feat: named and unnamed type assignment 3 of 3 (gnolang#2367)

This is the last part of the solution for issue gnolang#1141. 
The  1 of 3 of the solution can be found in PR  gnolang#1143.
The  2 of 3 of the solution can be found in PR gnolang#1246 

It decomposes function calls that return multiple values in the
preprocess.

### Here is the problem to solve: 

`  u1, n2 = x() `

How do we ensure that the returned multiple values from a function call
adhere to named and unnamed type assignment specifications?
Additionally, we want to solve this problem during preprocessing instead
of at runtime to minimize the impact on runtime performance.

### The main ideas:

    u1, n2 = x()  << decompose the statement to the following two lines 
    // .tmp_1, .tmp_2 := x() 
    // u1, n2 = .tmp_1, .tmp_2

then we can apply name and unname type conversion specs to the second
line.
    u1, n2 = _tmp_1, _tmp_2


### Here are the example code and the explanation

```
// decompose_filetest.gno
package main

  type nat []int

  func x() (nat, []int) {
    a := nat{1}
    b := []int{2}
    return a, b
  }

  func main() {
    var u1 []int
    var n2 nat

    u1, n2 = x() 
    // .tmp_1, .tmp_2 := x() 
    // u1, n2 = .tmp_1, .tmp_2

    println(u1)
    println(n2)

  }

  // Output:
  // slice[(1 int)]
  // (slice[(2 int)] main.nat)

```

### Here is the simplified recursive tree of the transformation in the
preprocess

<img width="1336" alt="image"
src="https://github.com/gnolang/gno/assets/90544084/306a4770-457d-4131-a82a-2de5c6b0dadf">

### Here are the major steps involved in this decomposition during
preprocessing:

- Create hidden temporary name expressions .tmp1, .tmp2. In Go, a
leading dot is not valid in variable names, ensuring that users cannot
create names that clash with these hidden variables.


- Create two statements in the block: one for defining and one for
assigning.

```
 .tmp1, .tmp2 := x() 
 u1, n2 = .tmp_1, .tmp_2
```

- Preprocess each newly created statements

- Replace the original statement with the two newly created statements. 


### Here are two additional changes to facilitate above.

- Update the FuncValue's body in `updates := pn.PrepareNewValues(pv)
`since its source Body has been changed during preprocessing.

- Replace all ` for index := range Body` with `for i:=0; i < len(Body);
i++` in transcribe.go since the body length might change due to the
decomposition.


<!-- please provide a detailed description of the changes made in this
pull request. -->

<details><summary>Contributors' checklist...</summary>

- [x] Added new tests, or not needed, or not feasible
- [x] Provided an example (e.g. screenshot) to aid review or the PR is
self-explanatory
- [ ] Updated the official documentation or not needed
- [x] No breaking changes were made
- [x] Added references to related issues and PRs
- [ ] Provided any useful hints for running manual tests
- [ ] Added new benchmarks to [generated
graphs](https://gnoland.github.io/benchmarks), if any. More info
[here](https://github.com/gnolang/gno/blob/master/.benchmarks/README.md).
</details>

---------

Co-authored-by: Miloš Živković <[email protected]>
Co-authored-by: Morgan <[email protected]>

gnovm/Makefile

@@ -53,7 +53,7 @@ lint:
 	$(rundep) github.com/golangci/golangci-lint/cmd/golangci-lint run --config ../.github/golangci.yml ./...
 
