Censor user warnings according to the documentation

README.md

@@ -1394,12 +1394,12 @@ workspace:
       #      censor warnings if the code matches this code
       # - { byPrefix } values:
       #      censor warnings if the warning's message
-      #      starts with the given text
+      #      starts with the given text.
       - CodeName
       # Note: when using `byPrefix`, use the `>` for block-string:
       # see https://yaml-multiline.info/
       - byPrefix: >
-        "Data.Map"'s `Semigroup instance`
+          Data.Map's `Semigroup` instance
 
     # Specify whether to show statistics at the end of the compilation,
     # and how verbose they should be.
@@ -1450,12 +1450,12 @@ package:
       #      censor warnings if the code matches this code
       # - { byPrefix } values:
       #      censor warnings if the warning's message
-      #      starts with the given text
+      #      starts with the given text.
       - CodeName
       # Note: when using `byPrefix`, use the `>` for block-string:
       # see https://yaml-multiline.info/
       - byPrefix: >
-        "Data.Map"'s `Semigroup instance`
+          Data.Map's `Semigroup` instance
     # Convert compiler warnings for files in this package's src code
     # into errors that can fail the build.
     # Optional and defaults to false
@@ -1518,12 +1518,12 @@ package:
       #      censor warnings if the code matches this code
       # - { byPrefix } values:
       #      censor warnings if the warning's message
-      #      starts with the given text
+      #      starts with the given text.
       - CodeName
       # Note: when using `byPrefix`, use the `>` for block-string:
       # see https://yaml-multiline.info/
       - byPrefix: >
-        "Data.Map"'s `Semigroup instance`
+          Data.Map's `Semigroup` instance
     # Convert compiler warnings for files from this package's test code
     # into errors that can fail the build.
     # Optional and defaults to false

src/Spago/Psa.purs

@@ -189,8 +189,29 @@ shouldPrintWarning = case _ of
       let
         tests = arr <#> case _ of
           ByCode c -> \code _ -> c == code
-          ByMessagePrefix prefix -> \_ msg -> isJust $ String.stripPrefix (String.Pattern $ String.trim prefix) (String.trim msg)
+          ByMessagePrefix prefix -> \code msg ->
+            let
+              trimmedPrefix = String.trim prefix
+              trimmedMsg = String.trim msg
+              -- Try direct match first
+              directMatch = isJust $ String.stripPrefix (String.Pattern trimmedPrefix) trimmedMsg
+              -- For UserDefinedWarning, also try matching without the compiler preamble
+              strippedMatch = code == "UserDefinedWarning"
+                && isJust (String.stripPrefix (String.Pattern trimmedPrefix) (stripUserWarningPreamble trimmedMsg))
+            in
+              directMatch || strippedMatch
       -- We return `true` to print the warning.
       -- If an element was found (i.e. `Just` is returned), then one of the tests succeeded,
       -- so we should not print the warning and return false here.
       \code msg -> isNothing $ NonEmptyArray.find (\f -> f code msg) tests
+
+-- | Strip the preamble that the PureScript compiler adds to UserDefinedWarning messages.
+-- | This allows byPrefix to match just the user-defined content.
+stripUserWarningPreamble :: String -> String
+stripUserWarningPreamble msg =
+  let
+    preamble = "A custom warning occurred while solving type class constraints:"
+  in
+    case String.stripPrefix (String.Pattern preamble) msg of
+      Just rest -> String.trim rest
+      Nothing -> msg

test-fixtures/build/censor-user-defined-warning/spago-no-censor.yaml

@@ -0,0 +1,8 @@
+package:
+  name: censor-user-warning-test
+  dependencies:
+    - prelude
+
+workspace:
+  packageSet:
+    registry: 41.5.0

test-fixtures/build/censor-user-defined-warning/spago.yaml

@@ -0,0 +1,11 @@
+package:
+  name: censor-user-warning-test
+  dependencies:
+    - prelude
+  build:
+    censorProjectWarnings:
+      - byPrefix: "This is a custom warning"
+
+workspace:
+  packageSet:
+    registry: 41.5.0

test-fixtures/build/censor-user-defined-warning/src/Main.purs

@@ -0,0 +1,17 @@
+module Main where
+
+import Prelude
+import Prim.TypeError (class Warn, Text)
+
+-- A type class with a Warn constraint that triggers a UserDefinedWarning
+class MyClass a where
+  myFunction :: a -> a
+
+instance warnedInstance ::
+  ( Warn (Text "This is a custom warning that should be censored")
+  ) => MyClass Int where
+  myFunction = identity
+
+-- Using the instance triggers the warning at compile time
+main :: Int
+main = myFunction 42

test/Spago/Build.purs

@@ -143,6 +143,36 @@ spec = Spec.around withTempDir do
         , result: isRight
         }
 
+    Spec.it "should censor UserDefinedWarning with byPrefix matching just the user content" \{ spago, fixture, testCwd } -> do
+      FS.copyTree { src: fixture "build/censor-user-defined-warning", dst: testCwd </> "." }
+
+      let
+        warningTexts =
+          [ escapePathInErrMsg [ "src", "Main.purs" ]
+          , "UserDefinedWarning"
+          , "A custom warning occurred while solving type class constraints"
+          , "This is a custom warning that should be censored"
+          ]
+        shouldHaveWarning = assertWarning warningTexts true
+        shouldNotHaveWarning = assertWarning warningTexts false
+
+      -- First, verify the warning IS censored with byPrefix matching user content
+      spago [ "build" ] >>= check
+        { stdout: mempty
+        , stderr: shouldNotHaveWarning
+        , result: isRight
+        }
+
+      -- Now remove the censor config and verify the warning DOES appear
+      FS.unlink $ testCwd </> "spago.yaml"
+      FS.moveSync { src: testCwd </> "spago-no-censor.yaml", dst: testCwd </> "spago.yaml" }
+      rmRf $ testCwd </> "output"
+      spago [ "build" ] >>= check
+        { stdout: mempty
+        , stderr: shouldHaveWarning
+        , result: isRight
+        }
+
     Spec.describe "lockfile" do
       Spec.it "building with a lockfile doesn't need the Registry repo" \{ spago, fixture, testCwd } -> do
         spago [ "init", "--name", "aaa", "--package-set", "33.0.0" ] >>= shouldBeSuccess