Better LSP completions inside of backticks

This improves the presentation compiler name completions inside of
backticks. The existing gaps which motivate doing this are outlined in
[this Metals feature request][0].

[0]: scalameta/metals-feature-requests#418

Author: harpocrates

Diff for: compiler/src/dotty/tools/dotc/interactive/Completion.scala

@@ -147,11 +147,12 @@ object Completion:
         checkBacktickPrefix(ident.source.content(), ident.span.start, ident.span.end)
 
       case (tree: untpd.RefTree) :: _ if tree.name != nme.ERROR =>
-        tree.name.toString.take(pos.span.point - tree.span.point)
-
-      case _ => naiveCompletionPrefix(pos.source.content().mkString, pos.point)
-
+        val nameStart = tree.span.point
+        val start = if pos.source.content().lift(nameStart).contains('`') then nameStart + 1 else nameStart
+        tree.name.toString.take(pos.span.point - start)
 
+      case _ =>
+        naiveCompletionPrefix(pos.source.content().mkString, pos.point)
   end completionPrefix
 
   private object GenericImportSelector:

Diff for: presentation-compiler/src/main/dotty/tools/pc/completions/CompletionPos.scala

@@ -23,7 +23,9 @@ case class CompletionPos(
   query: String,
   originalCursorPosition: SourcePosition,
   sourceUri: URI,
-  withCURSOR: Boolean
+  withCURSOR: Boolean,
+  hasLeadingBacktick: Boolean,
+  hasTrailingBacktick: Boolean
 ):
   def queryEnd: Int = originalCursorPosition.point
   def stripSuffixEditRange: l.Range = new l.Range(originalCursorPosition.offsetToPos(queryStart), originalCursorPosition.offsetToPos(identEnd))
@@ -38,16 +40,35 @@ object CompletionPos:
       adjustedPath: List[Tree],
       wasCursorApplied: Boolean
   )(using Context): CompletionPos =
-    val identEnd = adjustedPath match
+    def hasBacktickAt(offset: Int): Boolean =
+      sourcePos.source.content().lift(offset).contains('`')
+
+    val (identEnd, hasTrailingBacktick) = adjustedPath match
       case (refTree: RefTree) :: _ if refTree.name.toString.contains(Cursor.value) =>
-        refTree.span.end - Cursor.value.length
-      case (refTree: RefTree) :: _  => refTree.span.end
-      case _ => sourcePos.end
+        val refTreeEnd = refTree.span.end
+        val hasTrailingBacktick = hasBacktickAt(refTreeEnd - 1)
+        val identEnd = refTreeEnd - Cursor.value.length
+        (if hasTrailingBacktick then identEnd - 1 else identEnd, hasTrailingBacktick)
+      case (refTree: RefTree) :: _  =>
+        val refTreeEnd = refTree.span.end
+        val hasTrailingBacktick = hasBacktickAt(refTreeEnd - 1)
+        (if hasTrailingBacktick then refTreeEnd - 1 else refTreeEnd, hasTrailingBacktick)
+      case _ => (sourcePos.end, false)
 
     val query = Completion.completionPrefix(adjustedPath, sourcePos)
     val start = sourcePos.end - query.length()
+    val hasLeadingBacktick = hasBacktickAt(start - 1)
 
-    CompletionPos(start, identEnd, query.nn, sourcePos, offsetParams.uri.nn, wasCursorApplied)
+    CompletionPos(
+      start,
+      identEnd,
+      query.nn,
+      sourcePos,
+      offsetParams.uri.nn,
+      wasCursorApplied,
+      hasLeadingBacktick,
+      hasTrailingBacktick
+    )
 
   /**
    * Infer the indentation by counting the number of spaces in the given line.

Diff for: presentation-compiler/src/main/dotty/tools/pc/completions/CompletionProvider.scala

@@ -248,10 +248,17 @@ class CompletionProvider(
         range: Option[LspRange] = None
     ): CompletionItem =
       val oldText = params.text().nn.substring(completionPos.queryStart, completionPos.identEnd)
-      val editRange = if newText.startsWith(oldText) then completionPos.stripSuffixEditRange
+      val trimmedNewText = {
+        var nt = newText
+        if (completionPos.hasLeadingBacktick) nt = nt.stripPrefix("`")
+        if (completionPos.hasTrailingBacktick) nt = nt.stripSuffix("`")
+        nt
+      }
+
+      val editRange = if trimmedNewText.startsWith(oldText) then completionPos.stripSuffixEditRange
         else completionPos.toEditRange
 
-      val textEdit = new TextEdit(range.getOrElse(editRange), wrapInBracketsIfRequired(newText))
+      val textEdit = new TextEdit(range.getOrElse(editRange), wrapInBracketsIfRequired(trimmedNewText))
 
       val item = new CompletionItem(label)
       item.setSortText(f"${idx}%05d")

Diff for: presentation-compiler/test/dotty/tools/pc/tests/completion/CompletionBacktickSuite.scala

@@ -179,3 +179,60 @@ class CompletionBacktickSuite extends BaseCompletionSuite:
          |}
          |""".stripMargin
     )
+
+  @Test def `add-backticks-around-identifier` =
+    checkEdit(
+      """|object Main {
+         |  def `Foo Bar` = 123
+         |  Foo@@
+         |}
+         |""".stripMargin,
+      """|object Main {
+         |  def `Foo Bar` = 123
+         |  `Foo Bar`
+         |}
+         |""".stripMargin
+    )
+
+  @Test def `complete-inside-backticks` =
+    checkEdit(
+      """|object Main {
+         |  def `Foo Bar` = 123
+         |  `Foo@@`
+         |}
+         |""".stripMargin,
+      """|object Main {
+         |  def `Foo Bar` = 123
+         |  `Foo Bar`
+         |}
+         |""".stripMargin
+    )
+
+  @Test def `complete-inside-backticks-after-space` =
+    checkEdit(
+      """|object Main {
+         |  def `Foo Bar` = 123
+         |  `Foo B@@a`
+         |}
+         |""".stripMargin,
+      """|object Main {
+         |  def `Foo Bar` = 123
+         |  `Foo Bar`
+         |}
+         |""".stripMargin
+    )
+
+  @Test def `complete-inside-empty-backticks` =
+    checkEdit(
+      """|object Main {
+         |  def `Foo Bar` = 123
+         |  `@@`
+         |}
+         |""".stripMargin,
+      """|object Main {
+         |  def `Foo Bar` = 123
+         |  `Foo Bar`
+         |}
+         |""".stripMargin,
+      filter = _ == "Foo Bar: Int"
+    )