Make the tree structure more explicit in parse dumps (#7591)

It's hard to track 2-space indent levels across large vertical gaps, and
it can be hard to suppress the instinct to read the dump as if it were
preorder. This change introduces a new dump mode that addresses both
problems by using box-drawing characters to explicitly represent the
parent-child edges of the tree. This new mode is the default, but the
old behavior remains available with
`--parse-dump-format=yaml-postorder`.

toolchain/parse/testdata/package_expr/basic.carbon

@@ -11,20 +11,19 @@
 var x: package.Foo = package.Bar;
 
 // CHECK:STDOUT: - filename: basic.carbon
-// CHECK:STDOUT:   parse_tree: [
-// CHECK:STDOUT:     {kind: 'FileStart', text: ''},
-// CHECK:STDOUT:       {kind: 'VariableIntroducer', text: 'var'},
-// CHECK:STDOUT:           {kind: 'IdentifierNameNotBeforeSignature', text: 'x'},
-// CHECK:STDOUT:           {kind: 'BindingPatternTypeStart', text: ':'},
-// CHECK:STDOUT:             {kind: 'PackageExpr', text: 'package'},
-// CHECK:STDOUT:             {kind: 'IdentifierNameNotBeforeSignature', text: 'Foo'},
-// CHECK:STDOUT:           {kind: 'MemberAccessExpr', text: '.', subtree_size: 3},
-// CHECK:STDOUT:         {kind: 'VarBindingPattern', text: ':', subtree_size: 6},
-// CHECK:STDOUT:       {kind: 'VariablePattern', text: 'var', subtree_size: 7},
-// CHECK:STDOUT:       {kind: 'VariableInitializer', text: '='},
-// CHECK:STDOUT:         {kind: 'PackageExpr', text: 'package'},
-// CHECK:STDOUT:         {kind: 'IdentifierNameNotBeforeSignature', text: 'Bar'},
-// CHECK:STDOUT:       {kind: 'MemberAccessExpr', text: '.', subtree_size: 3},
-// CHECK:STDOUT:     {kind: 'VariableDecl', text: ';', subtree_size: 13},
-// CHECK:STDOUT:     {kind: 'FileEnd', text: ''},
-// CHECK:STDOUT:   ]
+// CHECK:STDOUT:   ╭─FileStart ''
+// CHECK:STDOUT:   │ ╭─VariableIntroducer 'var'
+// CHECK:STDOUT:   │ │   ╭─IdentifierNameNotBeforeSignature 'x'
+// CHECK:STDOUT:   │ │   ├─BindingPatternTypeStart ':'
+// CHECK:STDOUT:   │ │   │ ╭─PackageExpr 'package'
+// CHECK:STDOUT:   │ │   │ ├─IdentifierNameNotBeforeSignature 'Foo'
+// CHECK:STDOUT:   │ │   ├─MemberAccessExpr '.'
+// CHECK:STDOUT:   │ │ ╭─VarBindingPattern ':'
+// CHECK:STDOUT:   │ ├─VariablePattern 'var'
+// CHECK:STDOUT:   │ ├─VariableInitializer '='
+// CHECK:STDOUT:   │ │ ╭─PackageExpr 'package'
+// CHECK:STDOUT:   │ │ ├─IdentifierNameNotBeforeSignature 'Bar'
+// CHECK:STDOUT:   │ ├─MemberAccessExpr '.'
+// CHECK:STDOUT:   ├─VariableDecl ';'
+// CHECK:STDOUT:   ├─FileEnd ''
+// CHECK:STDOUT:   (root)

toolchain/parse/testdata/package_expr/core.carbon

@@ -11,20 +11,19 @@
 var x: Core.Foo = Core.Bar;
 
