diff --git a/.github/workflows/build.yml b/.github/workflows/build.yml
index 143cf350ff..4749e189a1 100644
--- a/.github/workflows/build.yml
+++ b/.github/workflows/build.yml
@@ -80,13 +80,6 @@ jobs:
           - thumbv7m-none-eabi
           - thumbv8m.base-none-eabi
           - thumbv8m.main-none-eabi
-        toolchain:
-          - stable
-          - nightly
-        features:
-          - ""
-          - "cas,portable-atomic/critical-section"
-          - "serde"
     steps:
       - name: Checkout
         uses: actions/checkout@v4
@@ -113,14 +106,17 @@ jobs:
             ${{ runner.OS }}-build-${{ hashFiles('**/Cargo.lock') }}
             ${{ runner.OS }}-build-
 
-      - name: Install Rust ${{ matrix.toolchain }} with target (${{ matrix.target }})
+      - name: Install Rust with target (${{ matrix.target }})
         uses: dtolnay/rust-toolchain@master
         with:
-          toolchain: ${{ matrix.toolchain }}
+          toolchain: stable
           targets: ${{ matrix.target }}
 
       - name: cargo check
-        run: cargo check --target=${{ matrix.target }} --no-default-features --features=${{ matrix.features }}
+        run: |
+          cargo check --target=${{ matrix.target }}
+          cargo check --target=${{ matrix.target }} --features="portable-atomic-critical-section"
+          cargo check --target=${{ matrix.target }} --features="ufmt serde defmt-03 mpmc_large"
 
   doc:
     name: doc
@@ -130,10 +126,6 @@ jobs:
         target:
           - x86_64-unknown-linux-gnu
           - thumbv7m-none-eabi
-        features:
-          - ""
-          - "cas"
-          - "serde"
     steps:
       - name: Checkout
         uses: actions/checkout@v4
@@ -166,7 +158,7 @@ jobs:
           targets: ${{ matrix.target }}
 
       - name: cargo doc
-        run: cargo doc --target=${{ matrix.target }} --no-default-features --features=${{ matrix.features }}
+        run: cargo doc --target=${{ matrix.target }} --features="ufmt serde defmt-03 mpmc_large portable-atomic-critical-section"
 
   # Run cpass tests
   testcpass:
diff --git a/CHANGELOG.md b/CHANGELOG.md
index 0174ee4960..76553c6f50 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -35,6 +35,10 @@ and this project adheres to [Semantic Versioning](http://semver.org/).
 - [breaking-change] this crate now uses `portable-atomic` v1.0 instead of `atomic-polyfill` for emulating
   CAS instructions on targets where they're not natively available.
 - [breaking-change] `From<&str>` for `String` was replaced with `TryFrom<&str>` because the `From` trait must not fail.
+- [breaking-change] Renamed Cargo features
+  - `defmt-impl` is now `defmt-03`
+  - `ufmt-impl` is now `ufmt`
+  - `cas` is removed, atomic polyfilling is now opt-in via the `portable-atomic` feature.
 
 ### Fixed
 
@@ -92,31 +96,31 @@ and this project adheres to [Semantic Versioning](http://semver.org/).
 
 ### Added
 
-* Added support for AVR architecture.
-* Add `entry` API to `IndexMap`
-* Implement `IntoIterator` trait for `Indexmap`
-* Implement `FromIterator` for `String`
-* Add `first` and `last` methods to `IndexMap` and `IndexSet`
-* Add `pop_{front_back}_unchecked` methods to `Deque`
+- Added support for AVR architecture.
+- Add `entry` API to `IndexMap`
+- Implement `IntoIterator` trait for `Indexmap`
+- Implement `FromIterator` for `String`
+- Add `first` and `last` methods to `IndexMap` and `IndexSet`
+- Add `pop_{front_back}_unchecked` methods to `Deque`
 
