hcl: Include Expression reference in diagnostics

If a diagnostic occurs while we're evaluating an expression, we'll now
include a reference to that expression in the diagnostic object. We
previously added the corresponding EvalContext here too, and so with these
together it is now possible for a diagnostic renderer to see not only
what was in scope when the problem occurred but also what parts of that
scope the expression was relying on (via method Expression.Variables).

ext/dynblock/expand_spec.go

@@ -47,6 +47,7 @@ func (b *expandBody) decodeSpec(blockS *hcl.BlockHeaderSchema, rawSpec *hcl.Bloc
 			Summary:     "Invalid dynamic for_each value",
 			Detail:      fmt.Sprintf("Cannot use a value of type %s in for_each. An iterable collection is required.", eachVal.Type()),
 			Subject:     eachAttr.Expr.Range().Ptr(),
+			Expression:  eachAttr.Expr,
 			EvalContext: b.forEachCtx,
 		})
 		return nil, diags
@@ -57,6 +58,7 @@ func (b *expandBody) decodeSpec(blockS *hcl.BlockHeaderSchema, rawSpec *hcl.Bloc
 			Summary:     "Invalid dynamic for_each value",
 			Detail:      "Cannot use a null value in for_each.",
 			Subject:     eachAttr.Expr.Range().Ptr(),
+			Expression:  eachAttr.Expr,
 			EvalContext: b.forEachCtx,
 		})
 		return nil, diags
@@ -165,6 +167,7 @@ func (s *expandSpec) newBlock(i *iteration, ctx *hcl.EvalContext) (*hcl.Block, h
 				Summary:     "Invalid dynamic block label",
 				Detail:      fmt.Sprintf("Cannot use this value as a dynamic block label: %s.", convErr),
 				Subject:     labelExpr.Range().Ptr(),
+				Expression:  labelExpr,
 				EvalContext: lCtx,
 			})
 			return nil, diags
@@ -175,6 +178,7 @@ func (s *expandSpec) newBlock(i *iteration, ctx *hcl.EvalContext) (*hcl.Block, h
 				Summary:     "Invalid dynamic block label",
 				Detail:      "Cannot use a null value as a dynamic block label.",
 				Subject:     labelExpr.Range().Ptr(),
+				Expression:  labelExpr,
 				EvalContext: lCtx,
 			})
 			return nil, diags
@@ -185,6 +189,7 @@ func (s *expandSpec) newBlock(i *iteration, ctx *hcl.EvalContext) (*hcl.Block, h
 				Summary:     "Invalid dynamic block label",
 				Detail:      "This value is not yet known. Dynamic block labels must be immediately-known values.",
 				Subject:     labelExpr.Range().Ptr(),
+				Expression:  labelExpr,
 				EvalContext: lCtx,
 			})
 			return nil, diags

hcl/diagnostic.go

@@ -46,22 +46,22 @@ type Diagnostic struct {
 	Subject *Range
 	Context *Range
 
-	// For diagnostics that occur when evaluating an expression, EvalContext
-	// may point to the EvalContext that was active when evaluating that
-	// expression, which may allow for the inclusion of additional useful
-	// information when rendering a diagnostic message to the user.
+	// For diagnostics that occur when evaluating an expression, Expression
+	// may refer to that expression and EvalContext may point to the
+	// EvalContext that was active when evaluating it. This may allow for the
+	// inclusion of additional useful information when rendering a diagnostic
+	// message to the user.
 	//
 	// It is not always possible to select a single EvalContext for a
 	// diagnostic, and so in some cases this field may be nil even when an
-	// expression causes a problem. Therefore it is not valid to use the
-	// nil-ness of this field to definitively decide whether a diagnostic
-	// relates to an expression.
+	// expression causes a problem.
 	//
 	// EvalContexts form a tree, so the given EvalContext may refer to a parent
 	// which in turn refers to another parent, etc. For a full picture of all
 	// of the active variables and functions the caller must walk up this
 	// chain, preferring definitions that are "closer" to the expression in
 	// case of colliding names.
+	Expression  Expression
 	EvalContext *EvalContext
 }
 

hcl/hclsyntax/diagnostics.go

@@ -13,9 +13,10 @@ import (
 // been seen by a caller. Its purpose is to apply additional context to a
 // set of diagnostics produced by a "deeper" component as the stack unwinds
 // during expression evaluation.
-func setDiagEvalContext(diags hcl.Diagnostics, ctx *hcl.EvalContext) {
+func setDiagEvalContext(diags hcl.Diagnostics, expr hcl.Expression, ctx *hcl.EvalContext) {
 	for _, diag := range diags {
-		if diag.EvalContext == nil {
+		if diag.Expression == nil {
+			diag.Expression = expr
 			diag.EvalContext = ctx
 		}
 	}

hcl/hclsyntax/expression.go

@@ -106,7 +106,7 @@ func (e *ScopeTraversalExpr) walkChildNodes(w internalWalkFunc) {
 
 func (e *ScopeTraversalExpr) Value(ctx *hcl.EvalContext) (cty.Value, hcl.Diagnostics) {
 	val, diags := e.Traversal.TraverseAbs(ctx)
-	setDiagEvalContext(diags, ctx)
+	setDiagEvalContext(diags, e, ctx)
 	return val, diags
 }
 
