Make with type operator a migration warning

* In future-migration we emit the deprecation warning and enable the patch with -rewrite.
* In future we emit we make this syntax an error

compiler/src/dotty/tools/dotc/parsing/Parsers.scala

@@ -1756,8 +1756,12 @@ object Parsers {
         if in.token == LBRACE || in.token == INDENT then
           t
         else
-          if sourceVersion.isAtLeast(future) then
-            deprecationWarning(DeprecatedWithOperator(), withOffset)
+          report.errorOrMigrationWarning(
+            DeprecatedWithOperator(rewriteNotice(`future-migration`)),
+            in.sourcePos(withOffset),
+            from = future)
+          if sourceVersion == `future-migration` then
+            patch(source, Span(withOffset, withOffset + 4), "&")
           atSpan(startOffset(t)) { makeAndType(t, withType()) }
       else t
 

compiler/src/dotty/tools/dotc/reporting/messages.scala

@@ -138,10 +138,10 @@ extends EmptyCatchOrFinallyBlock(tryBody, EmptyCatchAndFinallyBlockID) {
         |its body in a block; no exceptions are handled."""
 }
 
-class DeprecatedWithOperator()(using Context)
+class DeprecatedWithOperator(rewrite: String)(using Context)
 extends SyntaxMsg(DeprecatedWithOperatorID) {
   def msg(using Context) =
-    i"""${hl("with")} as a type operator has been deprecated; use ${hl("&")} instead"""
+    i"""${hl("with")} as a type operator has been deprecated; use ${hl("&")} instead$rewrite"""
   def explain(using Context) =
     i"""|Dotty introduces intersection types - ${hl("&")} types. These replace the
         |use of the ${hl("with")} keyword. There are a few differences in

compiler/test/dotty/tools/dotc/CompilationTests.scala

@@ -60,6 +60,7 @@ class CompilationTests {
       compileFile("tests/rewrites/rewrites.scala", defaultOptions.and("-source", "3.0-migration").and("-rewrite", "-indent")),
       compileFile("tests/rewrites/rewrites3x.scala", defaultOptions.and("-rewrite", "-source", "future-migration")),
       compileFile("tests/rewrites/rewrites3x-fatal-warnings.scala", defaultOptions.and("-rewrite", "-source", "future-migration", "-Xfatal-warnings")),
+      compileFile("tests/rewrites/with-type-operator.scala", defaultOptions.and("-rewrite", "-source", "future-migration")),
       compileFile("tests/rewrites/filtering-fors.scala", defaultOptions.and("-rewrite", "-source", "3.2-migration")),
       compileFile("tests/rewrites/refutable-pattern-bindings.scala", defaultOptions.and("-rewrite", "-source", "3.2-migration")),
       compileFile("tests/rewrites/i8982.scala", defaultOptions.and("-indent", "-rewrite")),

tests/neg/with-type-operator-future-migration.check

@@ -0,0 +1,7 @@
+-- [E003] Syntax Error: tests/neg/with-type-operator-future-migration.scala:5:13 ---------------------------------------
+5 |def foo: Int with String = ??? // error
+  |             ^
+  |             with as a type operator has been deprecated; use & instead
+  |             This construct can be rewritten automatically under -rewrite -source future-migration.
+  |
+  | longer explanation available when compiling with `-explain`

tests/neg/with-type-operator-future-migration.scala

@@ -0,0 +1,5 @@
+//> using options -Werror
+
+import scala.language.`future-migration`
+
+def foo: Int with String = ??? // error

tests/neg/with-type-operator-future.scala

@@ -0,0 +1,3 @@
+import scala.language.`future`
+
+def foo: Int with String = ??? // error

tests/pos/with-type-operator-future-migration.scala

@@ -0,0 +1,3 @@
+import scala.language.`future-migration`
+
+def foo: Int with String = ??? // warn

tests/pos/with-type-operator.scala

@@ -0,0 +1,3 @@
+//> using options -Werror
+
+def foo: Int with String = ??? // warn

tests/rewrites/with-type-operator.check

@@ -0,0 +1 @@
+def foo: Int & String = ???

tests/rewrites/with-type-operator.scala

@@ -0,0 +1 @@
+def foo: Int with String = ???