 .PHONY: fmt
-fmt: 
+fmt:
 	go run ./cmd/gno fmt $(GNOFMT_FLAGS) ./stdlibs/...
 	$(rundep) mvdan.cc/gofumpt $(GOFMT_FLAGS) .
 

gnovm/pkg/gnolang/nodes.go

@@ -672,6 +672,10 @@ func (ss Body) GetBody() Body {
 	return ss
 }
 
+func (ss *Body) SetBody(nb Body) {
+	*ss = nb
+}
+
 func (ss Body) GetLabeledStmt(label Name) (stmt Stmt, idx int) {
 	for idx, stmt = range ss {
 		if label == stmt.GetLabel() {
@@ -1375,6 +1379,13 @@ func (x *PackageNode) PrepareNewValues(pv *PackageValue) []TypedValue {
 			panic("PackageNode.PrepareNewValues() package mismatch")
 		}
 	}
+	// The FuncValue Body may have been altered during the preprocessing.
+	// We need to update body field from the source in the FuncValue accordingly.
+	for _, tv := range x.Values {
+		if fv, ok := tv.V.(*FuncValue); ok {
+			fv.UpdateBodyFromSource()
+		}
+	}
 	pvl := len(block.Values)
 	pnl := len(x.Values)
 	// copy new top-level defined values/types.
@@ -1480,6 +1491,7 @@ type BlockNode interface {
 	Define(Name, TypedValue)
 	Define2(bool, Name, Type, TypedValue)
 	GetBody() Body
+	SetBody(Body)
 }
 
 // ----------------------------------------
@@ -1873,18 +1885,34 @@ func (x *IfStmt) GetBody() Body {
 	panic("IfStmt has no body (but .Then and .Else do)")
 }
 
+func (x *IfStmt) SetBody(b Body) {
+	panic("IfStmt has no body (but .Then and .Else do)")
+}
+
 func (x *SwitchStmt) GetBody() Body {
 	panic("SwitchStmt has no body (but its cases do)")
 }
 
+func (x *SwitchStmt) SetBody(b Body) {
+	panic("SwitchStmt has no body (but its cases do)")
+}
+
 func (x *FileNode) GetBody() Body {
 	panic("FileNode has no body (but it does have .Decls)")
 }
 
+func (x *FileNode) SetBody(b Body) {
+	panic("FileNode has no body (but it does have .Decls)")
+}
+
 func (x *PackageNode) GetBody() Body {
 	panic("PackageNode has no body")
 }
 
+func (x *PackageNode) SetBody(b Body) {
+	panic("PackageNode has no body")
+}
+
 // ----------------------------------------
 // Value Path
 

gnovm/pkg/gnolang/preprocess.go

@@ -4,6 +4,7 @@ import (
 	"fmt"
 	"math/big"
 	"reflect"
+	"strings"
 	"sync/atomic"
 
 	"github.com/gnolang/gno/tm2/pkg/errors"
@@ -426,6 +427,24 @@ func Preprocess(store Store, ctx BlockNode, n Node) Node {
 			switch n := n.(type) {
 			// TRANS_ENTER -----------------------
 			case *AssignStmt:
+				if n.Op == DEFINE {
+					for _, lx := range n.Lhs {
+						ln := lx.(*NameExpr).Name
+						if ln == blankIdentifier {
+							// ignore.
+						} else if strings.HasPrefix(string(ln), ".decompose_") {
+							_, ok := last.GetLocalIndex(ln)
+							if !ok {
+								// initial declaration to be re-defined.
+								last.Predefine(false, ln)
+							} else {
+								// do not redeclare.
+							}
+						}
+					}
+				} else {
+					// nothing defined.
+				}
 
