improvement: Improve efficiency of completions (#23355)

There are two improvements here:
- isAbsent(force = false) which is already used in the codebase when we
care about performance
- we don't search for extension or implicit methods on empty query, as
we would have inverseSemanticdb invoked on every possible extension
method and it's quite inefficient

Author: tgodzik

compiler/src/dotty/tools/dotc/interactive/Completion.scala

@@ -335,7 +335,7 @@ object Completion:
       (sym.companionClass.exists && sym.companionClass.is(Enum))
 
     sym.exists &&
-    !sym.isAbsent() &&
+    !sym.isAbsent(canForce = false) &&
     !sym.isPrimaryConstructor &&
     sym.sourceSymbol.exists &&
     (!sym.is(Package) || sym.is(ModuleClass)) &&

presentation-compiler/src/main/dotty/tools/pc/completions/Completions.scala

@@ -588,7 +588,7 @@ class Completions(
         else false,
       )
       Some(search.search(query, buildTargetIdentifier, visitor).nn)
-    else if completionMode.is(Mode.Member) then
+    else if completionMode.is(Mode.Member) && query.nonEmpty then
       val visitor = new CompilerSearchVisitor(sym =>
         def isExtensionMethod = sym.is(ExtensionMethod) &&
           qualType.widenDealias <:< sym.extensionParam.info.widenDealias

presentation-compiler/test/dotty/tools/pc/tests/completion/CompletionExtensionSuite.scala

@@ -228,6 +228,10 @@ class CompletionExtensionSuite extends BaseCompletionSuite:
         |""".stripMargin
     )
 
+  /**
+   * For optimization, we don't show any completions here as it would bring
+   * every extension method into the completion list.
+   */
   @Test def `simple-empty` =
     check(
       """|package example
@@ -238,11 +242,13 @@ class CompletionExtensionSuite extends BaseCompletionSuite:
          |
          |def main = 100.@@
          |""".stripMargin,
-      """|incr: Int (extension)
-         |""".stripMargin,
+      "",
       filter = _.contains("(extension)")
     )
 
+  /**
+   * Some as above, but for implicit completions.
+   */
   @Test def `simple-empty-old` =
     check(
       """|package example
@@ -253,8 +259,7 @@ class CompletionExtensionSuite extends BaseCompletionSuite:
          |
          |def main = 100.@@
          |""".stripMargin,
-      """|testOps(b: Int): String (implicit)
-         |""".stripMargin,
+      "",
       filter = _.contains("(implicit)")
     )