`--watch` does not pick up sub-Compilation file system inputs such as compiler_rt, glibc, or musl source files

[andrewrk]

Extracted from #20580.

Zig version: 0.14.0-dev.283+1d20ff11d

Steps to reproduce:

In an empty directory:

$ zig init
info: created build.zig
info: created build.zig.zon
info: created src/main.zig
info: created src/root.zig
info: see `zig build --help` for a menu of options
$ zig build --watch
andy@bark ~/t/abc> zig build --watch
Build Summary: 5/5 steps succeeded
install success
├─ install abc success
│  └─ zig build-lib abc Debug native success 702ms MaxRSS:195M
└─ install abc success
   └─ zig build-exe abc Debug native success 759ms MaxRSS:198M
Watching 21 Directories

Then go make a change to lib/zig/compiler_rt.zig in the lib directory corresponding to the zig compiler you are using.

The file is not being watched; no changes are picked up. Instead, changes to compiler_rt and other sub-compilations should be noticed and trigger a recompilation.

The mechanism behind this is in Compilation.update:

zig/src/Compilation.zig

Line 2013 in 1d20ff1

pub fn update(comp: *Compilation, main_progress_node: std.Progress.Node) !void {

In this function:

• Pay attention to how comp.file_system_inputs is modified
• Note this comment:

  zig/src/Compilation.zig

  Lines 2116 to 2121 in 1d20ff1

  	// From this point we add a preliminary set of file system inputs that
  	// affects both incremental and whole cache mode. For incremental cache
  	// mode, the long-lived compiler state will track additional file system
  	// inputs discovered after this point. For whole cache mode, we rely on
  	// these inputs to make it past AstGen, and once there, we can rely on
  	// learning file system inputs from the Cache object.

• Notice how the Cache Manifest object is the authority on file inputs, but only when using whole cache mode, and it is either a hit, or a successful compilation.

In a future enhancement, the build runner should pass in the set of modified file system inputs to the update() function directly, so that the fstat calls are avoided. That should be a follow-up issue when closing this one.