Rewrite the Rust expression parser

The Rust expression parser was written to construct its own AST, then
lower this to GDB expressions.  I did this primarily because the old
expressions were difficult to work with; after rewriting those, I
realized I could remove the AST from the Rust parser.

After looking at this, I realized it might be simpler to rewrite the
parser.  This patch reimplements it as a recursive-descent parser.  I
kept a fair amount of the existing code -- the lexer is pulled in
nearly unchanged.

There are several benefits to this approach:

* The parser is shorter now (from 2882 LOC to 2351).
* The parser is just ordinary C++ code that can be debugged in the
  usual way.
* Memory management in the parser is now straightforward, as
  parsing methods simply return a unique pointer or vector.

This required a couple of minor changes to the test suite, as some
errors have changed.

While this passes the tests, it's possible there are lurking bugs,
particularly around error handling.

gdb/ChangeLog
2021-04-16  Tom Tromey  <tom@tromey.com>

	* rust-parse.c: New file.
	* rust-exp.y: Remove.
	* Makefile.in (COMMON_SFILES): Add rust-parse.c.
	(SFILES): Remove rust-exp.y.
	(YYFILES, local-maintainer-clean): Remove rust-exp.c.

gdb/testsuite/ChangeLog
2021-04-16  Tom Tromey  <tom@tromey.com>

	* gdb.rust/simple.exp: Change error text.
	* gdb.rust/expr.exp: Change error text.

gdb/ChangeLog

@@ -1,3 +1,11 @@
+2021-04-16  Tom Tromey  <tom@tromey.com>
+
+	* rust-parse.c: New file.
+	* rust-exp.y: Remove.
+	* Makefile.in (COMMON_SFILES): Add rust-parse.c.
+	(SFILES): Remove rust-exp.y.
+	(YYFILES, local-maintainer-clean): Remove rust-exp.c.
+
 2021-04-16  Luis Machado  <luis.machado@linaro.org>
 
 	* arch-utils.c (default_floatformat_for_type): Handle bfloat16.

gdb/Makefile.in

@@ -1134,6 +1134,7 @@ COMMON_SFILES = \
 	reverse.c \
 	run-on-main-thread.c \
 	rust-lang.c \
+	rust-parse.c \
 	sentinel-frame.c \
 	ser-event.c \
 	serial.c \
@@ -1200,7 +1201,6 @@ SFILES = \
 	m2-exp.y \
 	p-exp.y \
 	proc-service.list \
-	rust-exp.y \
 	ser-base.c \
 	ser-unix.c \
 	sol-thread.c \
@@ -1608,8 +1608,7 @@ YYFILES = \
 	f-exp.c \
 	go-exp.c \
 	m2-exp.c \
-	p-exp.c \
-	rust-exp.c
+	p-exp.c
 
 # ada-lex.c is included by another file, so it shouldn't wind up as a
 # .o itself.
@@ -1969,7 +1968,7 @@ local-maintainer-clean:
 	rm -f c-exp.c \
 		cp-name-parser.c \
 		ada-lex.c ada-exp.c \
-		d-exp.c f-exp.c go-exp.c m2-exp.c p-exp.c rust-exp.c
+		d-exp.c f-exp.c go-exp.c m2-exp.c p-exp.c
 	rm -f TAGS
 	rm -f $(YYFILES)
 	rm -f nm.h config.status

gdb/testsuite/ChangeLog

@@ -1,3 +1,8 @@
+2021-04-16  Tom Tromey  <tom@tromey.com>
+
+	* gdb.rust/simple.exp: Change error text.
+	* gdb.rust/expr.exp: Change error text.
+
 2021-04-16  Tom Tromey  <tom@tromey.com>
 
 	* gdb.rust/simple.exp: Add parens to 'as' test.

gdb/testsuite/gdb.rust/expr.exp

@@ -136,6 +136,10 @@ gdb_test "print \[mut 23usize; 4\]" " = \\\[23, 23, 23, 23\\\]"
 # Test lexer corner cases.
 gdb_test "print 0x0 as *mut ()" " = \\\(\\*mut \\\(\\\)\\\) 0x0"
 gdb_test "print 0x0 as fn(i64) -> ()" " = \\\(\\*mut fn \\\(i64\\\) -> \\\(\\\)\\\) 0x0"
-gdb_test "print r#" "syntax error in expression, near `#'\\."
+
+# The lexer doesn't treat this as a failure, but rather as two tokens,
+# and we error out while trying to look up 'r'.  This is fine, though
+# -- what's important is that it isn't accepted.
+gdb_test "print r#" "No symbol 'r' in current context"
 
 gdb_test "printf \"%d %d\\n\", 23+1, 23-1" "24 22"

gdb/testsuite/gdb.rust/simple.exp

@@ -165,7 +165,7 @@ gdb_test "print simple::HiBob(0xff, 5)" \
     "Type simple::HiBob is not a tuple struct"
 gdb_test "print sizeof(simple::HiBob)" " = \[0-9\]+"
 gdb_test "print simple::HiBob + 5" \
-    "Found type 'simple::HiBob', which can't be evaluated in this context"
+    "Attempt to use a type name as an expression"
 gdb_test "print nosuchsymbol" \
     "No symbol 'nosuchsymbol' in current context"