Add no_std support to diesel

This commit adds support for using diesel with the SQLite backend
 in a no-std (core + alloc) environment. 

This mainly means fixing a lot of imports to use the relevant items from
core or alloc instead. Notable exceptions are:

* There is no hashmap in alloc, so we need to pull in hashbrown for this
* There is no panicing/abort support in core so we need our custom shims
  for these.
* Global instrumentation setting is not supported yet (cfged away) as it
  requires lock which are not supported on embedded targets.

This commit also introduces a new `std` default feature for enabling `std` lib
support. If disabled the users need to manually enable the `hashbrown`
feature to pull in hashbrown for hashmap support. Cargo doesn't support
pulling in dependencies for disabled features.

Finally this commit also adds a `example/sqlite/embedded` example
project that demonstrates how to use this on a esp32-c6.

Author: weiznich

.github/workflows/ci.yml

@@ -555,3 +555,24 @@ jobs:
       - name: Run cargo semver-checks
         run: |
           cargo xtask semver-checks
+  Sqlite-no-std:
+    name: Sqlite (no-std)
+    runs-on: ubuntu-latest
+    needs: [rustfmt_and_clippy]
+    steps:
+      - uses: actions/checkout@8e8c483db84b4bee98b60c0593521ed34d9990e8 # v6.0.1
+        with:
+          persist-credentials: false
+      - uses: dtolnay/rust-toolchain@f7ccc83f9ed1e5b9c81d8a67d7ad1a747e22a561 # master
+        with:
+          toolchain: stable
+          targets: riscv32imac-unknown-none-elf
+      - name: Install cross compiler
+        run: |
+          curl -L https://github.com/nycu-arclab/riscv32-toolchain/releases/download/v14.2.0/riscv32-unknown-elf-gcc-14.2.0-linux.tar.gz -o riscv32-toolchain-linux.tar.gz
+          tar -xzvf riscv32-toolchain-linux.tar.gz -C /opt
+      - name: Check if diesel builds for no-std targets
+        run: |
+          cd examples/sqlite/embedded
+          rustup target add riscv32imac-unknown-none-elf
+          LIBSQLITE3_FLAGS="-DSQLITE_SMALL_STACK=1 -DSQLITE_THREADSAFE=0 -DSQLITE_OS_OTHER=1 -DSQLITE_OMIT_LOCALTIME=1 -USQLITE_ENABLE_FTS3 -UQLITE_ENABLE_DBSTAT_VTAB -UDSQLITE_ENABLE_JSON1 -USQLITE_ENABLE_FTS5 -USQLITE_ENABLE_STAT4 -DSQLITE_ENABLE_MEMSYS3=1" TARGET_CC=/opt/riscv/bin/riscv32-unknown-elf-gcc cargo build --target riscv32imac-unknown-none-elf

CHANGELOG.md

@@ -15,6 +15,8 @@ Increasing the minimal supported Rust version will always be coupled at least wi
 ### Changed
 
 * The minimal supported Rust version is now 1.88.0
+* Add support for libsqlite3-sys 0.36
+* Add support for no-std environments using the SQLite backend
 
 ## [2.3.6] 2026-01-23
 

diesel/Cargo.toml

@@ -29,7 +29,7 @@ mysqlclient-src = { version = ">=0.1.0, <0.3", optional = true }
 pq-sys = { version = ">=0.4.0, <0.8.0", optional = true }
 pq-src = { version = "0.3", optional = true }
 quickcheck = { version = "1.0.3", optional = true }
-serde_json = { version = ">=0.8.0, <2.0", optional = true }
+serde_json = { version = ">=0.8.0, <2.0", optional = true, default-features = false, features = ["alloc"] }
 url = { version = "2.1.0", optional = true }
 percent-encoding = { version = "2.1.0", optional = true }
 uuid = { version = ">=0.7.0, <2.0.0", optional = true }
@@ -43,7 +43,8 @@ bitflags = { version = "2.0.0", optional = true }
 r2d2 = { version = ">= 0.8.2, < 0.9.0", optional = true }
 itoa = { version = "1.0.0", optional = true }
 time = { version = "0.3.9", optional = true, features = ["macros"] }
-downcast-rs = "2.0.1"
+downcast-rs = { version = "2.0.1", default-features = false }
+hashbrown = { version = "0.16", optional = true }
 