 // CHECK:STDOUT: - filename: core.carbon
-// CHECK:STDOUT:   parse_tree: [
-// CHECK:STDOUT:     {kind: 'FileStart', text: ''},
-// CHECK:STDOUT:       {kind: 'VariableIntroducer', text: 'var'},
-// CHECK:STDOUT:           {kind: 'IdentifierNameNotBeforeSignature', text: 'x'},
-// CHECK:STDOUT:           {kind: 'BindingPatternTypeStart', text: ':'},
-// CHECK:STDOUT:             {kind: 'CoreNameExpr', text: 'Core'},
-// CHECK:STDOUT:             {kind: 'IdentifierNameNotBeforeSignature', text: 'Foo'},
-// CHECK:STDOUT:           {kind: 'MemberAccessExpr', text: '.', subtree_size: 3},
-// CHECK:STDOUT:         {kind: 'VarBindingPattern', text: ':', subtree_size: 6},
-// CHECK:STDOUT:       {kind: 'VariablePattern', text: 'var', subtree_size: 7},
-// CHECK:STDOUT:       {kind: 'VariableInitializer', text: '='},
-// CHECK:STDOUT:         {kind: 'CoreNameExpr', text: 'Core'},
-// CHECK:STDOUT:         {kind: 'IdentifierNameNotBeforeSignature', text: 'Bar'},
-// CHECK:STDOUT:       {kind: 'MemberAccessExpr', text: '.', subtree_size: 3},
-// CHECK:STDOUT:     {kind: 'VariableDecl', text: ';', subtree_size: 13},
-// CHECK:STDOUT:     {kind: 'FileEnd', text: ''},
-// CHECK:STDOUT:   ]
+// CHECK:STDOUT:   ╭─FileStart ''
+// CHECK:STDOUT:   │ ╭─VariableIntroducer 'var'
+// CHECK:STDOUT:   │ │   ╭─IdentifierNameNotBeforeSignature 'x'
+// CHECK:STDOUT:   │ │   ├─BindingPatternTypeStart ':'
+// CHECK:STDOUT:   │ │   │ ╭─CoreNameExpr 'Core'
+// CHECK:STDOUT:   │ │   │ ├─IdentifierNameNotBeforeSignature 'Foo'
+// CHECK:STDOUT:   │ │   ├─MemberAccessExpr '.'
+// CHECK:STDOUT:   │ │ ╭─VarBindingPattern ':'
+// CHECK:STDOUT:   │ ├─VariablePattern 'var'
+// CHECK:STDOUT:   │ ├─VariableInitializer '='
+// CHECK:STDOUT:   │ │ ╭─CoreNameExpr 'Core'
+// CHECK:STDOUT:   │ │ ├─IdentifierNameNotBeforeSignature 'Bar'
+// CHECK:STDOUT:   │ ├─MemberAccessExpr '.'
+// CHECK:STDOUT:   ├─VariableDecl ';'
+// CHECK:STDOUT:   ├─FileEnd ''
+// CHECK:STDOUT:   (root)

