello world'
+          ^
+'@ -replace "`r`n", "`n")
+    }
+
+    It "Shows arrow with dashes for mid-string difference" {
+        $err = { "abc" | Should-BeString "abcdef" } | Verify-AssertionFailed
+        $err.Exception.Message | Verify-Equal (@'
+Expected strings to be the same, but they were different.
+Expected length: 6
+Actual length:   3
+Strings differ at index 3.
+Expected: 'abcdef'
+But was:  'abc'
+          ---^
+'@ -replace "`r`n", "`n")
+    }
+
+    It "Shows expanded whitespace characters in diff" {
+        $err = { "abc`ndef" | Should-BeString "abc`r`ndef" } | Verify-AssertionFailed
+        $err.Exception.Message | Verify-Equal (@'
+Expected strings to be the same, but they were different.
+Expected length: 8
+Actual length:   7
+Strings differ at index 3.
+Expected: 'abc\r\ndef'
+But was:  'abc\ndef'
+          ----^
+'@ -replace "`r`n", "`n")
     }
 }