Merge #1287

1287: Add AST dump visitor r=CohenArthur a=CohenArthur

This adds a base for pretty printing our AST through a new visitor (Addresses #1261).

This adds enough code to pretty print "functions" without types in the parameters or return type. Looking for feedback on the implementation before we keep going with more code :)

Here's a tiny patch if you want to see the output whenever you're running `-frust-dump-all`:

```diff
diff --git a/gcc/rust/rust-session-manager.cc b/gcc/rust/rust-session-manager.cc
index 0b0e5af915f..3e39c6a6e17 100644
--- a/gcc/rust/rust-session-manager.cc
+++ b/gcc/rust/rust-session-manager.cc
`@@` -31,6 +31,7 `@@`
 #include "rust-cfg-parser.h"
 #include "rust-lint-scan-deadcode.h"
 #include "rust-lint-unused-var.h"
+#include "rust-ast-dump.h"
 
 #include "diagnostic.h"
 #include "input.h"
`@@` -1023,6 +1024,8 `@@` Session::dump_ast (Parser<Lexer> &parser, AST::Crate &crate) const
 
   parser.debug_dump_ast_output (crate, out);
   out.close ();
+
+  AST::Dump (std::cerr).go (crate);
 }
 
 void
```

output:
```rust
arthur@platypus ~/G/r/gccrs (add-ast-dump-visitor) [1]> cat test.rs
fn foo0(a: i32) {
    let a = 15;
    let b: i32 = 15;
}

fn foo1(a: &i32) {}

struct S;

fn foo2(a: S) {}
fn foo3(a: S) -> i32 {}

arthur@platypus ~/G/r/gccrs (add-ast-dump-visitor)> build/gcc/rust1 test.rs -frust-dump-all
fn foo0(a: ) {
	let a = ;
	let b:  = ;

}

fn foo1(a: ) {

}

fn foo2(a: ) {

}

fn foo3(a: ) -> {

}

test.rs:11:1: error: expected [i32] got [()]
   11 | fn foo3(a: S) -> i32 {}
      | ^                ~

Analyzing compilation unit

Time variable                                   usr           sys          wall           GGC
 phase setup                        :   0.00 (  0%)   0.00 (  0%)   0.01 (100%)   129k ( 94%)
 TOTAL                              :   0.00          0.00          0.01          137k
Extra diagnostic checks enabled; compiler may run slowly.
Configure with --enable-checking=release to disable checks.
```

