Avoid rebuilding rustc_codegen_spirv between example runners.

Cargo.lock

@@ -2002,8 +2002,10 @@ dependencies = [
  "indexmap",
  "libc",
  "num-traits",
+ "once_cell",
  "pipe",
  "pretty_assertions",
+ "regex",
  "rspirv",
  "rustc-demangle",
  "rustc_codegen_spirv-types",

crates/rustc_codegen_spirv/Cargo.toml

@@ -32,14 +32,21 @@ use-compiled-tools = ["spirv-tools/use-compiled-tools"]
 # Enable this feature to be able to experiment with other versions.
 skip-toolchain-check = []
 
+# HACK(eddyb) these are not real "build dependencies", but rather dependencies
+# of proc macros, which we want to enable extra features in (without having
+# a proc macro crate to do that through), for the same reasons as the list at
+# the start of `[dependencies]` (avoiding unnecessary rebuilds).
+[build-dependencies]
+syn = { version = "1", features = ["extra-traits", "full"] }
 
 [dependencies]
 # HACK(eddyb) these only exist to unify features across dependency trees,
 # in order to avoid multiple separate instances of `rustc_codegen_spirv`.
-hashbrown = { version = "0.11", features = ["default"] }
+hashbrown = "0.11"
 libc = { version = "0.2", features = ["align", "extra_traits"] }
 num-traits = { version = "0.2", features = ["libm"] }
-syn = { version = "1", features = ["visit", "visit-mut"] }
+once_cell = "1"
+regex = { version = "1", features = ["perf"] }
 
 # Normal dependencies.
 ar = "0.9.0"
@@ -50,7 +57,7 @@ rustc-demangle = "0.1.21"
 sanitize-filename = "0.4"
 serde = { version = "1.0", features = ["derive"] }
 serde_json = "1.0"
-smallvec = "1.6.1"
+smallvec = { version = "1.6.1", features = ["union"] }
 spirv-tools = { version = "0.8", default-features = false }
 rustc_codegen_spirv-types = { path = "../rustc_codegen_spirv-types", version = "0.4.0-alpha.15" }