Allocate tree-sitter's external scanners from the same Zig allocator as its runtime

syntax.zig points the tree-sitter runtime at a Zig allocator, but the
grammars' external scanners did not follow it. A grammar's
tree_sitter/alloc.h maps ts_malloc to the runtime's current allocator only
when TREE_SITTER_REUSE_ALLOCATOR is defined, and plain malloc otherwise, so
every scanner's state and its arrays came from libc. Five scanners (bash,
markdown, markdown_inline, python, typst) also call malloc, calloc, realloc
and free by name.

build.zig now compiles every grammar with TREE_SITTER_REUSE_ALLOCATOR and
forces in src/tree_sitter_heap.h, which includes stdlib.h and then defines
the four names as calls through ts_current_malloc and friends. A header
forced in from the build rather than a patch to the grammars keeps zig-pkg
pristine. No grammar archive refers to libc's allocator any more; only the
runtime does, for its defaults. A scanner frees only what it allocated
itself, and the syntax tests that create and destroy a parser for every
grammar under a leak-checking allocator pass.

Co-Authored-By: Claude Opus 5.5 (1M context) <[email protected]>

1 files changed, 11 insertions, 1 deletions

diff --git a/build.zig b/build.zig
index c70f52b2..f0675a70 100644
--- a/build.zig
+++ b/build.zig
@@ -390,7 +390,17 @@ pub fn build(b: *std.Build) void {
                     .root_module = b.createModule(.{ .target = target, .optimize = c_optimize, .link_libc = !is_web }),
                     .linkage = .static,
                 });
-                const ts_cflags = [_][]const u8{"-fno-sanitize=function"};
+                // Every allocation a grammar's external scanner makes goes to the
+                // runtime's allocator, which syntax.zig points at a Zig one:
+                // TREE_SITTER_REUSE_ALLOCATOR for the scanners that use
+                // ts_malloc, and src/tree_sitter_heap.h for the ones that call
+                // malloc themselves.
+                const ts_cflags = [_][]const u8{
+                    "-fno-sanitize=function",
+                    "-DTREE_SITTER_REUSE_ALLOCATOR",
+                    "-include",
+                    b.path("src/tree_sitter_heap.h").getPath(b),
+                };
                 lib.root_module.addCSourceFile(.{ .file = dep.path(g.src ++ "/parser.c"), .flags = &ts_cflags });
                 if (g.scanner) lib.root_module.addCSourceFile(.{ .file = dep.path(g.src ++ "/scanner.c"), .flags = &ts_cflags });
                 lib.root_module.addIncludePath(dep.path(g.src));