 ### Changed
 
-* Optimize the codegen of `Vec::clone`
-* `riscv32i` and `riscv32imc` targets unconditionally (e.g. `build --no-default-features`) depends on `atomic-polyfill`
+- Optimize the codegen of `Vec::clone`
+- `riscv32i` and `riscv32imc` targets unconditionally (e.g. `build --no-default-features`) depends on `atomic-polyfill`
 
 ### Fixed
 
-* Inserting an item that replaces an already present item will no longer
-fail with an error
+- Inserting an item that replaces an already present item will no longer
+  fail with an error
 
 ## [v0.7.11] - 2022-05-09
 
 ### Fixed
 
-* Fixed `pool` example in docstring.
-* Fixed undefined behavior in `Vec::truncate()`, `Vec::swap_remove_unchecked()`,
+- Fixed `pool` example in docstring.
+- Fixed undefined behavior in `Vec::truncate()`, `Vec::swap_remove_unchecked()`,
   and `Hole::move_to()` (internal to the binary heap implementation).
-* Fixed `BinaryHeap` elements are being dropped twice
+- Fixed `BinaryHeap` elements are being dropped twice
 
 ## [v0.7.10] - 2022-01-21
 
@@ -296,8 +300,8 @@ fail with an error
 ### Added
 
 - opt-out `cas` feature to disable parts of the API that use CAS instructions.
-Useful if using a custom (i.e. not built-in) rustc target that does not have CAS
-instructions.
+  Useful if using a custom (i.e. not built-in) rustc target that does not have CAS
+  instructions.
 
 - singleton `Pool` support on ARMv7-A devices
 
@@ -316,7 +320,7 @@ instructions.
 - `Pool` now implements the `Sync` trait when targeting ARMv7-R.
 
 - Most data structures can now be constructed in "const context" (e.g. `static
-  [mut]` variables) using a newtype in `heapless::i`.
+[mut]` variables) using a newtype in `heapless::i`.
 
 - `Pool` has gained a `grow_exact` method to more efficiently use statically
   allocated memory.
@@ -361,7 +365,7 @@ instructions.
 ### Added
 
 - Added a memory pool that's lock-free and interrupt-safe on the ARMv7-M
-architecture.
+  architecture.
 
 - `IndexMap` have gained `Eq` and `PartialEq` implementations.
 
@@ -549,7 +553,7 @@ architecture.
 - [breaking-change] The error type of all operations that may fail has changed from `()` to
   `BufferFullError`.
 
-- Both `RingBuffer` and `Vec` now support arrays of *any* size for their backup storage.
+- Both `RingBuffer` and `Vec` now support arrays of _any_ size for their backup storage.
 
 ## [v0.1.0] - 2017-04-27
 
diff --git a/Cargo.toml b/Cargo.toml
index 687f161dd4..7a4a222758 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -15,35 +15,32 @@ repository = "https://github.com/rust-embedded/heapless"
 version = "0.8.0"
 
 [features]
-default = ["cas"]
-cas = ["portable-atomic"]
-ufmt-impl = ["ufmt-write"]
-# only for tests
-__trybuild = []
-# Enable larger MPMC sizes.
-mpmc_large = []
-# This flag has no version guarantee, the `defmt` dependency can be updated in a patch release
-defmt-impl = ["defmt"]
+# Enable polyfilling of atomics via `portable-atomic`.
+# `portable-atomic` polyfills some functionality by default, but to get full atomics you must
+# enable one of its features to tell it how to do it. See `portable-atomic` documentation for details.
+portable-atomic = ["dep:portable-atomic"]
 
-[target.thumbv6m-none-eabi.dependencies]
-portable-atomic = { version = "1.0", optional = true }
+# Enable polyfilling of atomics via portable-atomic, using critical section for locking
+portable-atomic-critical-section = ["dep:portable-atomic", "portable-atomic?/critical-section"]
 