 [dependencies.diesel_derives]
 version = "~2.3.0"
@@ -69,7 +70,7 @@ tempfile = "3.10.1"
 diesel_test_helper = { path = "../diesel_test_helper" }
 
 [features]
-default = ["with-deprecated", "32-column-tables"]
+default = ["with-deprecated", "32-column-tables", "std"]
 extras = ["chrono", "time", "serde_json", "uuid", "network-address", "numeric", "r2d2"]
 unstable = ["diesel_derives/nightly"]
 large-tables = ["32-column-tables"]
@@ -78,22 +79,24 @@ huge-tables = ["64-column-tables"]
 64-column-tables = ["32-column-tables", "diesel_derives/64-column-tables"]
 128-column-tables = ["64-column-tables", "diesel_derives/128-column-tables"]
 postgres = ["dep:pq-sys", "postgres_backend"]
-sqlite = ["dep:libsqlite3-sys", "dep:sqlite-wasm-rs", "diesel_derives/sqlite", "time?/formatting", "time?/parsing"]
+sqlite-no-std = ["__sqlite-shared", "hashbrown"]
+sqlite = ["__sqlite-shared", "std"]
 mysql = ["dep:mysqlclient-sys", "dep:url", "dep:percent-encoding", "dep:bitflags", "mysql_backend"]
 without-deprecated = ["diesel_derives/without-deprecated"]
 with-deprecated = ["diesel_derives/with-deprecated"]
 network-address = ["dep:ipnetwork", "dep:libc"]
 ipnet-address = ["dep:ipnet", "dep:libc"]
 numeric = ["dep:num-bigint", "dep:bigdecimal", "dep:num-traits", "dep:num-integer", "diesel_derives/numeric"]
-postgres_backend = ["diesel_derives/postgres", "dep:bitflags", "dep:byteorder", "dep:itoa"]
-mysql_backend = ["diesel_derives/mysql", "dep:byteorder"]
+postgres_backend = ["diesel_derives/postgres", "dep:bitflags", "dep:byteorder", "dep:itoa", "std"]
+mysql_backend = ["diesel_derives/mysql", "dep:byteorder", "std"]
 returning_clauses_for_sqlite_3_35 = ["sqlite"]
 i-implement-a-third-party-backend-and-opt-into-breaking-changes = []
 r2d2 = ["diesel_derives/r2d2", "dep:r2d2"]
 chrono = ["diesel_derives/chrono", "dep:chrono"]
 time = ["diesel_derives/time", "dep:time"]
 uuid = ["dep:uuid"]
 serde_json = ["dep:serde_json"]
+__sqlite-shared = ["dep:libsqlite3-sys", "dep:sqlite-wasm-rs", "diesel_derives/sqlite", "time?/formatting", "time?/parsing"]
 __with_asan_tests = [
     "libsqlite3-sys?/bundled",
     "libsqlite3-sys?/with-asan",
@@ -102,6 +105,8 @@ __with_asan_tests = [
     "mysqlclient-sys?/bundled",
     "mysqlclient-src?/with-asan",
 ]
+std = ["serde_json?/std"]
+hashbrown = ["dep:hashbrown"]
 
 [package.metadata.docs.rs]
 features = ["postgres", "mysql", "sqlite", "extras"]

diesel/src/associations/belongs_to.rs

@@ -5,9 +5,10 @@ use crate::expression::array_comparison::AsInExpression;
 use crate::prelude::*;
 use crate::query_dsl::methods::FilterDsl;
 use crate::sql_types::SqlType;
+use alloc::vec::Vec;
 
-use std::borrow::Borrow;
-use std::hash::Hash;
+use core::borrow::Borrow;
+use core::hash::Hash;
 
 /// Indicates that a type belongs to `Parent`
 ///
@@ -18,7 +19,7 @@ use std::hash::Hash;
 /// This trait is not capable of supporting composite foreign keys
 pub trait BelongsTo<Parent> {
     /// The foreign key of this struct
-    type ForeignKey: Hash + ::std::cmp::Eq;
+    type ForeignKey: Hash + ::core::cmp::Eq;
     /// The database column representing the foreign key
     /// of the table this struct represents
     type ForeignKeyColumn: Column;
@@ -296,8 +297,8 @@ where
     Id<&'a Parent>: Borrow<Child::ForeignKey>,
 {
     fn grouped_by(self, parents: &'a [Parent]) -> Vec<Vec<Child>> {
-        use std::collections::HashMap;
-        use std::iter;
+        use crate::util::std_compat::HashMap;
+        use core::iter;
 
         let mut grouped: Vec<_> = iter::repeat_with(Vec::new).take(parents.len()).collect();
 