toolchain/parse/testdata/package_expr/fail_in_name.carbon

@@ -39,25 +39,24 @@ class package.C {
 }
 
 // CHECK:STDOUT: - filename: fail_in_name.carbon
-// CHECK:STDOUT:   parse_tree: [
-// CHECK:STDOUT:     {kind: 'FileStart', text: ''},
-// CHECK:STDOUT:       {kind: 'VariableIntroducer', text: 'var'},
-// CHECK:STDOUT:           {kind: 'PackageExpr', text: 'package'},
-// CHECK:STDOUT:           {kind: 'IdentifierNameNotBeforeSignature', text: 'n'},
-// CHECK:STDOUT:         {kind: 'MemberAccessExpr', text: '.', subtree_size: 3},
-// CHECK:STDOUT:       {kind: 'VariablePattern', text: 'var', has_error: yes, subtree_size: 4},
-// CHECK:STDOUT:     {kind: 'VariableDecl', text: ';', subtree_size: 6},
-// CHECK:STDOUT:       {kind: 'VariableIntroducer', text: 'var'},
-// CHECK:STDOUT:           {kind: 'PackageExpr', text: 'package'},
-// CHECK:STDOUT:           {kind: 'IdentifierNameNotBeforeSignature', text: 'val'},
-// CHECK:STDOUT:         {kind: 'MemberAccessExpr', text: '.', subtree_size: 3},
-// CHECK:STDOUT:       {kind: 'VariablePattern', text: 'var', has_error: yes, subtree_size: 4},
-// CHECK:STDOUT:     {kind: 'VariableDecl', text: ';', subtree_size: 6},
-// CHECK:STDOUT:       {kind: 'NamespaceStart', text: 'namespace'},
-// CHECK:STDOUT:       {kind: 'InvalidParse', text: 'package', has_error: yes},
-// CHECK:STDOUT:     {kind: 'Namespace', text: ';', has_error: yes, subtree_size: 3},
-// CHECK:STDOUT:       {kind: 'ClassIntroducer', text: 'class'},
-// CHECK:STDOUT:       {kind: 'InvalidParse', text: 'package', has_error: yes},
-// CHECK:STDOUT:     {kind: 'ClassDecl', text: '}', has_error: yes, subtree_size: 3},
-// CHECK:STDOUT:     {kind: 'FileEnd', text: ''},
-// CHECK:STDOUT:   ]
+// CHECK:STDOUT:   ╭─FileStart ''
+// CHECK:STDOUT:   │ ╭─VariableIntroducer 'var'
+// CHECK:STDOUT:   │ │   ╭─PackageExpr 'package'
+// CHECK:STDOUT:   │ │   ├─IdentifierNameNotBeforeSignature 'n'
+// CHECK:STDOUT:   │ │ ╭─MemberAccessExpr '.'
+// CHECK:STDOUT:   │ ├─VariablePattern 'var' has_error
+// CHECK:STDOUT:   ├─VariableDecl ';'
+// CHECK:STDOUT:   │ ╭─VariableIntroducer 'var'
+// CHECK:STDOUT:   │ │   ╭─PackageExpr 'package'
+// CHECK:STDOUT:   │ │   ├─IdentifierNameNotBeforeSignature 'val'
+// CHECK:STDOUT:   │ │ ╭─MemberAccessExpr '.'
+// CHECK:STDOUT:   │ ├─VariablePattern 'var' has_error
+// CHECK:STDOUT:   ├─VariableDecl ';'
+// CHECK:STDOUT:   │ ╭─NamespaceStart 'namespace'
+// CHECK:STDOUT:   │ ├─InvalidParse 'package' has_error
+// CHECK:STDOUT:   ├─Namespace ';' has_error
+// CHECK:STDOUT:   │ ╭─ClassIntroducer 'class'
+// CHECK:STDOUT:   │ ├─InvalidParse 'package' has_error
+// CHECK:STDOUT:   ├─ClassDecl '}' has_error
+// CHECK:STDOUT:   ├─FileEnd ''
+// CHECK:STDOUT:   (root)

toolchain/parse/testdata/package_expr/fail_standalone.carbon

@@ -17,15 +17,14 @@ fn F() {
 }
 
 // CHECK:STDOUT: - filename: fail_standalone.carbon
-// CHECK:STDOUT:   parse_tree: [
-// CHECK:STDOUT:     {kind: 'FileStart', text: ''},
-// CHECK:STDOUT:         {kind: 'FunctionIntroducer', text: 'fn'},
-// CHECK:STDOUT:         {kind: 'IdentifierNameMaybeBeforeSignature', text: 'F'},
-// CHECK:STDOUT:           {kind: 'ExplicitParamListStart', text: '('},
-// CHECK:STDOUT:         {kind: 'ExplicitParamList', text: ')', subtree_size: 2},
-// CHECK:STDOUT:       {kind: 'FunctionDefinitionStart', text: '{', subtree_size: 5},
-// CHECK:STDOUT:         {kind: 'PackageExpr', text: 'package'},
-// CHECK:STDOUT:       {kind: 'ExprStatement', text: ';', has_error: yes, subtree_size: 2},
-// CHECK:STDOUT:     {kind: 'FunctionDefinition', text: '}', subtree_size: 8},
-// CHECK:STDOUT:     {kind: 'FileEnd', text: ''},
-// CHECK:STDOUT:   ]
+// CHECK:STDOUT:   ╭─FileStart ''
+// CHECK:STDOUT:   │   ╭─FunctionIntroducer 'fn'
+// CHECK:STDOUT:   │   ├─IdentifierNameMaybeBeforeSignature 'F'
+// CHECK:STDOUT:   │   │ ╭─ExplicitParamListStart '('
+// CHECK:STDOUT:   │   ├─ExplicitParamList ')'
+// CHECK:STDOUT:   │ ╭─FunctionDefinitionStart '{'
+// CHECK:STDOUT:   │ │ ╭─PackageExpr 'package'
+// CHECK:STDOUT:   │ ├─ExprStatement ';' has_error
+// CHECK:STDOUT:   ├─FunctionDefinition '}'
+// CHECK:STDOUT:   ├─FileEnd ''
+// CHECK:STDOUT:   (root)