hclsyntax: report expr and ctx correctly in ForExpr diagnostics

We previously weren't returning appropriate Expression and EvalContext
references inside many of the diagnostics for ForExpr.

First, it was using the top-level expression instead of one of the nested
expressions in many cases. Secondly, it was using the given context
rather than the child context when talking about expressions that get
evaluated once per iteration.

As a result of this reporting we must now produce a new EvalContext for
each iteration, rather than sharing and mutating as we did before, but
in retrospect that's less likely to cause other confusing bugs anyway,
since we don't generally expect EvalContexts to be mutated.

hcl/hclsyntax/expression.go

@@ -859,6 +859,7 @@ func (e *ForExpr) Value(ctx *hcl.EvalContext) (cty.Value, hcl.Diagnostics) {
 			Detail:      "A null value cannot be used as the collection in a 'for' expression.",
 			Subject:     e.CollExpr.Range().Ptr(),
 			Context:     &e.SrcRange,
+			Expression:  e.CollExpr,
 			EvalContext: ctx,
 		})
 		return cty.DynamicVal, diags
@@ -876,6 +877,7 @@ func (e *ForExpr) Value(ctx *hcl.EvalContext) (cty.Value, hcl.Diagnostics) {
 			),
 			Subject:     e.CollExpr.Range().Ptr(),
 			Context:     &e.SrcRange,
+			Expression:  e.CollExpr,
 			EvalContext: ctx,
 		})
 		return cty.DynamicVal, diags
@@ -884,14 +886,13 @@ func (e *ForExpr) Value(ctx *hcl.EvalContext) (cty.Value, hcl.Diagnostics) {
 		return cty.DynamicVal, diags
 	}
 