@@ -323,8 +324,8 @@ where
         self,
         parents: &'a [Parent],
     ) -> Result<Vec<Vec<Child>>, TryGroupedByError<Child>> {
-        use std::collections::HashMap;
-        use std::iter;
+        use crate::util::std_compat::HashMap;
+        use core::iter;
 
         let mut grouped: Vec<_> = iter::repeat_with(Vec::new).take(parents.len()).collect();
         let mut ungrouped: Vec<_> = Vec::new();

diesel/src/associations/mod.rs

@@ -403,7 +403,7 @@
 //! be used to construct the complex behavior applications need.
 mod belongs_to;
 
-use std::hash::Hash;
+use core::hash::Hash;
 
 use crate::query_source::Table;
 
@@ -440,23 +440,23 @@ impl<T: HasTable> HasTable for Option<T> {
     }
 }
 
-impl<T: HasTable> HasTable for Box<T> {
+impl<T: HasTable> HasTable for alloc::boxed::Box<T> {
     type Table = T::Table;
 
     fn table() -> Self::Table {
         T::table()
     }
 }
 
-impl<T: HasTable> HasTable for std::rc::Rc<T> {
+impl<T: HasTable> HasTable for alloc::rc::Rc<T> {
     type Table = T::Table;
 
     fn table() -> Self::Table {
         T::table()
     }
 }
 
-impl<T: HasTable> HasTable for std::sync::Arc<T> {
+impl<T: HasTable> HasTable for alloc::sync::Arc<T> {
     type Table = T::Table;
 
     fn table() -> Self::Table {

diesel/src/backend.rs

@@ -7,7 +7,7 @@ use crate::sql_types::{self, HasSqlType, TypeMetadata};
     not(any(
         feature = "postgres_backend",
         feature = "mysql_backend",
-        feature = "sqlite",
+        feature = "__sqlite-shared",
         feature = "i-implement-a-third-party-backend-and-opt-into-breaking-changes"
     )),
     allow(unused_imports)

diesel/src/collation/mod.rs

@@ -36,13 +36,13 @@ impl QueryId for Custom {
 /// This collation is binary, case-sensitive, and locale-free.
 /// It is supported by SQLite.
 #[derive(Debug, Clone, Copy, PartialEq, Eq, Hash, Default, QueryId)]
-#[cfg(feature = "sqlite")]
+#[cfg(feature = "__sqlite-shared")]
 pub struct Binary;
 
-#[cfg(feature = "sqlite")]
+#[cfg(feature = "__sqlite-shared")]
 impl Collation for Binary {}
 
-#[cfg(feature = "sqlite")]
+#[cfg(feature = "__sqlite-shared")]
 impl QueryFragment<crate::sqlite::Sqlite> for Binary {
     fn walk_ast<'b>(&'b self, mut out: AstPass<'_, 'b, crate::sqlite::Sqlite>) -> QueryResult<()> {
         out.push_sql("BINARY");
@@ -74,13 +74,13 @@ impl QueryFragment<crate::pg::Pg> for C {
 /// This collation is ASCII-only case-insensitive.
 /// It is supported by SQLite.
 #[derive(Debug, Clone, Copy, PartialEq, Eq, Hash, Default, QueryId)]
-#[cfg(feature = "sqlite")]
+#[cfg(feature = "__sqlite-shared")]
 pub struct NoCase;
 
-#[cfg(feature = "sqlite")]
+#[cfg(feature = "__sqlite-shared")]
 impl Collation for NoCase {}
 
-#[cfg(feature = "sqlite")]
+#[cfg(feature = "__sqlite-shared")]
 impl QueryFragment<crate::sqlite::Sqlite> for NoCase {
     fn walk_ast<'b>(&'b self, mut out: AstPass<'_, 'b, crate::sqlite::Sqlite>) -> QueryResult<()> {
         out.push_sql("NOCASE");
@@ -112,13 +112,13 @@ impl QueryFragment<crate::pg::Pg> for Posix {
 /// This collation ignores trailing spaces.
 /// It is supported by SQLite.
 #[derive(Debug, Clone, Copy, PartialEq, Eq, Hash, Default, QueryId)]
-#[cfg(feature = "sqlite")]
+#[cfg(feature = "__sqlite-shared")]
 pub struct RTrim;
 
