tools/gopls: argument in function bodies marked as parameter by semantic tokens

In func f(x int) int {return x;} the second x used to be marked as a
variable, but now is marked as a parameter, visually tying it to its
definition.

Fixes golang/go#56257

Change-Id: I8aa506b1ddff5ed9a3d2716d48c64521bdea0fd5
Reviewed-on: https://go-review.googlesource.com/c/tools/+/445095
Reviewed-by: Robert Findley <rfindley@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Peter Weinberger <pjw@google.com>
gopls-CI: kokoro <noreply+kokoro@google.com>

gopls/internal/lsp/semantic.go

@@ -526,9 +526,14 @@ func (e *encoded) ident(x *ast.Ident) {
 			tok(x.Pos(), len(x.Name), tokFunction, nil)
 		} else if _, ok := y.Type().(*typeparams.TypeParam); ok {
 			tok(x.Pos(), len(x.Name), tokTypeParam, nil)
+		} else if e.isParam(use.Pos()) {
+			// variable, unless use.pos is the pos of a Field in an ancestor FuncDecl
+			// or FuncLit and then it's a parameter
+			tok(x.Pos(), len(x.Name), tokParameter, nil)
 		} else {
 			tok(x.Pos(), len(x.Name), tokVariable, nil)
 		}
+
 	default:
 		// can't happen
 		if use == nil {
@@ -543,6 +548,30 @@ func (e *encoded) ident(x *ast.Ident) {
 	}
 }
 
+func (e *encoded) isParam(pos token.Pos) bool {
+	for i := len(e.stack) - 1; i >= 0; i-- {
+		switch n := e.stack[i].(type) {
+		case *ast.FuncDecl:
+			for _, f := range n.Type.Params.List {
+				for _, id := range f.Names {
+					if id.Pos() == pos {
+						return true
+					}
+				}
+			}
+		case *ast.FuncLit:
+			for _, f := range n.Type.Params.List {
+				for _, id := range f.Names {
+					if id.Pos() == pos {
+						return true
+					}
+				}
+			}
+		}
+	}
+	return false
+}
+
 func isSignature(use types.Object) bool {
 	if true {
 		return false //PJW: fix after generics seem ok
@@ -640,7 +669,7 @@ func (e *encoded) unkIdent(x *ast.Ident) (tokenType, []string) {
 				if nd.Tok != token.DEFINE {
 					def = nil
 				}
-				return tokVariable, def
+				return tokVariable, def // '_' in _ = ...
 			}
 		}
 		// RHS, = x

gopls/internal/lsp/testdata/semantic/a.go.golden

@@ -65,7 +65,7 @@
 	/*⇒2,variable,[definition]*/ff /*⇒2,operator,[]*/:= /*⇒4,keyword,[]*/func() {}
 	/*⇒5,keyword,[]*/defer /*⇒2,variable,[]*/ff()
 	/*⇒2,keyword,[]*/go /*⇒3,namespace,[]*/utf./*⇒9,function,[]*/RuneCount(/*⇒2,string,[]*/"")
-	/*⇒2,keyword,[]*/go /*⇒4,namespace,[]*/utf8./*⇒9,function,[]*/RuneCount(/*⇒2,variable,[]*/vv.(/*⇒6,type,[]*/string))
+	/*⇒2,keyword,[]*/go /*⇒4,namespace,[]*/utf8./*⇒9,function,[]*/RuneCount(/*⇒2,parameter,[]*/vv.(/*⇒6,type,[]*/string))
 	/*⇒2,keyword,[]*/if /*⇒4,variable,[readonly]*/true {
 	} /*⇒4,keyword,[]*/else {
 	}
@@ -73,9 +73,9 @@
 	/*⇒3,keyword,[]*/for /*⇒1,variable,[definition]*/i /*⇒2,operator,[]*/:= /*⇒1,number,[]*/0; /*⇒1,variable,[]*/i /*⇒1,operator,[]*/< /*⇒2,number,[]*/10; {
 		/*⇒5,keyword,[]*/break Never
 	}
-	_, /*⇒2,variable,[definition]*/ok /*⇒2,operator,[]*/:= /*⇒2,variable,[]*/vv[/*⇒1,number,[]*/0].(/*⇒1,type,[]*/A)
+	_, /*⇒2,variable,[definition]*/ok /*⇒2,operator,[]*/:= /*⇒2,parameter,[]*/vv[/*⇒1,number,[]*/0].(/*⇒1,type,[]*/A)
 	/*⇒2,keyword,[]*/if /*⇒1,operator,[]*/!/*⇒2,variable,[]*/ok {
-		/*⇒6,keyword,[]*/switch /*⇒1,variable,[definition]*/x /*⇒2,operator,[]*/:= /*⇒2,variable,[]*/vv[/*⇒1,number,[]*/0].(/*⇒4,keyword,[]*/type) {
+		/*⇒6,keyword,[]*/switch /*⇒1,variable,[definition]*/x /*⇒2,operator,[]*/:= /*⇒2,parameter,[]*/vv[/*⇒1,number,[]*/0].(/*⇒4,keyword,[]*/type) {
 		}
 		/*⇒4,keyword,[]*/goto Never
 	}