feature: completions inside of backticks

Scala 2 only - the [corresponding Scala 3 change][0] has already been
merged. Completions can now be triggered from inside of backticks.

In addition, the backtick itself is added as a completion character.
Especially when the editor is configured to auto-close backticks, this
plays nicely with completions now producing useful results inside of
backticks.

Closes [this feature request][1]

[0]: scala/scala3#22555
[1]: scalameta/metals-feature-requests#418

metals/src/main/scala/scala/meta/internal/metals/WorkspaceLspService.scala

@@ -1211,7 +1211,7 @@ class WorkspaceLspService(
         capabilities.setCompletionProvider(
           new lsp4j.CompletionOptions(
             clientConfig.isCompletionItemResolve(),
-            List(".", "*", "$").asJava,
+            List(".", "*", "$", "`").asJava,
           )
         )
         capabilities.setCallHierarchyProvider(true)

mtags/src/main/scala-2/scala/meta/internal/pc/CompletionProvider.scala

@@ -57,10 +57,15 @@ class CompletionProvider(
     val pos = unit.position(params.offset)
     val isSnippet = isSnippetEnabled(pos, params.text())
 
-    val (i, completion, editRange, query) = safeCompletionsAt(pos, params.uri())
-
-    val start = inferIdentStart(pos, params.text())
-    val end = inferIdentEnd(pos, params.text())
+    val (i, completion, identOffsets, editRange, query) =
+      safeCompletionsAt(pos, params.uri())
+
+    val InferredIdentOffsets(
+      start,
+      end,
+      hadLeadingBacktick,
+      hadTrailingBacktick
+    ) = identOffsets
     val oldText = params.text().substring(start, end)
     val stripSuffix = pos.withStart(start).withEnd(end).toLsp
 
@@ -275,7 +280,12 @@ class CompletionProvider(
       if (item.getTextEdit == null) {
         val editText = member match {
           case _: NamedArgMember => item.getLabel
-          case _ => ident
+          case _ =>
+            val ident0 =
+              if (hadLeadingBacktick) ident.stripPrefix("`") else ident
+            val ident1 =
+              if (hadTrailingBacktick) ident0.stripSuffix("`") else ident0
+            ident1
         }
         item.setTextEdit(textEdit(editText))
       }
@@ -502,9 +512,16 @@ class CompletionProvider(
   private def safeCompletionsAt(
       pos: Position,
       source: URI
-  ): (InterestingMembers, CompletionPosition, l.Range, String) = {
+  ): (
+      InterestingMembers,
+      CompletionPosition,
+      InferredIdentOffsets,
+      l.Range,
+      String
+  ) = {
+    lazy val inferredIdentOffsets = inferIdentOffsets(pos, params.text())
     lazy val editRange = pos
-      .withStart(inferIdentStart(pos, params.text()))
+      .withStart(inferredIdentOffsets.start)
       .withEnd(pos.point)
       .toLsp
     val noQuery = "$a"
@@ -522,6 +539,7 @@ class CompletionProvider(
           (
             InterestingMembers(Nil, SymbolSearch.Result.COMPLETE),
             NoneCompletion,
+            inferredIdentOffsets,
             editRange,
             noQuery
           )
@@ -532,6 +550,7 @@ class CompletionProvider(
               SymbolSearch.Result.COMPLETE
             ),
             completion,
+            inferredIdentOffsets,
             editRange,
             noQuery
           )
@@ -579,7 +598,7 @@ class CompletionProvider(
         params.text()
       )
       params.checkCanceled()
-      (items, completion, editRange, query)
+      (items, completion, inferredIdentOffsets, editRange, query)
     } catch {
       case e: CyclicReference
           if e.getMessage.contains("illegal cyclic reference") =>