-[target.riscv32i-unknown-none-elf.dependencies]
-portable-atomic = { version = "1.0" }
+# Enable polyfilling of atomics via portable-atomic, using disabling interrupts for locking.
+# WARNING: this is only sound for single-core bare-metal privileged-mode targets!
+portable-atomic-unsafe-assume-single-core = ["dep:portable-atomic", "portable-atomic?/unsafe-assume-single-core"]
 
-[target.riscv32imc-unknown-none-elf.dependencies]
-portable-atomic = { version = "1.0" }
+# implement serde traits.
+serde = ["dep:serde"]
 
-[target.msp430-none-elf.dependencies]
-portable-atomic = { version = "1.0" }
+# implement ufmt traits.
+ufmt = ["dep:ufmt-write"]
 
-[target.xtensa-esp32s2-none-elf.dependencies]
-portable-atomic = { version = "1.0", optional = true }
+# Implement defmt::Format from defmt v0.3
+defmt-03 = ["dep:defmt"]
 
-[target.'cfg(target_arch = "avr")'.dependencies]
-portable-atomic = { version = "1.0", optional = true }
+# Enable larger MPMC sizes.
+mpmc_large = []
 
 [dependencies]
+portable-atomic = { version = "1.0", optional = true }
 hash32 = "0.3.0"
 serde = { version = "1", optional = true, default-features = false }
 stable_deref_trait = { version = "1", default-features = false }
diff --git a/build.rs b/build.rs
index 1c9892e081..1373281224 100644
--- a/build.rs
+++ b/build.rs
@@ -11,82 +11,21 @@ use std::{
 fn main() -> Result<(), Box<dyn Error>> {
     let target = env::var("TARGET")?;
 
-    if target.starts_with("thumbv6m-") {
-        println!("cargo:rustc-cfg=armv6m");
-    } else if target.starts_with("thumbv7m-") {
-        println!("cargo:rustc-cfg=armv7m");
-    } else if target.starts_with("thumbv7em-") {
-        println!("cargo:rustc-cfg=armv7m");
-    } else if target.starts_with("armv7r-") | target.starts_with("armebv7r-") {
-        println!("cargo:rustc-cfg=armv7r");
-    } else if target.starts_with("thumbv8m.base") {
-        println!("cargo:rustc-cfg=armv8m_base");
-    } else if target.starts_with("thumbv8m.main") {
-        println!("cargo:rustc-cfg=armv8m_main");
-    } else if target.starts_with("armv7-") | target.starts_with("armv7a-") {
-        println!("cargo:rustc-cfg=armv7a");
-    }
-
-    let is_avr = env::var("CARGO_CFG_TARGET_ARCH").as_deref() == Ok("avr");
-
-    // Set some cfg's depending on the target.
-    // - has_atomics: atomic load/store is available (either natively or through portable-atomic)
-    // - has_cas: atomic CAS is available (either natively or through portable-atomic)
-    // - use_portable_atomic: Use portable-atomic for all atomics (load/store and CAS).
-    // - use_portable_atomic_cas: Use portable-atomic for CAS atomic operations. Load/store can keep using core::sync:atomic.
-
-    // built-in targets with no atomic / CAS support as of nightly-2022-01-13
-    // AND not supported by the portable-atomic crate
-    // see the `no-atomics.sh` / `no-cas.sh` script sitting next to this file
-    if is_avr {
-        // lacks cas
-    } else {
-        match &target[..] {
-            "avr-unknown-gnu-atmega328"
-                | "bpfeb-unknown-none"
-                | "bpfel-unknown-none"
-                // | "msp430-none-elf"              // supported by portable-atomic
-                // | "riscv32i-unknown-none-elf"    // supported by portable-atomic
-                // | "riscv32imc-unknown-none-elf"  // supported by portable-atomic
-                // | "thumbv4t-none-eabi"           // supported by portable-atomic
-                // | "thumbv6m-none-eabi"           // supported by portable-atomic
-                => {}
-
-            _ => {
-                println!("cargo:rustc-cfg=has_cas");
-            }
-        }
+    // Manually list targets that have atomic load/store, but no CAS.
+    // Remove when `cfg(target_has_atomic_load_store)` is stable.
+    // last updated nightly-2023-10-28
+    match &target[..] {
+        "armv4t-none-eabi"
+        | "armv5te-none-eabi"
+        | "avr-unknown-gnu-atmega328"
+        | "bpfeb-unknown-none"
+        | "bpfel-unknown-none"
+        | "thumbv4t-none-eabi"
+        | "thumbv5te-none-eabi"
+        | "thumbv6m-none-eabi" => println!("cargo:rustc-cfg=has_atomic_load_store"),
+        _ => {}
     };
 