@@ -138,7 +138,7 @@ func (e *RelativeTraversalExpr) walkChildNodes(w internalWalkFunc) {
 func (e *RelativeTraversalExpr) Value(ctx *hcl.EvalContext) (cty.Value, hcl.Diagnostics) {
 	src, diags := e.Source.Value(ctx)
 	ret, travDiags := e.Traversal.TraverseRel(src)
-	setDiagEvalContext(travDiags, ctx)
+	setDiagEvalContext(travDiags, e, ctx)
 	diags = append(diags, travDiags...)
 	return ret, diags
 }
@@ -214,6 +214,7 @@ func (e *FunctionCallExpr) Value(ctx *hcl.EvalContext) (cty.Value, hcl.Diagnosti
 					Summary:     "Function calls not allowed",
 					Detail:      "Functions may not be called here.",
 					Subject:     e.Range().Ptr(),
+					Expression:  e,
 					EvalContext: ctx,
 				},
 			}
@@ -235,6 +236,7 @@ func (e *FunctionCallExpr) Value(ctx *hcl.EvalContext) (cty.Value, hcl.Diagnosti
 				Detail:      fmt.Sprintf("There is no function named %q.%s", e.Name, suggestion),
 				Subject:     &e.NameRange,
 				Context:     e.Range().Ptr(),
+				Expression:  e,
 				EvalContext: ctx,
 			},
 		}
@@ -263,8 +265,9 @@ func (e *FunctionCallExpr) Value(ctx *hcl.EvalContext) (cty.Value, hcl.Diagnosti
 					Severity:    hcl.DiagError,
 					Summary:     "Invalid expanding argument value",
 					Detail:      "The expanding argument (indicated by ...) must not be null.",
-					Context:     expandExpr.Range().Ptr(),
-					Subject:     e.Range().Ptr(),
+					Subject:     expandExpr.Range().Ptr(),
+					Context:     e.Range().Ptr(),
+					Expression:  expandExpr,
 					EvalContext: ctx,
 				})
 				return cty.DynamicVal, diags
@@ -289,8 +292,9 @@ func (e *FunctionCallExpr) Value(ctx *hcl.EvalContext) (cty.Value, hcl.Diagnosti
 				Severity:    hcl.DiagError,
 				Summary:     "Invalid expanding argument value",
 				Detail:      "The expanding argument (indicated by ...) must be of a tuple, list, or set type.",
-				Context:     expandExpr.Range().Ptr(),
-				Subject:     e.Range().Ptr(),
+				Subject:     expandExpr.Range().Ptr(),
+				Context:     e.Range().Ptr(),
+				Expression:  expandExpr,
 				EvalContext: ctx,
 			})
 			return cty.DynamicVal, diags
@@ -313,6 +317,7 @@ func (e *FunctionCallExpr) Value(ctx *hcl.EvalContext) (cty.Value, hcl.Diagnosti
 				),
 				Subject:     &e.CloseParenRange,
 				Context:     e.Range().Ptr(),
+				Expression:  e,
 				EvalContext: ctx,
 			},
 		}
@@ -329,6 +334,7 @@ func (e *FunctionCallExpr) Value(ctx *hcl.EvalContext) (cty.Value, hcl.Diagnosti
 				),
 				Subject:     args[len(params)].StartRange().Ptr(),
 				Context:     e.Range().Ptr(),
+				Expression:  e,
 				EvalContext: ctx,
 			},
 		}
@@ -361,6 +367,7 @@ func (e *FunctionCallExpr) Value(ctx *hcl.EvalContext) (cty.Value, hcl.Diagnosti
 				),
 				Subject:     argExpr.StartRange().Ptr(),
 				Context:     e.Range().Ptr(),
+				Expression:  argExpr,
 				EvalContext: ctx,
 			})
 		}
@@ -399,6 +406,7 @@ func (e *FunctionCallExpr) Value(ctx *hcl.EvalContext) (cty.Value, hcl.Diagnosti
 				),
 				Subject:     argExpr.StartRange().Ptr(),
 				Context:     e.Range().Ptr(),
+				Expression:  argExpr,
 				EvalContext: ctx,
 			})
 
