diff --git a/alloc.h b/alloc.h
index 3e4e828db4..7a41bb9eb3 100644
--- a/alloc.h
+++ b/alloc.h
@@ -1,9 +1,11 @@
 #ifndef ALLOC_H
 #define ALLOC_H
 
+struct alloc_state;
 struct tree;
 struct commit;
 struct tag;
+struct repository;
 
 void *alloc_blob_node(struct repository *r);
 void *alloc_tree_node(struct repository *r);
diff --git a/apply.h b/apply.h
index b80d8ba736..0b70e1f3f5 100644
--- a/apply.h
+++ b/apply.h
@@ -1,6 +1,9 @@
 #ifndef APPLY_H
 #define APPLY_H
 
+#include "lockfile.h"
+#include "string-list.h"
+
 enum apply_ws_error_action {
 	nowarn_ws_error,
 	warn_on_ws_error,
diff --git a/archive.h b/archive.h
index 1f9954f7cd..7aba133635 100644
--- a/archive.h
+++ b/archive.h
@@ -1,6 +1,7 @@
 #ifndef ARCHIVE_H
 #define ARCHIVE_H
 
+#include "cache.h"
 #include "pathspec.h"
 
 struct archiver_args {
diff --git a/attr.h b/attr.h
index 442d464db6..3185538bda 100644
--- a/attr.h
+++ b/attr.h
@@ -7,6 +7,7 @@ struct git_attr;
 /* opaque structures used internally for attribute collection */
 struct all_attrs_item;
 struct attr_stack;
+struct index_state;
 
 /*
  * Given a string, return the gitattribute object that
diff --git a/bisect.h b/bisect.h
index a5d9248a47..34df209351 100644
--- a/bisect.h
+++ b/bisect.h
@@ -1,6 +1,8 @@
 #ifndef BISECT_H
 #define BISECT_H
 
+struct commit_list;
+
 /*
  * Find bisection. If something is found, `reaches` will be the number of
  * commits that the best commit reaches. `all` will be the count of
diff --git a/branch.h b/branch.h
index 473d0a93e9..7d9b330eba 100644
--- a/branch.h
+++ b/branch.h
@@ -1,6 +1,8 @@
 #ifndef BRANCH_H
 #define BRANCH_H
 
+struct strbuf;
+
 /* Functions for acting on the information about branches. */
 
 /*
diff --git a/bulk-checkin.h b/bulk-checkin.h
index a85527318b..f438f93811 100644
--- a/bulk-checkin.h
+++ b/bulk-checkin.h
@@ -4,6 +4,8 @@
 #ifndef BULK_CHECKIN_H
 #define BULK_CHECKIN_H
 
+#include "cache.h"
+
 extern int index_bulk_checkin(struct object_id *oid,
 			      int fd, size_t size, enum object_type type,
 			      const char *path, unsigned flags);
diff --git a/column.h b/column.h
index 0a61917fa7..2567a5cf4d 100644
--- a/column.h
+++ b/column.h
@@ -36,6 +36,7 @@ static inline int column_active(unsigned int colopts)
 	return (colopts & COL_ENABLE_MASK) == COL_ENABLED;
 }
 
+struct string_list;
 extern void print_columns(const struct string_list *list, unsigned int colopts,
 			  const struct column_options *opts);
 
diff --git a/commit-graph.h b/commit-graph.h
index 76e098934a..eea62f8c0e 100644
--- a/commit-graph.h
+++ b/commit-graph.h
@@ -4,6 +4,7 @@
 #include "git-compat-util.h"
 #include "repository.h"
 #include "string-list.h"
+#include "cache.h"
 
 struct commit;
 
diff --git a/config.h b/config.h
index bb2f506b27..75ba1d45ff 100644
--- a/config.h
+++ b/config.h
@@ -1,6 +1,11 @@
 #ifndef CONFIG_H
 #define CONFIG_H
 
+#include "hashmap.h"
+#include "string-list.h"
+
+struct object_id;
+
 /* git_config_parse_key() returns these negated: */
 #define CONFIG_INVALID_KEY 1
 #define CONFIG_NO_SECTION_OR_NAME 2
diff --git a/connected.h b/connected.h
index 322dc76372..e4c961817d 100644
--- a/connected.h
+++ b/connected.h
@@ -1,6 +1,7 @@
 #ifndef CONNECTED_H
 #define CONNECTED_H
 
+struct object_id;
 struct transport;
 
 /*
diff --git a/convert.h b/convert.h
index 01385d9288..76c654385d 100644
--- a/convert.h
+++ b/convert.h
@@ -7,6 +7,8 @@
 #include "string-list.h"
 
 struct index_state;
+struct object_id;
+struct strbuf;
 