-	childCtx := ctx.NewChild()
-	childCtx.Variables = map[string]cty.Value{}
-
 	// Before we start we'll do an early check to see if any CondExpr we've
 	// been given is of the wrong type. This isn't 100% reliable (it may
 	// be DynamicVal until real values are given) but it should catch some
 	// straightforward cases and prevent a barrage of repeated errors.
 	if e.CondExpr != nil {
+		childCtx := ctx.NewChild()
+		childCtx.Variables = map[string]cty.Value{}
 		if e.KeyVar != "" {
 			childCtx.Variables[e.KeyVar] = cty.DynamicVal
 		}
@@ -906,6 +907,7 @@ func (e *ForExpr) Value(ctx *hcl.EvalContext) (cty.Value, hcl.Diagnostics) {
 				Detail:      "The value of the 'if' clause must not be null.",
 				Subject:     e.CondExpr.Range().Ptr(),
 				Context:     &e.SrcRange,
+				Expression:  e.CondExpr,
 				EvalContext: ctx,
 			})
 			return cty.DynamicVal, diags
@@ -918,6 +920,7 @@ func (e *ForExpr) Value(ctx *hcl.EvalContext) (cty.Value, hcl.Diagnostics) {
 				Detail:      fmt.Sprintf("The 'if' clause value is invalid: %s.", err.Error()),
 				Subject:     e.CondExpr.Range().Ptr(),
 				Context:     &e.SrcRange,
+				Expression:  e.CondExpr,
 				EvalContext: ctx,
 			})
 			return cty.DynamicVal, diags
@@ -942,6 +945,8 @@ func (e *ForExpr) Value(ctx *hcl.EvalContext) (cty.Value, hcl.Diagnostics) {
 		known := true
 		for it.Next() {
 			k, v := it.Element()
+			childCtx := ctx.NewChild()
+			childCtx.Variables = map[string]cty.Value{}
 			if e.KeyVar != "" {
 				childCtx.Variables[e.KeyVar] = k
 			}
@@ -958,7 +963,8 @@ func (e *ForExpr) Value(ctx *hcl.EvalContext) (cty.Value, hcl.Diagnostics) {
 							Detail:      "The value of the 'if' clause must not be null.",
 							Subject:     e.CondExpr.Range().Ptr(),
 							Context:     &e.SrcRange,
-							EvalContext: ctx,
+							Expression:  e.CondExpr,
+							EvalContext: childCtx,
 						})
 					}
 					known = false
@@ -973,7 +979,8 @@ func (e *ForExpr) Value(ctx *hcl.EvalContext) (cty.Value, hcl.Diagnostics) {
 							Detail:      fmt.Sprintf("The 'if' clause value is invalid: %s.", err.Error()),
 							Subject:     e.CondExpr.Range().Ptr(),
 							Context:     &e.SrcRange,
-							EvalContext: ctx,
+							Expression:  e.CondExpr,
+							EvalContext: childCtx,
 						})
 					}
 					known = false
@@ -1000,7 +1007,8 @@ func (e *ForExpr) Value(ctx *hcl.EvalContext) (cty.Value, hcl.Diagnostics) {
 						Detail:      "Key expression in 'for' expression must not produce a null value.",
 						Subject:     e.KeyExpr.Range().Ptr(),
 						Context:     &e.SrcRange,
-						EvalContext: ctx,
+						Expression:  e.KeyExpr,
+						EvalContext: childCtx,
 					})
 				}
 				known = false
@@ -1020,7 +1028,8 @@ func (e *ForExpr) Value(ctx *hcl.EvalContext) (cty.Value, hcl.Diagnostics) {
 						Detail:      fmt.Sprintf("The key expression produced an invalid result: %s.", err.Error()),
 						Subject:     e.KeyExpr.Range().Ptr(),
 						Context:     &e.SrcRange,
-						EvalContext: ctx,
+						Expression:  e.KeyExpr,
+						EvalContext: childCtx,
 					})
 				}
 				known = false
@@ -1045,7 +1054,8 @@ func (e *ForExpr) Value(ctx *hcl.EvalContext) (cty.Value, hcl.Diagnostics) {
 						),
 						Subject:     e.KeyExpr.Range().Ptr(),
 						Context:     &e.SrcRange,
-						EvalContext: ctx,
+						Expression:  e.KeyExpr,
+						EvalContext: childCtx,
 					})
 				} else {
 					vals[key.AsString()] = val
@@ -1075,6 +1085,8 @@ func (e *ForExpr) Value(ctx *hcl.EvalContext) (cty.Value, hcl.Diagnostics) {
 		known := true
 		for it.Next() {
 			k, v := it.Element()
+			childCtx := ctx.NewChild()
+			childCtx.Variables = map[string]cty.Value{}
 			if e.KeyVar != "" {
 				childCtx.Variables[e.KeyVar] = k
 			}
@@ -1091,7 +1103,8 @@ func (e *ForExpr) Value(ctx *hcl.EvalContext) (cty.Value, hcl.Diagnostics) {
 							Detail:      "The value of the 'if' clause must not be null.",
 							Subject:     e.CondExpr.Range().Ptr(),
 							Context:     &e.SrcRange,
-							EvalContext: ctx,
+							Expression:  e.CondExpr,
+							EvalContext: childCtx,
 						})
 					}
 					known = false
@@ -1114,7 +1127,8 @@ func (e *ForExpr) Value(ctx *hcl.EvalContext) (cty.Value, hcl.Diagnostics) {
 							Detail:      fmt.Sprintf("The 'if' clause value is invalid: %s.", err.Error()),
 							Subject:     e.CondExpr.Range().Ptr(),
 							Context:     &e.SrcRange,
-							EvalContext: ctx,
+							Expression:  e.CondExpr,
+							EvalContext: childCtx,
 						})
 					}
 					known = false
@@ -1205,6 +1219,7 @@ func (e *SplatExpr) Value(ctx *hcl.EvalContext) (cty.Value, hcl.Diagnostics) {
 			Detail:      "Splat expressions (with the * symbol) cannot be applied to null values.",
 			Subject:     e.Source.Range().Ptr(),
 			Context:     hcl.RangeBetween(e.Source.Range(), e.MarkerRange).Ptr(),
+			Expression:  e.Source,
 			EvalContext: ctx,
 		})
 		return cty.DynamicVal, diags