-#[cfg(feature = "sqlite")]
+#[cfg(feature = "__sqlite-shared")]
 impl Collation for RTrim {}
 
-#[cfg(feature = "sqlite")]
+#[cfg(feature = "__sqlite-shared")]
 impl QueryFragment<crate::sqlite::Sqlite> for RTrim {
     fn walk_ast<'b>(&'b self, mut out: AstPass<'_, 'b, crate::sqlite::Sqlite>) -> QueryResult<()> {
         out.push_sql("RTRIM");

diesel/src/connection/instrumentation.rs

@@ -1,8 +1,10 @@
+use alloc::boxed::Box;
+use core::fmt::{Debug, Display};
+use core::num::NonZeroU32;
+use core::ops::{Deref, DerefMut};
 use downcast_rs::Downcast;
-use std::fmt::{Debug, Display};
-use std::num::NonZeroU32;
-use std::ops::{Deref, DerefMut};
 
+#[cfg(feature = "std")]
 static GLOBAL_INSTRUMENTATION: std::sync::RwLock<fn() -> Option<Box<dyn Instrumentation>>> =
     std::sync::RwLock::new(|| None);
 
@@ -34,7 +36,7 @@ impl<'query> StrQueryHelper<'query> {
     )]
     #[cfg(any(
         feature = "postgres",
-        feature = "sqlite",
+        feature = "__sqlite-shared",
         feature = "mysql",
         feature = "i-implement-a-third-party-backend-and-opt-into-breaking-changes"
     ))]
@@ -44,13 +46,13 @@ impl<'query> StrQueryHelper<'query> {
 }
 
 impl Debug for StrQueryHelper<'_> {
-    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+    fn fmt(&self, f: &mut core::fmt::Formatter<'_>) -> core::fmt::Result {
         Debug::fmt(self.s, f)
     }
 }
 
 impl Display for StrQueryHelper<'_> {
-    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+    fn fmt(&self, f: &mut core::fmt::Formatter<'_>) -> core::fmt::Result {
         Display::fmt(&self.s, f)
     }
 }
@@ -254,10 +256,13 @@ downcast_rs::impl_downcast!(Instrumentation);
 /// This function is mostly useful for crates implementing
 /// their own connection types
 pub fn get_default_instrumentation() -> Option<Box<dyn Instrumentation>> {
+    #[cfg(feature = "std")]
     match GLOBAL_INSTRUMENTATION.read() {
         Ok(f) => (*f)(),
         Err(_) => None,
     }
+    #[cfg(not(feature = "std"))]
+    None
 }
 
 /// Set a custom constructor for the default [`Instrumentation`]
@@ -277,6 +282,7 @@ pub fn get_default_instrumentation() -> Option<Box<dyn Instrumentation>> {
 ///
 /// set_default_instrumentation(simple_logger);
 /// ```
+#[cfg(feature = "std")]
 pub fn set_default_instrumentation(
     default: fn() -> Option<Box<dyn Instrumentation>>,
 ) -> crate::QueryResult<()> {
@@ -358,7 +364,7 @@ impl DynInstrumentation {
     )]
     #[cfg(any(
         feature = "postgres",
-        feature = "sqlite",
+        feature = "__sqlite-shared",
         feature = "mysql",
         feature = "i-implement-a-third-party-backend-and-opt-into-breaking-changes"
     ))]
@@ -375,7 +381,7 @@ impl DynInstrumentation {
     )]
     #[cfg(any(
         feature = "postgres",
-        feature = "sqlite",
+        feature = "__sqlite-shared",
         feature = "mysql",
         feature = "i-implement-a-third-party-backend-and-opt-into-breaking-changes"
     ))]
@@ -392,7 +398,7 @@ impl DynInstrumentation {
     )]
     #[cfg(any(
         feature = "postgres",
-        feature = "sqlite",
+        feature = "__sqlite-shared",
         feature = "mysql",
         feature = "i-implement-a-third-party-backend-and-opt-into-breaking-changes"
     ))]