enable on-demand rustc incremental compilation

Summary:
Users can now enable incremental compilation by running:
```
buck2 build -c rust.incremental=relative/path/to/target <TARGET>
```
This will generate an `incremental/<BUILD_MODE>/` directory which can be reused between builds. Since `rustc` is quite opinionated about incremental artifact hashing, file naming and placement, we enforce incremental compilation to run locally by skipping RE.

## Benchmarking (quick)

From a few local experiments run builds with and without incremental compilation turned on after making minor code changes, I see *> 80%* reduction in build times when using incremental compilation:

**example #1**: `//common/rust/tools/rust-expand:rust-expand`
```
|---------------------+-------------------+-------------------------|
| Build type          | 1st Build (clean) | 2nd Build (with change) |
|---------------------+-------------------+-------------------------|
| Without incremental | 9.2sec            | 14.2sec                 |
| With incremental    | 15.2sec           | 2.1sec                  |
|---------------------+-------------------+-------------------------|
```

**example #2**: `//common/rust/tools/rustfix2:buck`
```
|---------------------+-------------------+-------------------------|
| Build type          | 1st Build (clean) | 2nd Build (with change) |
|---------------------+-------------------+-------------------------|
| Without incremental | 7.6sec            | 3.4sec                  |
| With incremental    | 9sec              | 0.72sec                 |
|---------------------+-------------------+-------------------------|
```

For both examples above, 1st build gets run & timed like so:
```
for i in {1..5}; do buck2 clean && time buck2 build --show-full-output //common/rust/tools/rustfix2:buck; done
```
and 2nd build gets run & timed like so:
```
cd ~/fbcode
for i in {1..5}; do sed -i 's/println!("some some something 1");/&\n    println!("some some something 2");/' common/rust/tools/rustfix2/src/buck.rs && time buck2 build --show-full-output //common/rust/tools/rustfix2:buck; done
```
adding and removing `-c rust.incremental` to generate results with and without incremental compilation.

Reviewed By: krallin

Differential Revision: D38374692

fbshipit-source-id: 0f7412249da6e3dad2a381f0945e8cc8f9412065

prelude/rules_impl.bzl

@@ -442,6 +442,8 @@ extra_attributes = struct(
     },
     #rust
     rust_binary = {
+        "incremental_build_mode": attrs.option(attrs.string()),
+        "incremental_enabled": attrs.bool(default = False),
         "resources": attrs.named_set(attrs.one_of(attrs.dep(), attrs.source()), sorted = True, default = []),
         "_cxx_toolchain": _cxx_toolchain(),
         "_rust_toolchain": _rust_toolchain(),
@@ -457,6 +459,8 @@ extra_attributes = struct(
         # them and it simplifies the implementation of Rust rules since they
         # don't have to know whether we're building a rust_binary or a
         # rust_library.
+        "incremental_build_mode": attrs.option(attrs.string()),
+        "incremental_enabled": attrs.bool(default = False),
         "linker_flags": attrs.list(attrs.arg(), default = []),
         "preferred_linkage": attrs.enum(Linkage, default = "any"),
         "resources": attrs.named_set(attrs.one_of(attrs.dep(), attrs.source()), sorted = True, default = []),
@@ -465,6 +469,8 @@ extra_attributes = struct(
     },
     rust_test = {
         "framework": attrs.bool(default = True),
+        "incremental_build_mode": attrs.option(attrs.string()),
+        "incremental_enabled": attrs.bool(default = False),
         "resources": attrs.named_set(attrs.one_of(attrs.dep(), attrs.source()), sorted = True, default = []),
         "_cxx_toolchain": _cxx_toolchain(),
         "_rust_toolchain": _rust_toolchain(),

prelude/rust/build.bzl

@@ -659,8 +659,14 @@ def _rustc_emits(
 
     if emit not in (Emit("expand"), Emit("save-analysis")):
         extra_dir = subdir + "/extras/" + output_filename(crate, emit, params)
-        extra_dir = ctx.actions.declare_output(extra_dir)
-        emit_args.add("--out-dir", extra_dir.as_output())
+        extra_out = ctx.actions.declare_output(extra_dir)
+        emit_args.add("--out-dir", extra_out.as_output())
+
+        if ctx.attrs.incremental_enabled:
+            build_mode = ctx.attrs.incremental_build_mode
+            incremental_out = ctx.actions.declare_output("{}/extras/incremental/{}".format(subdir, build_mode))
+            incremental_cmd = cmd_args(incremental_out.as_output(), format = "-Cincremental={}")
+            emit_args.add(incremental_cmd)
 
     return (outputs, emit_args)
 
@@ -712,9 +718,17 @@ def _rustc_invoke(
     compile_cmd.add(rustc_cmd)
     compile_cmd.hidden(toolchain_info.compiler, compile_ctx.symlinked_srcs)
 
-    local_only = is_binary and link_cxx_binary_locally(ctx)
+    incremental_enabled = ctx.attrs.incremental_enabled
+    local_only = (is_binary and link_cxx_binary_locally(ctx)) or incremental_enabled
     identifier = "{} {} [{}]".format(prefix, short_cmd, diag)
-    ctx.actions.run(compile_cmd, env = plain_env, local_only = local_only, category = "rustc", identifier = identifier)
+    ctx.actions.run(
+        compile_cmd,
+        env = plain_env,
+        local_only = local_only,
+        category = "rustc",
+        identifier = identifier,
+        no_outputs_cleanup = incremental_enabled,
+    )
 
     return ({diag + ".json": json_diag, diag + ".txt": txt_diag}, build_status)