 #define CONV_EOL_RNDTRP_DIE   (1<<0) /* Die if CRLF to LF to CRLF is different */
 #define CONV_EOL_RNDTRP_WARN  (1<<1) /* Warn if CRLF to LF to CRLF is different */
diff --git a/csum-file.h b/csum-file.h
index c5a2e335e7..3bf7184736 100644
--- a/csum-file.h
+++ b/csum-file.h
@@ -1,6 +1,8 @@
 #ifndef CSUM_FILE_H
 #define CSUM_FILE_H
 
+#include "hash.h"
+
 struct progress;
 
 /* A SHA1-protected file */
diff --git a/diffcore.h b/diffcore.h
index a30da161da..0dd1e9a63a 100644
--- a/diffcore.h
+++ b/diffcore.h
@@ -4,6 +4,10 @@
 #ifndef DIFFCORE_H
 #define DIFFCORE_H
 
+#include "cache.h"
+
+struct diff_options;
+
 /* This header file is internal between diff.c and its diff transformers
  * (e.g. diffcore-rename, diffcore-pickaxe).  Never include this header
  * in anything else.
diff --git a/dir-iterator.h b/dir-iterator.h
index 27739e6c29..970793d07a 100644
--- a/dir-iterator.h
+++ b/dir-iterator.h
@@ -1,6 +1,8 @@
 #ifndef DIR_ITERATOR_H
 #define DIR_ITERATOR_H
 
+#include "strbuf.h"
+
 /*
  * Iterate over a directory tree.
  *
diff --git a/fsck.h b/fsck.h
index c3cf5e0034..0c7e8c9428 100644
--- a/fsck.h
+++ b/fsck.h
@@ -6,6 +6,7 @@
 #define FSCK_IGNORE 3
 
 struct fsck_options;
+struct object;
 
 void fsck_set_msg_type(struct fsck_options *options,
 		const char *msg_id, const char *msg_type);
diff --git a/fsmonitor.h b/fsmonitor.h
index 65f3743636..01017c43aa 100644
--- a/fsmonitor.h
+++ b/fsmonitor.h
@@ -1,6 +1,9 @@
 #ifndef FSMONITOR_H
 #define FSMONITOR_H
 
+#include "cache.h"
+#include "dir.h"
+
 extern struct trace_key trace_fsmonitor;
 
 /*
diff --git a/gpg-interface.h b/gpg-interface.h
index 5ecff4aa0c..acf50c4610 100644
--- a/gpg-interface.h
+++ b/gpg-interface.h
@@ -1,6 +1,8 @@
 #ifndef GPG_INTERFACE_H
 #define GPG_INTERFACE_H
 
+struct strbuf;
+
 #define GPG_VERIFY_VERBOSE		1
 #define GPG_VERIFY_RAW			2
 #define GPG_VERIFY_OMIT_STATUS	4
diff --git a/khash.h b/khash.h
index c0da40daa7..07b4cc2e67 100644
--- a/khash.h
+++ b/khash.h
@@ -26,6 +26,9 @@
 #ifndef __AC_KHASH_H
 #define __AC_KHASH_H
 
+#include "cache.h"
+#include "hashmap.h"
+
 #define AC_VERSION_KHASH_H "0.2.8"
 
 typedef uint32_t khint32_t;
diff --git a/list-objects-filter.h b/list-objects-filter.h
index a963d0274c..a6f6b4990b 100644
--- a/list-objects-filter.h
+++ b/list-objects-filter.h
@@ -1,6 +1,10 @@
 #ifndef LIST_OBJECTS_FILTER_H
 #define LIST_OBJECTS_FILTER_H
 
+struct list_objects_filter_options;
+struct object;
+struct oidset;
+
 /*
  * During list-object traversal we allow certain objects to be
  * filtered (omitted) from the result.  The active filter uses
diff --git a/list-objects.h b/list-objects.h
index aa618d7f45..ad40762926 100644
--- a/list-objects.h
+++ b/list-objects.h
@@ -1,6 +1,10 @@
 #ifndef LIST_OBJECTS_H
 #define LIST_OBJECTS_H
 
+struct commit;
+struct object;
+struct rev_info;
+
 typedef void (*show_commit_fn)(struct commit *, void *);
 typedef void (*show_object_fn)(struct object *, const char *, void *);
 void traverse_commit_list(struct rev_info *, show_commit_fn, show_object_fn, void *);
diff --git a/ll-merge.h b/ll-merge.h
index 244a31f55a..b72b19921e 100644
--- a/ll-merge.h
+++ b/ll-merge.h
@@ -5,6 +5,8 @@
 #ifndef LL_MERGE_H
 #define LL_MERGE_H
 
