Add a skill for writing mutations

Mutannot/Program.fs

@@ -20,6 +20,7 @@ type MutationOutcome =
     | BuildFailed
 
 type Command =
+    | Validate
     | List
     | Show of string
     | Run of string list
@@ -91,7 +92,7 @@ let repoRoot =
 
 let parseArgs (args: string list) =
     let usage () =
-        fail "Usage: mutannot <path/to/project.fsproj> [--configuration Debug|Release] [--build-arg <value> ...] [--no-build] [--list | --show <id> | --run [id...]]"
+        fail "Usage: mutannot <path/to/project.fsproj> [--configuration Debug|Release] [--build-arg <value> ...] [--no-build] [--validate | --list | --show <id> | --run [id...]]"
 
     let rec loop configuration projectPath buildArgs noBuild remaining =
         match remaining with
@@ -104,6 +105,12 @@ let parseArgs (args: string list) =
         | "--configuration" :: value :: tail -> loop value projectPath buildArgs noBuild tail
         | "--build-arg" :: value :: tail -> loop configuration projectPath (value :: buildArgs) noBuild tail
         | "--no-build" :: tail -> loop configuration projectPath buildArgs true tail
+        | "--validate" :: tail when tail.IsEmpty ->
+            { Configuration = configuration
+              ProjectPath = projectPath
+              BuildArgs = List.rev buildArgs
+              NoBuild = noBuild
+              Command = Validate }
         | "--list" :: tail when tail.IsEmpty ->
             { Configuration = configuration
               ProjectPath = projectPath
@@ -278,6 +285,76 @@ let replaceOccurrenceOnLine (mutation: MutationCase) (text: string) =
     let absoluteOffset = lineStart + lineOffset
     text.Remove(absoluteOffset, mutation.Find.Length).Insert(absoluteOffset, mutation.Replace)
 
+let countOccurrencesOnLine (needle: string) (lineText: string) =
+    if String.IsNullOrEmpty needle then
+        0
+    else
+        let mutable count = 0
+        let mutable startIndex = 0
+        let mutable keepSearching = true
+
+        while keepSearching do
+            let index = lineText.IndexOf(needle, startIndex, StringComparison.Ordinal)
+
+            if index < 0 then
+                keepSearching <- false
+            else
+                count <- count + 1
+                startIndex <- index + needle.Length
+
+        count
+
+let trimLineForDisplay (lineText: string) =
+    let trimmed = lineText.Trim()
+
+    if String.IsNullOrEmpty trimmed then
+        "<empty line>"
+    else
+        trimmed
+
+let staleMutationMessage (mutation: MutationCase) (detail: string) =
+    $"Mutation '{mutation.Id}' is stale: the recorded target at {mutation.File}:{mutation.Line} no longer matches the current source. {detail} Re-locate the intended mutation and update its file/line/find metadata."
+
+let validateMutation (repoRoot: string) (mutation: MutationCase) =
+    let targetFile = Path.Combine(repoRoot, mutation.File.Replace('/', Path.DirectorySeparatorChar))
+
+    if String.IsNullOrEmpty mutation.Find then
+        Some(staleMutationMessage mutation "The recorded find text is empty.")
+    elif not (File.Exists targetFile) then
+        Some(staleMutationMessage mutation $"The target file '{mutation.File}' no longer exists.")
+    else
+        let originalText = File.ReadAllText targetFile
+
+        try
+            let lineStart, lineEnd = lineSpan mutation.File originalText mutation.Line
+            let lineText = originalText.Substring(lineStart, lineEnd - lineStart)
+            let occurrenceCount = countOccurrencesOnLine mutation.Find lineText
+
+            match occurrenceCount with
+            | 0 ->
+                Some
+                    (staleMutationMessage
+                        mutation
+                        $"Expected to find '{mutation.Find}' on that line, but it has changed to '{trimLineForDisplay lineText}'.")
+            | 1 -> None
+            | count ->
+                Some
+                    (staleMutationMessage
+                        mutation
+                        $"The text '{mutation.Find}' now appears {count} times on that line, so the recorded target is no longer unique. Current line: '{trimLineForDisplay lineText}'.")
+        with
+        | UserError message -> Some(staleMutationMessage mutation message)
+
+let printValidationErrors errors =
+    eprintfn "Validation failed for %d mutation(s):" (errors |> List.length)
+
+    for mutation, message in errors do
+        eprintfn "[%s] %s:%d %s" mutation.Id mutation.File mutation.Line message
+
+let collectValidationErrors repoRoot mutations =
+    mutations
+    |> List.choose (fun mutation -> validateMutation repoRoot mutation |> Option.map (fun message -> mutation, message))
+
 let printMutation mutation =
     printfn "id: %s" mutation.Id
     printfn "test: %s.%s" mutation.DeclaringType mutation.TestName
@@ -412,6 +489,15 @@ let main argv =
             fail "No mutation cases were discovered in the test assembly."
 
         match options.Command with
+        | Validate ->
+            let validationErrors = mutations |> Array.toList |> collectValidationErrors repoRoot
+
+            if validationErrors.IsEmpty then
+                printfn "Validated %d mutation(s): all still apply syntactically." mutations.Length
+                0
+            else
+                printValidationErrors validationErrors
+                1
         | List ->
             mutations
             |> Array.iter (fun mutation ->
@@ -428,6 +514,13 @@ let main argv =
                 | [] -> mutations |> Array.toList
                 | _ -> ids |> List.map (fun id -> findMutation id mutations)
 
+            let validationErrors = collectValidationErrors repoRoot requested
+
+            if not validationErrors.IsEmpty then
+                printValidationErrors validationErrors
+                1
+            else
+
                 let tempRoot = Path.Combine(Path.GetTempPath(), $"fscheck-mutants.{Guid.NewGuid():N}")
                 let worktreePath = Path.Combine(tempRoot, "worktree")
                 let patchPath = Path.Combine(tempRoot, "current.patch")

skills/write-mutations/SKILL.md

@@ -0,0 +1,25 @@
+---
+name: Write mutations
+description: Trigger when asked to write mutations
+---
+
+Annotate tests with one or more mutations (`MutationCase`s) that will cause the test to fail.
+
+Example:
+
+```fs
+[<Fact>]
+[<MutationCase("calc-operator-mixup", "Calculator/Calculator.fs", 4, "value + 1", "value - 1")>]
+member _.AddOne_increments() =
+    Assert.Equal(42, Calculator.addOne 41)
+```
+
+In this example,
+
+* `calc-operator-mixup` is the mutation name,
+* `Calculator/Calculator.fs` is the path to the production code (relative to the repository root),
+* `4` is the number of the line to mutate,
+* `value + 1` is the string to find, and
+* `value - 1` is the string to replace it with.
+
+Verify your work with `mutannot --run <mutation id 1> <mutation id 2> <...>`.