Add a check discouraging \s, unless it's a sequence of \s terminating a line.

core/src/main/java/com/google/errorprone/bugpatterns/MisleadingEscapedSpace.java

@@ -0,0 +1,76 @@
+/*
+ * Copyright 2024 The Error Prone Authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package com.google.errorprone.bugpatterns;
+
+import static com.google.errorprone.BugPattern.SeverityLevel.ERROR;
+import static com.google.errorprone.matchers.Description.NO_MATCH;
+import static com.google.errorprone.util.SourceVersion.supportsTextBlocks;
+
+import com.google.errorprone.BugPattern;
+import com.google.errorprone.VisitorState;
+import com.google.errorprone.bugpatterns.BugChecker.LiteralTreeMatcher;
+import com.google.errorprone.matchers.Description;
+import com.sun.source.tree.LiteralTree;
+
+/** See the summary. */
+@BugPattern(
+    summary =
+        "Using \\s anywhere except at the end of a line in a text block is potentially misleading.",
+    severity = ERROR)
+public final class MisleadingEscapedSpace extends BugChecker implements LiteralTreeMatcher {
+  @Override
+  public Description matchLiteral(LiteralTree tree, VisitorState state) {
+    if (!supportsTextBlocks(state.context)) {
+      return NO_MATCH;
+    }
+    if (tree.getValue() instanceof Character) {
+      if (tree.getValue().equals(' ') && state.getSourceForNode(tree).equals("'\\s'")) {
+        return describeMatch(tree);
+      }
+    }
+    if (tree.getValue() instanceof String) {
+      // Fast path out and avoid scanning through source code if there are simply no spaces in the
+      // literal.
+      String value = (String) tree.getValue();
+      if (!value.contains(" ")) {
+        return NO_MATCH;
+      }
+      String source = state.getSourceForNode(tree);
+      boolean seenEscape = false;
+      for (int i = 0; i < source.length(); ++i) {
+        switch (source.charAt(i)) {
+          case '\n':
+            seenEscape = false;
+            break;
+          case '\\':
+            i++;
+            if (source.charAt(i) == 's') {
+              seenEscape = true;
+              break;
+            }
+          // fall through
+          default:
+            if (seenEscape) {
+              return describeMatch(tree);
+            }
+            break;
+        }
+      }
+    }
+    return NO_MATCH;
+  }
+}

core/src/main/java/com/google/errorprone/scanner/BuiltInCheckerSuppliers.java

@@ -236,6 +236,7 @@
 import com.google.errorprone.bugpatterns.MemoizeConstantVisitorStateLookups;
 import com.google.errorprone.bugpatterns.MethodCanBeStatic;
 import com.google.errorprone.bugpatterns.MisformattedTestData;
+import com.google.errorprone.bugpatterns.MisleadingEscapedSpace;
 import com.google.errorprone.bugpatterns.MissingBraces;
 import com.google.errorprone.bugpatterns.MissingCasesInEnumSwitch;
 import com.google.errorprone.bugpatterns.MissingDefault;
@@ -767,6 +768,7 @@ public static ScannerSupplier warningChecks() {
           LossyPrimitiveCompare.class,
           MathRoundIntLong.class,
           MislabeledAndroidString.class,
+          MisleadingEscapedSpace.class,
           MisplacedScopeAnnotations.class,
           MissingSuperCall.class,
           MissingTestCall.class,

core/src/test/java/com/google/errorprone/bugpatterns/MisleadingEscapedSpaceTest.java

@@ -0,0 +1,129 @@
+/*
+ * Copyright 2024 The Error Prone Authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package com.google.errorprone.bugpatterns;
+
+import static com.google.common.truth.TruthJUnit.assume;
+
+import com.google.errorprone.CompilationTestHelper;
+import org.junit.Test;
+import org.junit.runner.RunWith;
+import org.junit.runners.JUnit4;
+
+@RunWith(JUnit4.class)
+public final class MisleadingEscapedSpaceTest {
+  private final CompilationTestHelper testHelper =
+      CompilationTestHelper.newInstance(MisleadingEscapedSpace.class, getClass());
+
+  @Test
+  public void misleadingEscape() {
+    assume().that(Runtime.version().feature()).isAtLeast(14);
+
+    testHelper
+        .addSourceLines(
+            "Test.class",
+            """
+            class Test {
+              // BUG: Diagnostic contains:
+              private static final String FOO = " \\s ";
+            }""")
+        .doTest();
+  }
+
+  @Test
+  public void literalBackslashS() {
+    assume().that(Runtime.version().feature()).isAtLeast(14);
+
+    testHelper
+        .addSourceLines(
+            "Test.class",
+            """
+            class Test {
+              private static final String FOO = " \\\\s ";
+            }""")
+        .doTest();
+  }
+
+  @Test
+  public void asSingleCharacter_misleading() {
+    assume().that(Runtime.version().feature()).isAtLeast(14);
+
+    testHelper
+        .addSourceLines(
+            "Test.class",
+            """
+            class Test {
+              // BUG: Diagnostic contains:
+              private static final char x = '\\s';
+            }""")
+        .doTest();
+  }
+
+  @Test
+  public void withinTextBlock_notAtEndOfLine_misleading() {
+    assume().that(Runtime.version().feature()).isAtLeast(14);
+
+    testHelper
+        .addSourceLines(
+            "Test.class",
+            """
+            class Test {
+              // BUG: Diagnostic contains:
+              private static final String FOO = \"""
+              foo   \\s  bar
+              \""";
+              // BUG: Diagnostic contains:
+              private static final String BAZ = \"""
+              foo   \\s
+              bar  \\s baz
+              \""";
+            }""")
+        .doTest();
+  }
+
+  @Test
+  public void atEndOfLine_notMisleading() {
+    assume().that(Runtime.version().feature()).isAtLeast(14);
+
+    testHelper
+        .addSourceLines(
+            "Test.class",
+            """
+            class Test {
+              private static final String FOO = \"""
+              foo   \\s
+              bar     \\s
+              \""";
+            }""")
+        .doTest();
+  }
+
+  @Test
+  public void multipleAtEndOfLine_notMisleading() {
+    assume().that(Runtime.version().feature()).isAtLeast(14);
+
+    testHelper
+        .addSourceLines(
+            "Test.class",
+            """
+            class Test {
+              private static final String FOO = \"""
+              foo   \\s\\s\\s\\s
+              \""";
+            }""")
+        .doTest();
+  }
+}

docs/bugpattern/MisleadingEscapedSpace.md

@@ -0,0 +1,22 @@
+When Java introduced text blocks as a feature, it also introduced a new string
+escape sequence `\s`. This escape sequence is another way to write a normal
+space, but it has the advantage that it can be used at the end of a line in a
+text block, where a normal space would be stripped.
+
+This new escape sequence can easily be confused with the regex `\s`, which is a
+metacharacter that matches any kind of whitespace character. To write that
+metacharacter in a Java string, you must still write `\\s`: an escaped backslash
+followed by an `s`.
+
+There is little reason to ever write the Java escape `\s` except at the end of a
+line. Either use a normal space, or switch to `\\s` if you are trying to write
+the regex metacharacter.
+
+```java
+// Each line here is five characters long.
+String colors = """
+    one \s
+    two \s
+    three
+    """;
+```