 			// TRANS_ENTER -----------------------
 			case *ImportDecl, *ValueDecl, *TypeDecl, *FuncDecl:
@@ -1872,7 +1891,88 @@ func Preprocess(store Store, ctx BlockNode, n Node) Node {
 				} else { // ASSIGN, or assignment operation (+=, -=, <<=, etc.)
 					// NOTE: Keep in sync with DEFINE above.
 					if len(n.Lhs) > len(n.Rhs) {
-						// check is done in assertCompatible
+						// check is done in assertCompatible where we also
+						// asserted we have at lease one element in Rhs
+						if cx, ok := n.Rhs[0].(*CallExpr); ok {
+							// we decompose the a,b = x(...) for named and unamed
+							// type value return in an assignments
+							// Call case: a, b = x(...)
+							ift := evalStaticTypeOf(store, last, cx.Func)
+							cft := getGnoFuncTypeOf(store, ift)
+							// check if we we need to decompose for named typed conversion in the function return results
+							var decompose bool
+
+							for i, rhsType := range cft.Results {
+								lt := evalStaticTypeOf(store, last, n.Lhs[i])
+								if lt != nil && isNamedConversion(rhsType.Type, lt) {
+									decompose = true
+									break
+								}
+							}
+							if decompose {
+								// only enter this section if cft.Results to be converted to Lhs type for named type conversion.
+								// decompose a,b = x()
+								// .decompose1, .decompose2 := x()  assignment statement expression (Op=DEFINE)
+								// a,b = .decompose1, .decompose2   assignment statement expression ( Op=ASSIGN )
+								// add the new statement to last.Body
+
+								// step1:
+								// create a hidden var with leading . (dot) the curBodyLen increase every time when there is a decomposition
+								// because there could be multiple decomposition happens
+								// we use both stmt index and return result number to differentiate the .decompose variables created in each assignment decompostion
+								// ex. .decompose_3_2: this variable is created as the 3rd statement in the block, the 2nd parameter returned from x(),
+								// create .decompose_1_1, .decompose_1_2 .... based on number of result from x()
+								tmpExprs := make(Exprs, 0, len(cft.Results))
+								for i := range cft.Results {
+									rn := fmt.Sprintf(".decompose_%d_%d", index, i)
+									tmpExprs = append(tmpExprs, Nx(rn))
+								}
+								// step2:
+								// .decompose1, .decompose2 := x()
+								dsx := &AssignStmt{
+									Lhs: tmpExprs,
+									Op:  DEFINE,
+									Rhs: n.Rhs,
+								}
+								dsx.SetLine(n.Line)
+								dsx = Preprocess(store, last, dsx).(*AssignStmt)
+
+								// step3:
+
+								// a,b = .decompose1, .decompose2
+								// assign stmt expression
+								// The right-hand side will be converted to a call expression for named/unnamed conversion.
+								// tmpExprs is a []Expr; we make a copy of tmpExprs to prevent dsx.Lhs in the previous statement (dsx) from being changed by side effects.
+								// If we don't copy tmpExprs, when asx.Rhs is converted to a const call expression during the preprocessing of the AssignStmt asx,
+								// dsx.Lhs will change from []NameExpr to []CallExpr.
+								// This side effect would cause a panic when the machine executes the dsx statement, as it expects Lhs to be []NameExpr.
+
+								asx := &AssignStmt{
+									Lhs: n.Lhs,
+									Op:  ASSIGN,
+									Rhs: copyExprs(tmpExprs),
+								}
+								asx.SetLine(n.Line)
+								asx = Preprocess(store, last, asx).(*AssignStmt)
+
+								// step4:
+								// replace the original stmt with two new stmts
+								body := last.GetBody()
+								// we need to do an in-place replacement while leaving the current node
+								n.Attributes = dsx.Attributes
+								n.Lhs = dsx.Lhs
+								n.Op = dsx.Op
+								n.Rhs = dsx.Rhs
+
+								//  insert a assignment statement a,b = .decompose1,.decompose2 AFTER the current statement in the last.Body.
+								body = append(body[:index+1], append(Body{asx}, body[index+1:]...)...)
+								last.SetBody(body)
+							} // end of the decomposition
+
+							// Last step: we need to insert the statements to FuncValue.body of PackageNopde.Values[i].V
+							// updating FuncValue.body=FuncValue.Source.Body in updates := pn.PrepareNewValues(pv) during preprocess.
+							// we updated FuncValue from source.
+						}
 					} else { // len(Lhs) == len(Rhs)
 						if n.Op == SHL_ASSIGN || n.Op == SHR_ASSIGN {
 							if len(n.Lhs) != 1 || len(n.Rhs) != 1 {
@@ -3160,7 +3260,7 @@ func predefineNow2(store Store, last BlockNode, d Decl, m map[Name]struct{}) (De
 			} else {
 				panic("should not happen")
 			}
-
+			// The body may get altered during preprocessing later.
 			if !dt.TryDefineMethod(&FuncValue{
 				Type:       ft,
 				IsMethod:   true,
@@ -3388,6 +3488,7 @@ func tryPredefine(store Store, last BlockNode, d Decl) (un Name) {
 			pkg := skipFile(last).(*PackageNode)
 			// define a FuncValue w/ above type as d.Name.
 			// fill in later during *FuncDecl:BLOCK.
+			// The body may get altered during preprocessing later.
 			fv := &FuncValue{
 				Type:       ft,
 				IsMethod:   false,