@@ -412,6 +420,7 @@ func (e *FunctionCallExpr) Value(ctx *hcl.EvalContext) (cty.Value, hcl.Diagnosti
 				),
 				Subject:     e.StartRange().Ptr(),
 				Context:     e.Range().Ptr(),
+				Expression:  e,
 				EvalContext: ctx,
 			})
 		}
@@ -481,6 +490,7 @@ func (e *ConditionalExpr) Value(ctx *hcl.EvalContext) (cty.Value, hcl.Diagnostic
 				),
 				Subject:     hcl.RangeBetween(e.TrueResult.Range(), e.FalseResult.Range()).Ptr(),
 				Context:     &e.SrcRange,
+				Expression:  e,
 				EvalContext: ctx,
 			},
 		}
@@ -495,6 +505,7 @@ func (e *ConditionalExpr) Value(ctx *hcl.EvalContext) (cty.Value, hcl.Diagnostic
 			Detail:      "The condition value is null. Conditions must either be true or false.",
 			Subject:     e.Condition.Range().Ptr(),
 			Context:     &e.SrcRange,
+			Expression:  e.Condition,
 			EvalContext: ctx,
 		})
 		return cty.UnknownVal(resultType), diags
@@ -510,6 +521,7 @@ func (e *ConditionalExpr) Value(ctx *hcl.EvalContext) (cty.Value, hcl.Diagnostic
 			Detail:      fmt.Sprintf("The condition expression must be of type bool."),
 			Subject:     e.Condition.Range().Ptr(),
 			Context:     &e.SrcRange,
+			Expression:  e.Condition,
 			EvalContext: ctx,
 		})
 		return cty.UnknownVal(resultType), diags
@@ -531,6 +543,7 @@ func (e *ConditionalExpr) Value(ctx *hcl.EvalContext) (cty.Value, hcl.Diagnostic
 					),
 					Subject:     e.TrueResult.Range().Ptr(),
 					Context:     &e.SrcRange,
+					Expression:  e.TrueResult,
 					EvalContext: ctx,
 				})
 				trueResult = cty.UnknownVal(resultType)
@@ -551,8 +564,9 @@ func (e *ConditionalExpr) Value(ctx *hcl.EvalContext) (cty.Value, hcl.Diagnostic
 						"The false result value has the wrong type: %s.",
 						err.Error(),
 					),
-					Subject:     e.TrueResult.Range().Ptr(),
+					Subject:     e.FalseResult.Range().Ptr(),
 					Context:     &e.SrcRange,
+					Expression:  e.FalseResult,
 					EvalContext: ctx,
 				})
 				falseResult = cty.UnknownVal(resultType)
@@ -697,6 +711,7 @@ func (e *ObjectConsExpr) Value(ctx *hcl.EvalContext) (cty.Value, hcl.Diagnostics
 				Summary:     "Null value as key",
 				Detail:      "Can't use a null value as a key.",
 				Subject:     item.ValueExpr.Range().Ptr(),
+				Expression:  item.KeyExpr,
 				EvalContext: ctx,
 			})
 			known = false
@@ -711,6 +726,7 @@ func (e *ObjectConsExpr) Value(ctx *hcl.EvalContext) (cty.Value, hcl.Diagnostics
 				Summary:     "Incorrect key type",
 				Detail:      fmt.Sprintf("Can't use this value as a key: %s.", err.Error()),
 				Subject:     item.ValueExpr.Range().Ptr(),
+				Expression:  item.ValueExpr,
 				EvalContext: ctx,
 			})
 			known = false

