Rewrite gdb pretty-printer registration

Currently, the Rust pretty-printers are registered in gdb using the
uninformative name "lookup":

    (gdb) info pretty-printer
    global pretty-printers:
    [...]
    objfile /home/tromey/[...]
      lookup

It's nicer for users if the top-level registration is given a clear
name.  Additionally, gdb lets users individually enable and disable
specific printers, provided they are registered correctly.

This patch implements both these ideas.  Now the output looks like:

    (gdb) info pretty-printer
    global pretty-printers:
    [...]
    objfile /home/tromey/[...]
      rust
	StdArc
	StdBTreeMap
	StdBTreeSet
	StdCell
	StdHashMap
	StdHashSet
	StdNonZeroNumber
	StdOsString
	StdRc
	StdRef
	StdRefCell
	StdRefMut
	StdSlice
	StdStr
	StdString
	StdVec
	StdVecDeque

src/etc/gdb_lookup.py

@@ -1,4 +1,5 @@
 import gdb
+import gdb.printing
 import re
 
 from gdb_providers import *
@@ -9,7 +10,7 @@
 gdb_version = [int(num) for num in _gdb_version_matched.groups()] if _gdb_version_matched else []
 
 def register_printers(objfile):
-    objfile.pretty_printers.append(lookup)
+    objfile.pretty_printers.append(printer)
 
 
 # BACKCOMPAT: rust 1.35
@@ -38,58 +39,80 @@ def check_enum_discriminant(valobj):
     return True
 
 
-def lookup(valobj):
-    rust_type = classify_rust_type(valobj.type)
-
-    if rust_type == RustType.ENUM:
-        # use enum provider only for GDB <7.12
-        if gdb_version[0] < 7 or (gdb_version[0] == 7 and gdb_version[1] < 12):
-            if check_enum_discriminant(valobj):
-                return EnumProvider(valobj)
-
-    if rust_type == RustType.STD_STRING:
-        return StdStringProvider(valobj)
-    if rust_type == RustType.STD_OS_STRING:
-        return StdOsStringProvider(valobj)
-    if rust_type == RustType.STD_STR:
-        return StdStrProvider(valobj)
-    if rust_type == RustType.STD_SLICE:
-        return StdSliceProvider(valobj)
-    if rust_type == RustType.STD_VEC:
-        return StdVecProvider(valobj)
-    if rust_type == RustType.STD_VEC_DEQUE:
-        return StdVecDequeProvider(valobj)
-    if rust_type == RustType.STD_BTREE_SET:
-        return StdBTreeSetProvider(valobj)
-    if rust_type == RustType.STD_BTREE_MAP:
-        return StdBTreeMapProvider(valobj)
-    if rust_type == RustType.STD_HASH_MAP:
-        if is_hashbrown_hashmap(valobj):
-            return StdHashMapProvider(valobj)
-        else:
-            return StdOldHashMapProvider(valobj)
-    if rust_type == RustType.STD_HASH_SET:
-        hash_map = valobj[valobj.type.fields()[0]]
-        if is_hashbrown_hashmap(hash_map):
-            return StdHashMapProvider(valobj, show_values=False)
-        else:
-            return StdOldHashMapProvider(hash_map, show_values=False)
-
-    if rust_type == RustType.STD_RC:
-        return StdRcProvider(valobj)
-    if rust_type == RustType.STD_ARC:
-        return StdRcProvider(valobj, is_atomic=True)
-
-    if rust_type == RustType.STD_CELL:
-        return StdCellProvider(valobj)
-    if rust_type == RustType.STD_REF:
-        return StdRefProvider(valobj)
-    if rust_type == RustType.STD_REF_MUT:
-        return StdRefProvider(valobj)
-    if rust_type == RustType.STD_REF_CELL:
-        return StdRefCellProvider(valobj)
-
-    if rust_type == RustType.STD_NONZERO_NUMBER:
-        return StdNonZeroNumberProvider(valobj)
-
+# Helper for enum printing that checks the discriminant.  Only used in
+# older gdb.
+def enum_provider(valobj):
+    if check_enum_discriminant(valobj):
+        return EnumProvider(valobj)
     return None
+
+
+# Helper to handle both old and new hash maps.
+def hashmap_provider(valobj):
+    if is_hashbrown_hashmap(valobj):
+        return StdHashMapProvider(valobj)
+    else:
+        return StdOldHashMapProvider(valobj)
+
+
+# Helper to handle both old and new hash sets.
+def hashset_provider(valobj):
+    hash_map = valobj[valobj.type.fields()[0]]
+    if is_hashbrown_hashmap(hash_map):
+        return StdHashMapProvider(valobj, show_values=False)
+    else:
+        return StdOldHashMapProvider(hash_map, show_values=False)
+
+
+class PrintByRustType(gdb.printing.SubPrettyPrinter):
+    def __init__(self, name, provider):
+        super(PrintByRustType, self).__init__(name)
+        self.provider = provider
+
+    def __call__(self, val):
+        if self.enabled:
+            return self.provider(val)
+        return None
+
+
+class RustPrettyPrinter(gdb.printing.PrettyPrinter):
+    def __init__(self, name):
+        super(RustPrettyPrinter, self).__init__(name, [])
+        self.type_map = {}
+
+    def add(self, rust_type, provider):
+        # Just use the rust_type as the name.
+        printer = PrintByRustType(rust_type, provider)
+        self.type_map[rust_type] = printer
+        self.subprinters.append(printer)
+
+    def __call__(self, valobj):
+        rust_type = classify_rust_type(valobj.type)
+        if rust_type in self.type_map:
+            return self.type_map[rust_type](valobj)
+        return None
+
+
+printer = RustPrettyPrinter("rust")
+# use enum provider only for GDB <7.12
+if gdb_version[0] < 7 or (gdb_version[0] == 7 and gdb_version[1] < 12):
+    printer.add(RustType.ENUM, enum_provider)
+printer.add(RustType.STD_STRING, StdStringProvider)
+printer.add(RustType.STD_OS_STRING, StdOsStringProvider)
+printer.add(RustType.STD_STR, StdStrProvider)
+printer.add(RustType.STD_SLICE, StdSliceProvider)
+printer.add(RustType.STD_VEC, StdVecProvider)
+printer.add(RustType.STD_VEC_DEQUE, StdVecDequeProvider)
+printer.add(RustType.STD_BTREE_SET, StdBTreeSetProvider)
+printer.add(RustType.STD_BTREE_MAP, StdBTreeMapProvider)
+printer.add(RustType.STD_HASH_MAP, hashmap_provider)
+printer.add(RustType.STD_HASH_SET, hashset_provider)
+printer.add(RustType.STD_RC, StdRcProvider)
+printer.add(RustType.STD_ARC, lambda valobj: StdRcProvider(valobj, is_atomic=True))
+
+printer.add(RustType.STD_CELL, StdCellProvider)
+printer.add(RustType.STD_REF, StdRefProvider)
+printer.add(RustType.STD_REF_MUT, StdRefProvider)
+printer.add(RustType.STD_REF_CELL, StdRefCellProvider)
+
+printer.add(RustType.STD_NONZERO_NUMBER, StdNonZeroNumberProvider)