gnovm/pkg/gnolang/transcribe.go

@@ -271,7 +271,8 @@ func transcribe(t Transform, ns []Node, ftype TransField, index int, n Node, nc
 		} else {
 			cnn = cnn2.(*FuncLitExpr)
 		}
-		for idx := range cnn.Body {
+		// iterate over Body; its length can change if a statement is decomposed.
+		for idx := 0; idx < len(cnn.Body); idx++ {
 			cnn.Body[idx] = transcribe(t, nns, TRANS_FUNCLIT_BODY, idx, cnn.Body[idx], &c).(Stmt)
 			if isBreak(c) {
 				break
@@ -383,7 +384,8 @@ func transcribe(t Transform, ns []Node, ftype TransField, index int, n Node, nc
 		} else {
 			cnn = cnn2.(*BlockStmt)
 		}
-		for idx := range cnn.Body {
+		// iterate over Body; its length can change if a statement is decomposed.
+		for idx := 0; idx < len(cnn.Body); idx++ {
 			cnn.Body[idx] = transcribe(t, nns, TRANS_BLOCK_BODY, idx, cnn.Body[idx], &c).(Stmt)
 			if isBreak(c) {
 				break
@@ -393,7 +395,8 @@ func transcribe(t Transform, ns []Node, ftype TransField, index int, n Node, nc
 		}
 	case *BranchStmt:
 	case *DeclStmt:
-		for idx := range cnn.Body {
+		// iterate over Body; its length can change if a statement is decomposed.
+		for idx := 0; idx < len(cnn.Body); idx++ {
 			cnn.Body[idx] = transcribe(t, nns, TRANS_DECL_BODY, idx, cnn.Body[idx], &c).(SimpleDeclStmt)
 			if isBreak(c) {
 				break
@@ -438,7 +441,8 @@ func transcribe(t Transform, ns []Node, ftype TransField, index int, n Node, nc
 				return
 			}
 		}
-		for idx := range cnn.Body {
+		// iterate over Body; its length can change if a statement is decomposed.
+		for idx := 0; idx < len(cnn.Body); idx++ {
 			cnn.Body[idx] = transcribe(t, nns, TRANS_FOR_BODY, idx, cnn.Body[idx], &c).(Stmt)
 			if isBreak(c) {
 				break
@@ -488,7 +492,8 @@ func transcribe(t Transform, ns []Node, ftype TransField, index int, n Node, nc
 		} else {
 			cnn = cnn2.(*IfCaseStmt)
 		}
-		for idx := range cnn.Body {
+		// iterate over Body; its length can change if a statement is decomposed.
+		for idx := 0; idx < len(cnn.Body); idx++ {
 			cnn.Body[idx] = transcribe(t, nns, TRANS_IF_CASE_BODY, idx, cnn.Body[idx], &c).(Stmt)
 			if isBreak(c) {
 				break
@@ -525,7 +530,8 @@ func transcribe(t Transform, ns []Node, ftype TransField, index int, n Node, nc
 				return
 			}
 		}
-		for idx := range cnn.Body {
+		// iterate over Body; its length can change if a statement is decomposed.
+		for idx := 0; idx < len(cnn.Body); idx++ {
 			cnn.Body[idx] = transcribe(t, nns, TRANS_RANGE_BODY, idx, cnn.Body[idx], &c).(Stmt)
 			if isBreak(c) {
 				break
@@ -565,7 +571,8 @@ func transcribe(t Transform, ns []Node, ftype TransField, index int, n Node, nc
 		if isStopOrSkip(nc, c) {
 			return
 		}
-		for idx := range cnn.Body {
+		// iterate over Body; its length can change if a statement is decomposed.
+		for idx := 0; idx < len(cnn.Body); idx++ {
 			cnn.Body[idx] = transcribe(t, nns, TRANS_SELECTCASE_BODY, idx, cnn.Body[idx], &c).(Stmt)
 			if isBreak(c) {
 				break
@@ -640,7 +647,8 @@ func transcribe(t Transform, ns []Node, ftype TransField, index int, n Node, nc
 				return
 			}
 		}
-		for idx := range cnn.Body {
+		// iterate over Body; its length can change if a statement is decomposed.
+		for idx := 0; idx < len(cnn.Body); idx++ {
 			cnn.Body[idx] = transcribe(t, nns, TRANS_SWITCHCASE_BODY, idx, cnn.Body[idx], &c).(Stmt)
 			if isBreak(c) {
 				break
@@ -666,7 +674,8 @@ func transcribe(t Transform, ns []Node, ftype TransField, index int, n Node, nc
 		} else {
 			cnn = cnn2.(*FuncDecl)
 		}
-		for idx := range cnn.Body {
+		// iterate over Body; its length can change if a statement is decomposed.
+		for idx := 0; idx < len(cnn.Body); idx++ {
 			cnn.Body[idx] = transcribe(t, nns, TRANS_FUNC_BODY, idx, cnn.Body[idx], &c).(Stmt)
 			if isBreak(c) {
 				break

