Decouple serde from its derive crate

By not activating the `derive` feature on `serde`, the compilation speed
can be improved by a lot. This is because `serde` can then compile in
parallel to `serde_derive`, allowing it to finish compilation possibly
even before `serde_derive`, unblocking all the crates waiting for
`serde` to start compiling much sooner.

As it turns out the main deciding factor for how long the compile time of a
project is, is primarly determined by the depth of dependencies rather
than the width. In other words, a crate's compile times aren't affected
by how many crates it depends on, but rather by the longest chain of
dependencies that it needs to wait on. In many cases `serde` is part of
that long chain, as it is part of a long chain if the `derive` feature
is active:

`proc-macro2` compile build script > `proc-macro2` run build script >
`proc-macro2` > `quote` > `syn` > `serde_derive` > `serde` >
`serde_json` (or any crate that depends on serde)

By decoupling it from `serde_derive`, the chain is shortened and compile
times get much better.

Check this issue for a deeper elaboration:
serde-rs/serde#2584

For `wasmtime` I'm seeing a reduction from 24.75s to 22.45s when
compiling in `release` mode. This is because wasmtime through `gimli`
has a dependency on `indexmap` which can only start compiling when
`serde` is finished, which you want to happen as early as possible so
some of wasmtime's dependencies can start compiling.

To measure the full effect, the dependencies can't by themselves
activate the `derive` feature. I've upstreamed a patch for
`fxprof-processed-profile` which was the only dependency that activated
it for `wasmtime` (not yet published to crates.io). `wasmtime-cli` and
co. may need patches for their dependencies to see a similar
improvement.

Author: CryZe

Cargo.lock

@@ -42,6 +42,7 @@ once_cell = { workspace = true }
 listenfd = "1.0.0"
 wat = { workspace = true }
 serde = { workspace = true }
+serde_derive = { workspace = true }
 serde_json = { workspace = true }
 wasmparser = { workspace = true }
 wasm-encoder = { workspace = true }
@@ -235,7 +236,8 @@ async-trait = "0.1.71"
 heck = "0.4"
 similar = "2.1.0"
 toml = "0.5.9"
-serde = "1.0.94"
+serde = "1.0.188"
+serde_derive = "1.0.188"
 serde_json = "1.0.80"
 glob = "0.3.0"
 libfuzzer-sys = "0.4.0"

Cargo.toml

@@ -23,7 +23,8 @@ cranelift-control = { workspace = true }
 hashbrown = { workspace = true, features = ["raw"] }
 target-lexicon = { workspace = true }
 log = { workspace = true }
-serde = { version = "1.0.94", features = ["derive"], optional = true }
+serde = { version = "1.0.188", optional = true }
+serde_derive = { version = "1.0.188", optional = true }
 bincode = { version = "1.2.1", optional = true }
 gimli = { workspace = true, features = ["write"], optional = true }
 smallvec = { workspace = true }
@@ -87,6 +88,7 @@ all-arch = [
 # For dependent crates that want to serialize some parts of cranelift
 enable-serde = [
     "serde",
+    "serde_derive",
     "cranelift-entity/enable-serde",
     "regalloc2/enable-serde",
     "smallvec/serde"

cranelift/codegen/Cargo.toml

@@ -502,7 +502,7 @@ fn gen_opcodes(all_inst: &AllInstructions, fmt: &mut Formatter) {
     fmt.line(
         r#"#[cfg_attr(
             feature = "enable-serde",
-            derive(serde::Serialize, serde::Deserialize)
+            derive(serde_derive::Serialize, serde_derive::Deserialize)
         )]"#,
     );
 

cranelift/codegen/meta/src/gen_inst.rs

@@ -8,7 +8,7 @@ mod stack_map;
 pub use self::stack_map::StackMap;
 use core::fmt;
 #[cfg(feature = "enable-serde")]
-use serde::{Deserialize, Serialize};
+use serde_derive::{Deserialize, Serialize};
 
 /// Offset in bytes from the beginning of the function.
 ///

cranelift/codegen/src/binemit/mod.rs

@@ -67,7 +67,10 @@ const NUM_BITS: usize = core::mem::size_of::<Num>() * 8;
 /// live GC references in these slots. We record the `IncomingArg`s in the
 /// callee's stack map.
 #[derive(Clone, Debug, PartialEq, Eq)]
-#[cfg_attr(feature = "enable-serde", derive(serde::Deserialize, serde::Serialize))]
+#[cfg_attr(
+    feature = "enable-serde",
+    derive(serde_derive::Deserialize, serde_derive::Serialize)
+)]
 pub struct StackMap {
     bitmap: Vec<BitSet<Num>>,
     mapped_words: u32,

cranelift/codegen/src/binemit/stack_map.rs

@@ -12,7 +12,10 @@ use core::ops::{Add, BitOr, Shl, Sub};
 
 /// A small bitset built on a single primitive integer type
 #[derive(Clone, Copy, Debug, Default, PartialEq, Eq)]
-#[cfg_attr(feature = "enable-serde", derive(serde::Serialize, serde::Deserialize))]
+#[cfg_attr(
+    feature = "enable-serde",
+    derive(serde_derive::Serialize, serde_derive::Deserialize)
+)]
 pub struct BitSet<T>(pub T);
 
 impl<T> BitSet<T>

cranelift/codegen/src/bitset.rs

@@ -118,7 +118,7 @@ impl std::fmt::Display for CacheKeyHash {
     }
 }
 
-#[derive(serde::Serialize, serde::Deserialize)]
+#[derive(serde_derive::Serialize, serde_derive::Deserialize)]
 struct CachedFunc {
     // Note: The version marker must be first to ensure deserialization stops in case of a version
     // mismatch before attempting to deserialize the actual compiled code.
@@ -139,7 +139,7 @@ struct CacheKey<'a> {
     parameters: CompileParameters,
 }
 
-#[derive(Clone, PartialEq, Hash, serde::Serialize, serde::Deserialize)]
+#[derive(Clone, PartialEq, Hash, serde_derive::Serialize, serde_derive::Deserialize)]
 struct CompileParameters {
     isa: String,
     triple: String,

cranelift/codegen/src/incremental_cache.rs

@@ -2,7 +2,7 @@
 use core::fmt::{self, Display, Formatter};
 use core::str::FromStr;
 #[cfg(feature = "enable-serde")]
-use serde::{Deserialize, Serialize};
+use serde_derive::{Deserialize, Serialize};
 
 #[derive(Clone, Copy, PartialEq, Eq, Debug, Hash)]
 #[cfg_attr(feature = "enable-serde", derive(Serialize, Deserialize))]

cranelift/codegen/src/ir/atomic_rmw_op.rs

@@ -8,7 +8,7 @@ use core::fmt::{self, Display, Formatter};
 use core::str::FromStr;
 
 #[cfg(feature = "enable-serde")]
-use serde::{Deserialize, Serialize};
+use serde_derive::{Deserialize, Serialize};
 
 /// Common traits of condition codes.
 pub trait CondCode: Copy {