+#include "xdiff/xdiff.h"
+
 struct ll_merge_options {
 	unsigned virtual_ancestor : 1;
 	unsigned variant : 2;	/* favor ours, favor theirs, or union merge */
diff --git a/mailinfo.h b/mailinfo.h
index 04a25351d6..766c03dd1d 100644
--- a/mailinfo.h
+++ b/mailinfo.h
@@ -1,6 +1,8 @@
 #ifndef MAILINFO_H
 #define MAILINFO_H
 
+#include "strbuf.h"
+
 #define MAX_BOUNDARIES 5
 
 struct mailinfo {
diff --git a/mailmap.h b/mailmap.h
index ed7c93b05c..d0e65646cb 100644
--- a/mailmap.h
+++ b/mailmap.h
@@ -1,6 +1,8 @@
 #ifndef MAILMAP_H
 #define MAILMAP_H
 
+struct string_list;
+
 int read_mailmap(struct string_list *map, char **repo_abbrev);
 void clear_mailmap(struct string_list *map);
 
diff --git a/merge-recursive.h b/merge-recursive.h
index fa7bc6b683..0c46a5a4ff 100644
--- a/merge-recursive.h
+++ b/merge-recursive.h
@@ -1,8 +1,10 @@
 #ifndef MERGE_RECURSIVE_H
 #define MERGE_RECURSIVE_H
 
-#include "unpack-trees.h"
 #include "string-list.h"
+#include "unpack-trees.h"
+
+struct commit;
 
 struct merge_options {
 	const char *ancestor;
diff --git a/notes-merge.h b/notes-merge.h
index f815f23451..6c74e9385b 100644
--- a/notes-merge.h
+++ b/notes-merge.h
@@ -2,6 +2,10 @@
 #define NOTES_MERGE_H
 
 #include "notes-utils.h"
+#include "strbuf.h"
+
+struct commit;
+struct object_id;
 
 #define NOTES_MERGE_WORKTREE "NOTES_MERGE_WORKTREE"
 
diff --git a/notes-utils.h b/notes-utils.h
index 5d79cbef51..5408306528 100644
--- a/notes-utils.h
+++ b/notes-utils.h
@@ -3,6 +3,9 @@
 
 #include "notes.h"
 
+struct commit_list;
+struct object_id;
+
 /*
  * Create new notes commit from the given notes tree
  *
diff --git a/notes.h b/notes.h
index 0433f45db5..414bc6855a 100644
--- a/notes.h
+++ b/notes.h
@@ -3,6 +3,9 @@
 
 #include "string-list.h"
 
+struct object_id;
+struct strbuf;
+
 /*
  * Function type for combining two notes annotating the same object.
  *
diff --git a/object-store.h b/object-store.h
index e481f7ad41..b6f2c732c4 100644
--- a/object-store.h
+++ b/object-store.h
@@ -1,6 +1,7 @@
 #ifndef OBJECT_STORE_H
 #define OBJECT_STORE_H
 
+#include "cache.h"
 #include "oidmap.h"
 #include "list.h"
 #include "sha1-array.h"
diff --git a/object.h b/object.h
index 177b1a4571..6e28fdd0b4 100644
--- a/object.h
+++ b/object.h
@@ -1,6 +1,8 @@
 #ifndef OBJECT_H
 #define OBJECT_H
 
+#include "cache.h"
+
 struct buffer_slab;
 
 struct parsed_object_pool {
diff --git a/oidmap.h b/oidmap.h
index d3cd2bb590..72430b611e 100644
--- a/oidmap.h
+++ b/oidmap.h
@@ -1,6 +1,7 @@
 #ifndef OIDMAP_H
 #define OIDMAP_H
 
+#include "cache.h"
 #include "hashmap.h"
 
 /*
diff --git a/pack-bitmap.h b/pack-bitmap.h
index 4555907dee..8a04741e12 100644
--- a/pack-bitmap.h
+++ b/pack-bitmap.h
@@ -5,6 +5,9 @@
 #include "khash.h"
 #include "pack-objects.h"
 
+struct commit;
+struct rev_info;
+
 struct bitmap_disk_header {
 	char magic[4];
 	uint16_t version;
diff --git a/pack-objects.h b/pack-objects.h
index edf74dabdd..08c6b57d49 100644
--- a/pack-objects.h
+++ b/pack-objects.h
@@ -2,6 +2,7 @@
 #define PACK_OBJECTS_H
 
 #include "object-store.h"
+#include "pack.h"
 
