[gopls-release-branch.0.18] gopls/internal/analysis/modernize: rangeint: non-integer untyped constants

This CL fixes a bug in rangeint that caused it to replace

   const limit = 1e3
   for i := 0; i < limit; i++ {}

with

   for range limit {} // error: limit is not an integer

Now, we check that the type of limit is assignable to int, and if
not insert an explicit int(limit) conversion.

Updates golang/go#71847 (item d)

Change-Id: Icfaa96e5506fcb5a3e6f3ed8f911bf4bda9cf32f
Reviewed-on: https://go-review.googlesource.com/c/tools/+/653616
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Jonathan Amsterdam <jba@google.com>
Auto-Submit: Alan Donovan <adonovan@google.com>
(cherry picked from commit 0efa5e5)
Reviewed-on: https://go-review.googlesource.com/c/tools/+/655955
Reviewed-by: Robert Findley <rfindley@google.com>

Author: adonovan

gopls/internal/analysis/modernize/rangeint.go

@@ -31,6 +31,8 @@ import (
 //   - The ':=' may be replaced by '='.
 //   - The fix may remove "i :=" if it would become unused.
 //
+// TODO(adonovan): permit variants such as "i := int64(0)".
+//
 // Restrictions:
 //   - The variable i must not be assigned or address-taken within the
 //     loop, because a "for range int" loop does not respect assignments
@@ -108,6 +110,31 @@ func rangeint(pass *analysis.Pass) {
 							limit = call.Args[0]
 						}
 
+						// If the limit is a untyped constant of non-integer type,
+						// such as "const limit = 1e3", its effective type may
+						// differ between the two forms.
+						// In a for loop, it must be comparable with int i,
+						//    for i := 0; i < limit; i++
+						// but in a range loop it would become a float,
+						//    for i := range limit {}
+						// which is a type error. We need to convert it to int
+						// in this case.
+						//
+						// Unfortunately go/types discards the untyped type
+						// (but see Untyped in golang/go#70638) so we must
+						// re-type check the expression to detect this case.
+						var beforeLimit, afterLimit string
+						if v := info.Types[limit].Value; v != nil {
+							beforeLimit, afterLimit = "int(", ")"
+							info2 := &types.Info{Types: make(map[ast.Expr]types.TypeAndValue)}
+							if types.CheckExpr(pass.Fset, pass.Pkg, limit.Pos(), limit, info2) == nil {
+								tLimit := types.Default(info2.TypeOf(limit))
+								if types.AssignableTo(tLimit, types.Typ[types.Int]) {
+									beforeLimit, afterLimit = "", ""
+								}
+							}
+						}
+
 						pass.Report(analysis.Diagnostic{
 							Pos:      init.Pos(),
 							End:      inc.End(),
@@ -121,15 +148,30 @@ func rangeint(pass *analysis.Pass) {
 									//     -----              ---
 									//          -------
 									// for i := range  limit      {}
+
+									// Delete init.
 									{
 										Pos:     init.Rhs[0].Pos(),
 										End:     limit.Pos(),
 										NewText: []byte("range "),
 									},
+									// Add "int(" before limit, if needed.
+									{
+										Pos:     limit.Pos(),
+										End:     limit.Pos(),
+										NewText: []byte(beforeLimit),
+									},
+									// Delete inc.
 									{
 										Pos: limit.End(),
 										End: inc.End(),
 									},
+									// Add ")" after limit, if needed.
+									{
+										Pos:     limit.End(),
+										End:     limit.End(),
+										NewText: []byte(afterLimit),
+									},
 								}...),
 							}},
 						})

gopls/internal/analysis/modernize/testdata/src/rangeint/rangeint.go

@@ -51,3 +51,14 @@ func _(s string) {
 		}
 	}
 }
+
+// Non-integer untyped constants need to be explicitly converted to int.
+func issue71847d() {
+	const limit = 1e3            // float
+	for i := 0; i < limit; i++ { // want "for loop can be modernized using range over int"
+	}
+
+	const limit2 = 1 + 0i         // complex
+	for i := 0; i < limit2; i++ { // want "for loop can be modernized using range over int"
+	}
+}

gopls/internal/analysis/modernize/testdata/src/rangeint/rangeint.go.golden

@@ -51,3 +51,14 @@ func _(s string) {
 		}
 	}
 }
+
+// Non-integer untyped constants need to be explicitly converted to int.
+func issue71847d() {
+	const limit = 1e3            // float
+	for range int(limit) { // want "for loop can be modernized using range over int"
+	}
+
+	const limit2 = 1 + 0i         // complex
+	for range int(limit2) { // want "for loop can be modernized using range over int"
+	}
+}