hcl/hclsyntax/expression_ops.go

@@ -154,6 +154,7 @@ func (e *BinaryOpExpr) Value(ctx *hcl.EvalContext) (cty.Value, hcl.Diagnostics)
 			Detail:      fmt.Sprintf("Unsuitable value for left operand: %s.", err),
 			Subject:     e.LHS.Range().Ptr(),
 			Context:     &e.SrcRange,
+			Expression:  e.LHS,
 			EvalContext: ctx,
 		})
 	}
@@ -165,6 +166,7 @@ func (e *BinaryOpExpr) Value(ctx *hcl.EvalContext) (cty.Value, hcl.Diagnostics)
 			Detail:      fmt.Sprintf("Unsuitable value for right operand: %s.", err),
 			Subject:     e.RHS.Range().Ptr(),
 			Context:     &e.SrcRange,
+			Expression:  e.RHS,
 			EvalContext: ctx,
 		})
 	}
@@ -184,6 +186,7 @@ func (e *BinaryOpExpr) Value(ctx *hcl.EvalContext) (cty.Value, hcl.Diagnostics)
 			Summary:     "Operation failed",
 			Detail:      fmt.Sprintf("Error during operation: %s.", err),
 			Subject:     &e.SrcRange,
+			Expression:  e,
 			EvalContext: ctx,
 		})
 		return cty.UnknownVal(e.Op.Type), diags
@@ -227,6 +230,7 @@ func (e *UnaryOpExpr) Value(ctx *hcl.EvalContext) (cty.Value, hcl.Diagnostics) {
 			Detail:      fmt.Sprintf("Unsuitable value for unary operand: %s.", err),
 			Subject:     e.Val.Range().Ptr(),
 			Context:     &e.SrcRange,
+			Expression:  e.Val,
 			EvalContext: ctx,
 		})
 	}
@@ -246,6 +250,7 @@ func (e *UnaryOpExpr) Value(ctx *hcl.EvalContext) (cty.Value, hcl.Diagnostics) {
 			Summary:     "Operation failed",
 			Detail:      fmt.Sprintf("Error during operation: %s.", err),
 			Subject:     &e.SrcRange,
+			Expression:  e,
 			EvalContext: ctx,
 		})
 		return cty.UnknownVal(e.Op.Type), diags

hcl/hclsyntax/expression_template.go

@@ -39,6 +39,7 @@ func (e *TemplateExpr) Value(ctx *hcl.EvalContext) (cty.Value, hcl.Diagnostics)
 				),
 				Subject:     part.Range().Ptr(),
 				Context:     &e.SrcRange,
+				Expression:  part,
 				EvalContext: ctx,
 			})
 			continue
@@ -64,6 +65,7 @@ func (e *TemplateExpr) Value(ctx *hcl.EvalContext) (cty.Value, hcl.Diagnostics)
 				),
 				Subject:     part.Range().Ptr(),
 				Context:     &e.SrcRange,
+				Expression:  part,
 				EvalContext: ctx,
 			})
 			continue
@@ -130,6 +132,7 @@ func (e *TemplateJoinExpr) Value(ctx *hcl.EvalContext) (cty.Value, hcl.Diagnosti
 					"An iteration result is null. Cannot include a null value in a string template.",
 				),
 				Subject:     e.Range().Ptr(),
+				Expression:  e,
 				EvalContext: ctx,
 			})
 			continue
@@ -147,6 +150,7 @@ func (e *TemplateJoinExpr) Value(ctx *hcl.EvalContext) (cty.Value, hcl.Diagnosti
 					err.Error(),
 				),
 				Subject:     e.Range().Ptr(),
+				Expression:  e,
 				EvalContext: ctx,
 			})
 			continue

hcl/json/structure.go

@@ -432,7 +432,8 @@ func (e *expression) Value(ctx *hcl.EvalContext) (cty.Value, hcl.Diagnostics) {
 				Value:    jsonAttr.Name,
 				SrcRange: jsonAttr.NameRange,
 			}}).Value(ctx)
-			val, valDiags := (&expression{src: jsonAttr.Value}).Value(ctx)
+			valExpr := &expression{src: jsonAttr.Value}
+			val, valDiags := valExpr.Value(ctx)
 			diags = append(diags, nameDiags...)
 			diags = append(diags, valDiags...)
 
@@ -444,6 +445,7 @@ func (e *expression) Value(ctx *hcl.EvalContext) (cty.Value, hcl.Diagnostics) {
 					Summary:     "Invalid object key expression",
 					Detail:      fmt.Sprintf("Cannot use this expression as an object key: %s.", err),
 					Subject:     &jsonAttr.NameRange,
+					Expression:  valExpr,
 					EvalContext: ctx,
 				})
 				continue
@@ -454,6 +456,7 @@ func (e *expression) Value(ctx *hcl.EvalContext) (cty.Value, hcl.Diagnostics) {
 					Summary:     "Invalid object key expression",
 					Detail:      "Cannot use null value as an object key.",
 					Subject:     &jsonAttr.NameRange,
+					Expression:  valExpr,
 					EvalContext: ctx,
 				})
 				continue
@@ -477,6 +480,7 @@ func (e *expression) Value(ctx *hcl.EvalContext) (cty.Value, hcl.Diagnostics) {
 					Summary:     "Duplicate object attribute",
 					Detail:      fmt.Sprintf("An attribute named %q was already defined at %s.", nameStr, attrRanges[nameStr]),
 					Subject:     &jsonAttr.NameRange,
+					Expression:  e,
 					EvalContext: ctx,
 				})
 				continue