internal/lsp: add inlay hints for constant values

For golang/go#52343.
For golang/vscode-go#1631.

Change-Id: Iaef0beab2837502f6428767f457d1da21848fcb6
Reviewed-on: https://go-review.googlesource.com/c/tools/+/411101
Run-TryBot: Jamal Carvalho <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>
Reviewed-by: Suzy Mueller <[email protected]>
gopls-CI: kokoro <[email protected]>

Author: jamalc

internal/lsp/source/inlay_hint.go

@@ -8,8 +8,10 @@ import (
 	"context"
 	"fmt"
 	"go/ast"
+	"go/constant"
 	"go/token"
 	"go/types"
+	"strings"
 
 	"golang.org/x/tools/internal/event"
 	"golang.org/x/tools/internal/lsp/lsppos"
@@ -42,6 +44,8 @@ func InlayHint(ctx context.Context, snapshot Snapshot, fh FileHandle, _ protocol
 			hints = append(hints, assignVariableTypes(n, tmap, info, &q)...)
 		case *ast.RangeStmt:
 			hints = append(hints, rangeVariableTypes(n, tmap, info, &q)...)
+		case *ast.GenDecl:
+			hints = append(hints, constantValues(n, tmap, info)...)
 		}
 		return true
 	})
@@ -125,6 +129,56 @@ func variableType(e ast.Expr, tmap *lsppos.TokenMapper, info *types.Info, q *typ
 	}
 }
 
+func constantValues(node *ast.GenDecl, tmap *lsppos.TokenMapper, info *types.Info) []protocol.InlayHint {
+	if node.Tok != token.CONST {
+		return nil
+	}
+
+	var hints []protocol.InlayHint
+	for _, v := range node.Specs {
+		spec, ok := v.(*ast.ValueSpec)
+		if !ok {
+			continue
+		}
+		end, ok := tmap.Position(v.End())
+		if !ok {
+			continue
+		}
+		// Show hints when values are missing or at least one value is not
+		// a basic literal.
+		showHints := len(spec.Values) == 0
+		checkValues := len(spec.Names) == len(spec.Values)
+		var values []string
+		for i, w := range spec.Names {
+			obj, ok := info.ObjectOf(w).(*types.Const)
+			if !ok || obj.Val().Kind() == constant.Unknown {
+				return nil
+			}
+			if checkValues {
+				switch spec.Values[i].(type) {
+				case *ast.BadExpr:
+					return nil
+				case *ast.BasicLit:
+				default:
+					if obj.Val().Kind() != constant.Bool {
+						showHints = true
+					}
+				}
+			}
+			values = append(values, fmt.Sprintf("%v", obj.Val()))
+		}
+		if !showHints || len(values) == 0 {
+			continue
+		}
+		hints = append(hints, protocol.InlayHint{
+			Position:    &end,
+			Label:       buildLabel("= " + strings.Join(values, ", ")),
+			PaddingLeft: true,
+		})
+	}
+	return hints
+}
+
 func buildLabel(s string) []protocol.InlayHintLabelPart {
 	label := protocol.InlayHintLabelPart{
 		Value: s,

internal/lsp/testdata/inlay_hint/constant_values.go

@@ -0,0 +1,45 @@
+package inlayHint //@inlayHint("package")
+
+const True = true
+
+type Kind int
+
+const (
+	KindNone Kind = iota
+	KindPrint
+	KindPrintf
+	KindErrorf
+)
+
+const (
+	u         = iota * 4
+	v float64 = iota * 42
+	w         = iota * 42
+)
+
+const (
+	a, b = 1, 2
+	c, d
+	e, f = 5 * 5, "hello" + "world"
+	g, h
+	i, j = true, f
+)
+
+// No hint
+const (
+	Int     = 3
+	Float   = 3.14
+	Bool    = true
+	Rune    = '3'
+	Complex = 2.7i
+	String  = "Hello, world!"
+)
+
+var (
+	varInt     = 3
+	varFloat   = 3.14
+	varBool    = true
+	varRune    = '3' + '4'
+	varComplex = 2.7i
+	varString  = "Hello, world!"
+)

internal/lsp/testdata/inlay_hint/constant_values.go.golden

@@ -0,0 +1,47 @@
+-- inlayHint --
+package inlayHint //@inlayHint("package")
+
+const True = true
+
+type Kind int
+
+const (
+	KindNone Kind = iota<= 0>
+	KindPrint<= 1>
+	KindPrintf<= 2>
+	KindErrorf<= 3>
+)
+
+const (
+	u         = iota * 4<= 0>
+	v float64 = iota * 42<= 42>
+	w         = iota * 42<= 84>
+)
+
+const (
+	a, b = 1, 2
+	c, d<= 1, 2>
+	e, f = 5 * 5, "hello" + "world"<= 25, "helloworld">
+	g, h<= 25, "helloworld">
+	i, j = true, f<= true, "helloworld">
+)
+
+// No hint
+const (
+	Int     = 3
+	Float   = 3.14
+	Bool    = true
+	Rune    = '3'
+	Complex = 2.7i
+	String  = "Hello, world!"
+)
+
+var (
+	varInt     = 3
+	varFloat   = 3.14
+	varBool    = true
+	varRune    = '3' + '4'
+	varComplex = 2.7i
+	varString  = "Hello, world!"
+)
+