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>

0 files changed, 0 insertions, 0 deletions