 #define DEFAULT_DELTA_CACHE_SIZE (256 * 1024 * 1024)
 
diff --git a/patch-ids.h b/patch-ids.h
index bec0f727a7..79ac9a8498 100644
--- a/patch-ids.h
+++ b/patch-ids.h
@@ -1,6 +1,12 @@
 #ifndef PATCH_IDS_H
 #define PATCH_IDS_H
 
+#include "diff.h"
+#include "hashmap.h"
+
+struct commit;
+struct object_id;
+
 struct patch_id {
 	struct hashmap_entry ent;
 	struct object_id patch_id;
diff --git a/path.h b/path.h
index ed6732e5a2..b654ea8ff5 100644
--- a/path.h
+++ b/path.h
@@ -2,6 +2,7 @@
 #define PATH_H
 
 struct repository;
+struct strbuf;
 
 /*
  * The result to all functions which return statically allocated memory may be
diff --git a/pathspec.h b/pathspec.h
index 099a170c2e..a6525a6551 100644
--- a/pathspec.h
+++ b/pathspec.h
@@ -1,6 +1,8 @@
 #ifndef PATHSPEC_H
 #define PATHSPEC_H
 
+struct index_state;
+
 /* Pathspec magic */
 #define PATHSPEC_FROMTOP	(1<<0)
 #define PATHSPEC_MAXDEPTH	(1<<1)
diff --git a/pretty.h b/pretty.h
index 5c85d94e33..7359d318a9 100644
--- a/pretty.h
+++ b/pretty.h
@@ -1,7 +1,11 @@
 #ifndef PRETTY_H
 #define PRETTY_H
 
+#include "cache.h"
+#include "string-list.h"
+
 struct commit;
+struct strbuf;
 
 /* Commit formats */
 enum cmit_fmt {
diff --git a/reachable.h b/reachable.h
index 3c00fa0526..18b0f9f2f0 100644
--- a/reachable.h
+++ b/reachable.h
@@ -2,6 +2,8 @@
 #define REACHEABLE_H
 
 struct progress;
+struct rev_info;
+
 extern int add_unseen_recent_objects_to_traversal(struct rev_info *revs,
 						  timestamp_t timestamp);
 extern void mark_reachable_objects(struct rev_info *revs, int mark_reflog,
diff --git a/reflog-walk.h b/reflog-walk.h
index 7553c448fe..cb3e73755d 100644
--- a/reflog-walk.h
+++ b/reflog-walk.h
@@ -3,6 +3,7 @@
 
 #include "cache.h"
 
+struct commit;
 struct reflog_walk_info;
 
 extern void init_reflog_walk(struct reflog_walk_info **info);
diff --git a/refs.h b/refs.h
index cc2fb4c68c..bd52c1bbae 100644
--- a/refs.h
+++ b/refs.h
@@ -3,8 +3,10 @@
 
 struct object_id;
 struct ref_store;
+struct repository;
 struct strbuf;
 struct string_list;
+struct string_list_item;
 struct worktree;
 
 /*
diff --git a/remote.h b/remote.h
index 976292152c..88f8480c71 100644
--- a/remote.h
+++ b/remote.h
@@ -1,6 +1,7 @@
 #ifndef REMOTE_H
 #define REMOTE_H
 
+#include "cache.h"
 #include "parse-options.h"
 #include "hashmap.h"
 #include "refspec.h"
diff --git a/repository.h b/repository.h
index b9413be50c..13c8799678 100644
--- a/repository.h
+++ b/repository.h
@@ -1,6 +1,8 @@
 #ifndef REPOSITORY_H
 #define REPOSITORY_H
 
+#include "path.h"
+
 struct config_set;
 struct git_hash_algo;
 struct index_state;
diff --git a/resolve-undo.h b/resolve-undo.h
index 87291904bd..fbe348efac 100644
--- a/resolve-undo.h
+++ b/resolve-undo.h
@@ -1,6 +1,8 @@
 #ifndef RESOLVE_UNDO_H
 #define RESOLVE_UNDO_H
 
+#include "cache.h"
+
 struct resolve_undo_info {
 	unsigned int mode[3];
 	struct object_id oid[3];
diff --git a/revision.h b/revision.h
index c599c34da9..850e508783 100644
--- a/revision.h
+++ b/revision.h
@@ -1,6 +1,7 @@
 #ifndef REVISION_H
 #define REVISION_H
 
+#include "commit.h"
 #include "parse-options.h"
 #include "grep.h"
 #include "notes.h"
diff --git a/send-pack.h b/send-pack.h
index 6af71f7008..e148fcd960 100644
--- a/send-pack.h
+++ b/send-pack.h
@@ -3,6 +3,10 @@
 