Co-authored-by: Arthur Cohen <arthur.cohen@embecosm.com>

1 files changed, 239 insertions, 0 deletions

diff --git a/gcc/rust/ast/rust-ast-dump.h b/gcc/rust/ast/rust-ast-dump.h
new file mode 100644
index 0000000..436c2b5
--- /dev/null
+++ b/gcc/rust/ast/rust-ast-dump.h
@@ -0,0 +1,239 @@
+// Copyright (C) 2020-2022 Free Software Foundation, Inc.
+
+// This file is part of GCC.
+
+// GCC is free software; you can redistribute it and/or modify it under
+// the terms of the GNU General Public License as published by the Free
+// Software Foundation; either version 3, or (at your option) any later
+// version.
+
+// GCC is distributed in the hope that it will be useful, but WITHOUT ANY
+// WARRANTY; without even the implied warranty of MERCHANTABILITY or
+// FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
+// for more details.
+
+// You should have received a copy of the GNU General Public License
+// along with GCC; see the file COPYING3.  If not see
+// <http://www.gnu.org/licenses/>.
+
+#include "rust-ast-visitor.h"
+#include "rust-ast.h"
+#include "rust-ast-full.h"
+
+#ifndef RUST_AST_DUMP_H
+#define RUST_AST_DUMP_H
+
+namespace Rust {
+namespace AST {
+
+// TODO: We might want to reuse this class somewhere else
+class Indent
+{
+public:
+  Indent ();
+
+  friend std::ostream &operator<< (std::ostream &stream, const Indent &indent);
+
+  void increment ();
+  void decrement ();
+
+private:
+  size_t tabs;
+};
+
+class Dump : public ASTVisitor
+{
+public:
+  Dump (std::ostream &stream);
+
+  /**
+   * Run the visitor on an entire crate and its items
+   */
+  void go (AST::Crate &crate);
+
+private:
+  std::ostream &stream;
+  Indent indentation;
+
+  /**
+   * Format a function's definition parameter
+   */
+  void format_function_param (FunctionParam &param);
+
+  // rust-ast.h
+  void visit (Token &tok);
+  void visit (DelimTokenTree &delim_tok_tree);
+  void visit (AttrInputMetaItemContainer &input);
+  void visit (IdentifierExpr &ident_expr);
+  void visit (Lifetime &lifetime);
+  void visit (LifetimeParam &lifetime_param);
+
+  // rust-path.h
+  void visit (PathInExpression &path);
+  void visit (TypePathSegment &segment);
+  void visit (TypePathSegmentGeneric &segment);
+  void visit (TypePathSegmentFunction &segment);
+  void visit (TypePath &path);
+  void visit (QualifiedPathInExpression &path);
+  void visit (QualifiedPathInType &path);
+
+  // rust-expr.h
+  void visit (LiteralExpr &expr);
+  void visit (AttrInputLiteral &attr_input);
+  void visit (MetaItemLitExpr &meta_item);
+  void visit (MetaItemPathLit &meta_item);
+  void visit (BorrowExpr &expr);
+  void visit (DereferenceExpr &expr);
+  void visit (ErrorPropagationExpr &expr);
+  void visit (NegationExpr &expr);
+  void visit (ArithmeticOrLogicalExpr &expr);
+  void visit (ComparisonExpr &expr);
+  void visit (LazyBooleanExpr &expr);
+  void visit (TypeCastExpr &expr);
+  void visit (AssignmentExpr &expr);
+  void visit (CompoundAssignmentExpr &expr);
+  void visit (GroupedExpr &expr);
+  void visit (ArrayElemsValues &elems);
+  void visit (ArrayElemsCopied &elems);
+  void visit (ArrayExpr &expr);
+  void visit (ArrayIndexExpr &expr);
+  void visit (TupleExpr &expr);
+  void visit (TupleIndexExpr &expr);
+  void visit (StructExprStruct &expr);
+  void visit (StructExprFieldIdentifier &field);
+  void visit (StructExprFieldIdentifierValue &field);
+  void visit (StructExprFieldIndexValue &field);
+  void visit (StructExprStructFields &expr);
+  void visit (StructExprStructBase &expr);
+  void visit (CallExpr &expr);
+  void visit (MethodCallExpr &expr);
+  void visit (FieldAccessExpr &expr);
+  void visit (ClosureExprInner &expr);
+  void visit (BlockExpr &expr);
+  void visit (ClosureExprInnerTyped &expr);
+  void visit (ContinueExpr &expr);
+  void visit (BreakExpr &expr);
+  void visit (RangeFromToExpr &expr);
+  void visit (RangeFromExpr &expr);
+  void visit (RangeToExpr &expr);
+  void visit (RangeFullExpr &expr);
+  void visit (RangeFromToInclExpr &expr);
+  void visit (RangeToInclExpr &expr);
+  void visit (ReturnExpr &expr);
+  void visit (UnsafeBlockExpr &expr);
+  void visit (LoopExpr &expr);
+  void visit (WhileLoopExpr &expr);