gnovm/pkg/gnolang/values.go

@@ -601,6 +601,15 @@ func (fv *FuncValue) GetBodyFromSource(store Store) []Stmt {
 	return fv.body
 }
 
+func (fv *FuncValue) UpdateBodyFromSource() {
+	if fv.Source == nil {
+		panic(fmt.Sprintf(
+			"Source is missing for FuncValue %q",
+			fv.Name))
+	}
+	fv.body = fv.Source.GetBody()
+}
+
 func (fv *FuncValue) GetSource(store Store) BlockNode {
 	if rn, ok := fv.Source.(RefNode); ok {
 		source := store.GetBlockNode(rn.GetLocation())

gnovm/tests/files/assign_unnamed_type/decompose/decompose_unnamed1a_filetest.gno

@@ -0,0 +1,26 @@
+package main
+
+type nat []int
+
+func x() (nat, []int) {
+	a := nat{1}
+	b := []int{2}
+	return a, b
+}
+
+func main() {
+	var u1 []int
+	var n2 nat
+
+	_, n2 = x()
+	// .tmp1, .tmp_2  := x()
+	// _, u2 = .tmp1, .tmp_2
+
+	println(u1)
+	println(n2)
+
+}
+
+// Output:
+// (nil []int)
+// (slice[(2 int)] main.nat)

gnovm/tests/files/assign_unnamed_type/decompose/decompose_unnamed1b_filetest.gno

@@ -0,0 +1,26 @@
+package main
+
+type nat []int
+
+func x() (nat, []int) {
+	a := nat{1}
+	b := []int{2}
+	return a, b
+}
+
+func main() {
+	var u1 []int
+	var n2 nat
+
+	u1, _ = x()
+	// .tmp1, .tmp_2  := x()
+	// u1, _ = .tmp1, .tmp_2
+
+	println(u1)
+	println(n2)
+
+}
+
+// Output:
+// slice[(1 int)]
+// (nil main.nat)