 #include "string-list.h"
 
+struct child_process;
+struct oid_array;
+struct ref;
+
 /* Possible values for push_cert field in send_pack_args. */
 #define SEND_PACK_PUSH_CERT_NEVER 0
 #define SEND_PACK_PUSH_CERT_IF_ASKED 1
diff --git a/sequencer.h b/sequencer.h
index c5787c6b56..c751c9d6e4 100644
--- a/sequencer.h
+++ b/sequencer.h
@@ -1,6 +1,11 @@
 #ifndef SEQUENCER_H
 #define SEQUENCER_H
 
+#include "cache.h"
+#include "strbuf.h"
+
+struct commit;
+
 const char *git_path_commit_editmsg(void);
 const char *git_path_seq_dir(void);
 
diff --git a/shortlog.h b/shortlog.h
index 5d64cfe929..2fa61c4294 100644
--- a/shortlog.h
+++ b/shortlog.h
@@ -3,6 +3,8 @@
 
 #include "string-list.h"
 
+struct commit;
+
 struct shortlog {
 	struct string_list list;
 	int summary;
diff --git a/submodule.h b/submodule.h
index 4644683e6c..3adbc08915 100644
--- a/submodule.h
+++ b/submodule.h
@@ -1,11 +1,17 @@
 #ifndef SUBMODULE_H
 #define SUBMODULE_H
 
-struct repository;
-struct diff_options;
 struct argv_array;
+struct cache_entry;
+struct diff_options;
+struct index_state;
+struct object_id;
 struct oid_array;
+struct pathspec;
 struct remote;
+struct repository;
+struct string_list;
+struct strbuf;
 
 enum {
 	RECURSE_SUBMODULES_ONLY = -5,
diff --git a/tempfile.h b/tempfile.h
index 8959c5f1b5..36434eb6fa 100644
--- a/tempfile.h
+++ b/tempfile.h
@@ -2,6 +2,7 @@
 #define TEMPFILE_H
 
 #include "list.h"
+#include "strbuf.h"
 
 /*
  * Handle temporary files.
diff --git a/trailer.h b/trailer.h
index 6d7f8c2a52..9c10026c35 100644
--- a/trailer.h
+++ b/trailer.h
@@ -3,6 +3,8 @@
 
 #include "list.h"
 
+struct strbuf;
+
 enum trailer_where {
 	WHERE_DEFAULT,
 	WHERE_END,
diff --git a/tree-walk.h b/tree-walk.h
index 805f58f00f..196831007e 100644
--- a/tree-walk.h
+++ b/tree-walk.h
@@ -1,6 +1,8 @@
 #ifndef TREE_WALK_H
 #define TREE_WALK_H
 
+struct strbuf;
+
 struct name_entry {
 	const struct object_id *oid;
 	const char *path;
diff --git a/unpack-trees.h b/unpack-trees.h
index c2b434c606..48bc69cb47 100644
--- a/unpack-trees.h
+++ b/unpack-trees.h
@@ -1,11 +1,14 @@
 #ifndef UNPACK_TREES_H
 #define UNPACK_TREES_H
 
-#include "tree-walk.h"
+#include "cache.h"
 #include "argv-array.h"
+#include "string-list.h"
+#include "tree-walk.h"
 
 #define MAX_UNPACK_TREES 8
 
+struct cache_entry;
 struct unpack_trees_options;
 struct exclude_list;
 
diff --git a/url.h b/url.h
index abdaf6fa30..f311c40a46 100644
--- a/url.h
+++ b/url.h
@@ -1,6 +1,8 @@
 #ifndef URL_H
 #define URL_H
 
+struct strbuf;
+
 extern int is_url(const char *url);
 extern int is_urlschemechar(int first_flag, int ch);
 extern char *url_decode(const char *url);
diff --git a/utf8.h b/utf8.h
index db73a2d8d3..dd5ee1052a 100644
--- a/utf8.h
+++ b/utf8.h
@@ -1,6 +1,8 @@
 #ifndef GIT_UTF8_H
 #define GIT_UTF8_H
 
+struct strbuf;
+
 typedef unsigned int ucs_char_t;  /* assuming 32bit int */
 
 size_t display_mode_esc_sequence_len(const char *s);
diff --git a/worktree.h b/worktree.h
index fe38ce10c3..df3fc30f73 100644
--- a/worktree.h
+++ b/worktree.h
@@ -1,6 +1,7 @@
 #ifndef WORKTREE_H
 #define WORKTREE_H
 
+#include "cache.h"
 #include "refs.h"
 
 struct strbuf;