go/ssa: adds position info to implicit field selection instructions

Fixes golang/go#51986

Change-Id: I1fa5868d70ae53f57b6c2bc0c4dbe4ac524b89e6
Reviewed-on: https://go-review.googlesource.com/c/tools/+/396495
gopls-CI: kokoro <[email protected]>
Run-TryBot: Zvonimir Pavlinovic <[email protected]>
Reviewed-by: Robert Findley <[email protected]>
Reviewed-by: Tim King <[email protected]>
Trust: Zvonimir Pavlinovic <[email protected]>

Author: zpavlinovic

go/analysis/passes/nilness/testdata/src/a/a.go

@@ -181,3 +181,16 @@ func f11(a interface{}) {
 		return
 	}
 }
+
+type Y struct {
+	innerY
+}
+
+type innerY struct {
+	value int
+}
+
+func f12() {
+	var d *Y
+	print(d.value) // want "nil dereference in field selection"
+}

go/ssa/builder.go

@@ -741,7 +741,7 @@ func (b *builder) expr0(fn *Function, e ast.Expr, tv types.TypeAndValue) Value {
 			indices := sel.Index()
 			last := len(indices) - 1
 			v := b.expr(fn, e.X)
-			v = emitImplicitSelections(fn, v, indices[:last])
+			v = emitImplicitSelections(fn, v, indices[:last], e.Pos())
 			v = emitFieldSelection(fn, v, indices[last], false, e.Sel)
 			return v
 		}
@@ -832,7 +832,8 @@ func (b *builder) receiver(fn *Function, e ast.Expr, wantAddr, escaping bool, se
 	}
 
 	last := len(sel.Index()) - 1
-	v = emitImplicitSelections(fn, v, sel.Index()[:last])
+	// The position of implicit selection is the position of the inducing receiver expression.
+	v = emitImplicitSelections(fn, v, sel.Index()[:last], e.Pos())
 	if !wantAddr && isPointer(v.Type()) {
 		v = emitLoad(fn, v)
 	}

go/ssa/emit.go

@@ -373,7 +373,7 @@ func emitTailCall(f *Function, call *Call) {
 // a field; if it is the value of a struct, the result will be the
 // value of a field.
 //
-func emitImplicitSelections(f *Function, v Value, indices []int) Value {
+func emitImplicitSelections(f *Function, v Value, indices []int, pos token.Pos) Value {
 	for _, index := range indices {
 		fld := deref(v.Type()).Underlying().(*types.Struct).Field(index)
 
@@ -382,6 +382,7 @@ func emitImplicitSelections(f *Function, v Value, indices []int) Value {
 				X:     v,
 				Field: index,
 			}
+			instr.setPos(pos)
 			instr.setType(types.NewPointer(fld.Type()))
 			v = f.emit(instr)
 			// Load the field's value iff indirectly embedded.
@@ -393,6 +394,7 @@ func emitImplicitSelections(f *Function, v Value, indices []int) Value {
 				X:     v,
 				Field: index,
 			}
+			instr.setPos(pos)
 			instr.setType(fld.Type())
 			v = f.emit(instr)
 		}

go/ssa/ssa.go

@@ -795,7 +795,8 @@ type Slice struct {
 // Type() returns a (possibly named) *types.Pointer.
 //
 // Pos() returns the position of the ast.SelectorExpr.Sel for the
-// field, if explicit in the source.
+// field, if explicit in the source. For implicit selections, returns
+// the position of the inducing explicit selection.
 //
 // Example printed form:
 // 	t1 = &t0.name [#1]
@@ -813,7 +814,9 @@ type FieldAddr struct {
 // package-local identifiers and permit compact representations.
 //
 // Pos() returns the position of the ast.SelectorExpr.Sel for the
-// field, if explicit in the source.
+// field, if explicit in the source. For implicit selections, returns
+// the position of the inducing explicit selection.
+
 //
 // Example printed form:
 // 	t1 = t0.name [#1]

go/ssa/wrappers.go

@@ -22,6 +22,7 @@ package ssa
 import (
 	"fmt"
 
+	"go/token"
 	"go/types"
 )
 
@@ -112,7 +113,7 @@ func makeWrapper(prog *Program, sel *types.Selection) *Function {
 	// Load) in preference to value extraction (Field possibly
 	// preceded by Load).
 
-	v = emitImplicitSelections(fn, v, indices[:len(indices)-1])
+	v = emitImplicitSelections(fn, v, indices[:len(indices)-1], token.NoPos)
 
 	// Invariant: v is a pointer, either
 	//   value of implicit *C field, or