mtags/src/main/scala-2/scala/meta/internal/pc/completions/Completions.scala

@@ -905,57 +905,93 @@ trait Completions { this: MetalsGlobal =>
     result
   }
 
-  def inferStart(
+  case class InferredIdentOffsets(
+      start: Int,
+      end: Int,
+      strippedLeadingBacktick: Boolean,
+      strippedTrailingBacktick: Boolean
+  )
+
+  def inferIdentOffsets(
       pos: Position,
-      text: String,
-      charPred: Char => Boolean
-  ): Int = {
-    def fallback: Int = {
+      text: String
+  ): InferredIdentOffsets = {
+
+    // If we fail to find a tree, approximate with a heurstic about ident characters
+    def fallbackStart: Int = {
       var i = pos.point - 1
-      while (i >= 0 && charPred(text.charAt(i))) {
+      while (i >= 0 && Chars.isIdentifierPart(text.charAt(i))) {
         i -= 1
       }
       i + 1
     }
-    def loop(enclosing: List[Tree]): Int =
+    def fallbackEnd: Int = {
+      findEnd(false)
+    }
+
+    def findEnd(hasBacktick: Boolean): Int = {
+      val predicate: Char => Boolean = if (hasBacktick) { (ch: Char) =>
+        !Chars.isLineBreakChar(ch) && ch != '`'
+      } else {
+        Chars.isIdentifierPart(_)
+      }
+
+      var i = pos.point
+      while (i < text.length && predicate(text.charAt(i))) {
+        i += 1
+      }
+      i
+    }
+    def fallback =
+      InferredIdentOffsets(fallbackStart, fallbackEnd, false, false)
+
+    def refTreePos(refTree: RefTree): InferredIdentOffsets = {
+      val refTreePos = treePos(refTree)
+      var startPos = refTreePos.point
+      var strippedLeadingBacktick = false
+      if (text.charAt(startPos) == '`') {
+        startPos += 1
+        strippedLeadingBacktick = true
+      }
+
+      val endPos = findEnd(strippedLeadingBacktick)
+      var strippedTrailingBacktick = false
+      if (endPos < text.length) {
+        if (text.charAt(endPos) == '`') {
+          strippedTrailingBacktick = true
+        }
+      }
+      InferredIdentOffsets(
+        Math.min(startPos, pos.point),
+        endPos,
+        strippedLeadingBacktick,
+        strippedTrailingBacktick
+      )
+    }
+
+    def loop(enclosing: List[Tree]): InferredIdentOffsets =
       enclosing match {
         case Nil => fallback
         case head :: tl =>
           if (!treePos(head).includes(pos)) loop(tl)
           else {
             head match {
               case i: Ident =>
-                treePos(i).point
-              case Select(qual, _) if !treePos(qual).includes(pos) =>
-                treePos(head).point
+                refTreePos(i)
+              case sel @ Select(qual, _) if !treePos(qual).includes(pos) =>
+                refTreePos(sel)
               case _ => fallback
             }
           }
       }
-    val start = loop(lastVisitedParentTrees)
-    Math.min(start, pos.point)
+    loop(lastVisitedParentTrees)
   }
 
-  /** Can character form part of an alphanumeric Scala identifier? */
-  private def isIdentifierPart(c: Char) =
-    (c == '$') || Character.isUnicodeIdentifierPart(c)
-
   /**
    * Returns the start offset of the identifier starting as the given offset position.
    */
   def inferIdentStart(pos: Position, text: String): Int =
-    inferStart(pos, text, isIdentifierPart)
-
-  /**
-   * Returns the end offset of the identifier starting as the given offset position.
-   */
-  def inferIdentEnd(pos: Position, text: String): Int = {
-    var i = pos.point
-    while (i < text.length && Chars.isIdentifierPart(text.charAt(i))) {
-      i += 1
-    }
-    i
-  }
+    inferIdentOffsets(pos, text).start
 
   def isSnippetEnabled(pos: Position, text: String): Boolean = {
     pos.point < text.length() && {

tests/cross/src/test/scala/tests/pc/CompletionSuite.scala

@@ -2091,4 +2091,63 @@ class CompletionSuite extends BaseCompletionSuite {
     }
   )
 
+  val BacktickCompletionsTag: IgnoreScalaVersion =
+    IgnoreScala3.and(IgnoreScalaVersion.forLessThan("2.13.17"))
+
+  checkEdit(
+    "add-backticks-around-identifier".tag(BacktickCompletionsTag),
+    """|object Main {
+       |  def `Foo Bar` = 123
+       |  Foo@@
+       |}
+       |""".stripMargin,
+    """|object Main {
+       |  def `Foo Bar` = 123
+       |  `Foo Bar`
+       |}
+       |""".stripMargin
+  )
+
+  checkEdit(
+    "complete-inside-backticks".tag(BacktickCompletionsTag),
+    """|object Main {
+       |  def `Foo Bar` = 123
+       |  `Foo@@`
+       |}
+       |""".stripMargin,
+    """|object Main {
+       |  def `Foo Bar` = 123
+       |  `Foo Bar`
+       |}
+       |""".stripMargin
+  )
+
+  checkEdit(
+    "complete-inside-backticks-after-space".tag(BacktickCompletionsTag),
+    """|object Main {
+       |  def `Foo Bar` = 123
+       |  `Foo B@@`
+       |}
+       |""".stripMargin,
+    """|object Main {
+       |  def `Foo Bar` = 123
+       |  `Foo Bar`
+       |}
+       |""".stripMargin
+  )
+
+  checkEdit(
+    "complete-inside-empty-backticks".tag(BacktickCompletionsTag),
+    """|object Main {
+       |  def `Foo Bar` = 123
+       |  `@@`
+       |}
+       |""".stripMargin,
+    """|object Main {
+       |  def `Foo Bar` = 123
+       |  `Foo Bar`
+       |}
+       |""".stripMargin,
+    filter = _ == "`Foo Bar`: Int"
+  )
 }