+  void visit (WhileLetLoopExpr &expr);
+  void visit (ForLoopExpr &expr);
+  void visit (IfExpr &expr);
+  void visit (IfExprConseqElse &expr);
+  void visit (IfExprConseqIf &expr);
+  void visit (IfExprConseqIfLet &expr);
+  void visit (IfLetExpr &expr);
+  void visit (IfLetExprConseqElse &expr);
+  void visit (IfLetExprConseqIf &expr);
+  void visit (IfLetExprConseqIfLet &expr);
+  void visit (MatchExpr &expr);
+  void visit (AwaitExpr &expr);
+  void visit (AsyncBlockExpr &expr);
+
+  // rust-item.h
+  void visit (TypeParam &param);
+  void visit (LifetimeWhereClauseItem &item);
+  void visit (TypeBoundWhereClauseItem &item);
+  void visit (Method &method);
+  void visit (Module &module);
+  void visit (ExternCrate &crate);
+  void visit (UseTreeGlob &use_tree);
+  void visit (UseTreeList &use_tree);
+  void visit (UseTreeRebind &use_tree);
+  void visit (UseDeclaration &use_decl);
+  void visit (Function &function);
+  void visit (TypeAlias &type_alias);
+  void visit (StructStruct &struct_item);
+  void visit (TupleStruct &tuple_struct);
+  void visit (EnumItem &item);
+  void visit (EnumItemTuple &item);
+  void visit (EnumItemStruct &item);
+  void visit (EnumItemDiscriminant &item);
+  void visit (Enum &enum_item);
+  void visit (Union &union_item);
+  void visit (ConstantItem &const_item);
+  void visit (StaticItem &static_item);
+  void visit (TraitItemFunc &item);
+  void visit (TraitItemMethod &item);
+  void visit (TraitItemConst &item);
+  void visit (TraitItemType &item);
+  void visit (Trait &trait);
+  void visit (InherentImpl &impl);
+  void visit (TraitImpl &impl);
+  void visit (ExternalStaticItem &item);
+  void visit (ExternalFunctionItem &item);
+  void visit (ExternBlock &block);
+
+  // rust-macro.h
+  void visit (MacroMatchFragment &match);
+  void visit (MacroMatchRepetition &match);
+  void visit (MacroMatcher &matcher);
+  void visit (MacroRulesDefinition &rules_def);
+  void visit (MacroInvocation &macro_invoc);
+  void visit (MetaItemPath &meta_item);
+  void visit (MetaItemSeq &meta_item);
+  void visit (MetaWord &meta_item);
+  void visit (MetaNameValueStr &meta_item);
+  void visit (MetaListPaths &meta_item);
+  void visit (MetaListNameValueStr &meta_item);
+
+  // rust-pattern.h
+  void visit (LiteralPattern &pattern);
+  void visit (IdentifierPattern &pattern);
+  void visit (WildcardPattern &pattern);
+  // void visit(RangePatternBound& bound);
+  void visit (RangePatternBoundLiteral &bound);
+  void visit (RangePatternBoundPath &bound);
+  void visit (RangePatternBoundQualPath &bound);
+  void visit (RangePattern &pattern);
+  void visit (ReferencePattern &pattern);
+  // void visit(StructPatternField& field);
+  void visit (StructPatternFieldTuplePat &field);
+  void visit (StructPatternFieldIdentPat &field);
+  void visit (StructPatternFieldIdent &field);
+  void visit (StructPattern &pattern);
+  // void visit(TupleStructItems& tuple_items);
+  void visit (TupleStructItemsNoRange &tuple_items);
+  void visit (TupleStructItemsRange &tuple_items);
+  void visit (TupleStructPattern &pattern);
+  // void visit(TuplePatternItems& tuple_items);
+  void visit (TuplePatternItemsMultiple &tuple_items);
+  void visit (TuplePatternItemsRanged &tuple_items);
+  void visit (TuplePattern &pattern);
+  void visit (GroupedPattern &pattern);
+  void visit (SlicePattern &pattern);
+
+  // rust-stmt.h
+  void visit (EmptyStmt &stmt);
+  void visit (LetStmt &stmt);
+  void visit (ExprStmtWithoutBlock &stmt);
+  void visit (ExprStmtWithBlock &stmt);
+
+  // rust-type.h
+  void visit (TraitBound &bound);
+  void visit (ImplTraitType &type);
+  void visit (TraitObjectType &type);
+  void visit (ParenthesisedType &type);
+  void visit (ImplTraitTypeOneBound &type);
+  void visit (TraitObjectTypeOneBound &type);
+  void visit (TupleType &type);
+  void visit (NeverType &type);
+  void visit (RawPointerType &type);
+  void visit (ReferenceType &type);
+  void visit (ArrayType &type);
+  void visit (SliceType &type);
+  void visit (InferredType &type);
+  void visit (BareFunctionType &type);
+};
+
+} // namespace AST
+} // namespace Rust
+
+#endif // !RUST_AST_DUMP_H