-    if is_avr {
-        // lacks atomics
-    } else {
-        println!("cargo:rustc-cfg=has_atomics");
-    }
-
-    // Let the code know if it should use portable-atomic or not, for either
-    // only CAS, or for all atomics.
-    if is_avr {
-        println!("cargo:rustc-cfg=use_portable_atomic");
-        println!("cargo:rustc-cfg=use_portable_atomic_cas");
-    } else {
-        match &target[..] {
-            "riscv32i-unknown-none-elf"
-            | "riscv32imc-unknown-none-elf"
-            | "xtensa-esp32s2-none-elf"
-            | "thumbv4t-none-eabi"
-            | "msp430-none-elf" => {
-                println!("cargo:rustc-cfg=use_portable_atomic");
-                println!("cargo:rustc-cfg=use_portable_atomic_cas");
-            }
-
-            "thumbv6m-none-eabi" => {
-                println!("cargo:rustc-cfg=use_portable_atomic_cas");
-            }
-            _ => {}
-        }
-    }
-
     // AArch64 instruction set contains `clrex` but not `ldrex` or `strex`; the
     // probe will succeed when we already know to deny this target from LLSC.
     if !target.starts_with("aarch64") {
diff --git a/no-atomics.sh b/no-atomics.sh
deleted file mode 100644
index 697933eee8..0000000000
--- a/no-atomics.sh
+++ /dev/null
@@ -1,14 +0,0 @@
-#!/bin/bash
-
-set -euo pipefail
-
-main() {
-    IFS='
-'
-    for t in $(rustc --print target-list); do
-        rustc +nightly --print cfg --target $t | grep 'target_has_atomic_load_store=' >/dev/null || echo $t
-    done
-
-}
-
-main
diff --git a/no-cas.sh b/no-cas.sh
deleted file mode 100644
index 9ec576fcf3..0000000000
--- a/no-cas.sh
+++ /dev/null
@@ -1,14 +0,0 @@
-#!/bin/bash
-
-set -euo pipefail
-
-main() {
-    IFS='
-'
-    for t in $(rustc --print target-list); do
-        rustc +nightly --print cfg --target $t | grep 'target_has_atomic=' >/dev/null || echo $t
-    done
-
-}
-
-main
diff --git a/src/lib.rs b/src/lib.rs
index 6228233397..130120908d 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -61,7 +61,7 @@
 //!
 //! The `heapless` crate provides the following optional Cargo features:
 //!
-//! - `ufmt-impl`: Implement [`ufmt_write::uWrite`] for `String<N>` and `Vec<u8, N>`
+//! - `ufmt`: Implement [`ufmt_write::uWrite`] for `String<N>` and `Vec<u8, N>`
 //!
 //! [`ufmt_write::uWrite`]: https://docs.rs/ufmt-write/
 //!
@@ -108,17 +108,31 @@ mod de;
 mod ser;
 
 pub mod binary_heap;
-#[cfg(feature = "defmt-impl")]
+#[cfg(feature = "defmt-03")]
 mod defmt;
-#[cfg(all(has_cas, feature = "cas"))]
+#[cfg(any(
+    // assume we have all atomics available if we're using portable-atomic
+    feature = "portable-atomic",
+    // target has native atomic CAS (mpmc_large requires usize, otherwise just u8)
+    all(feature = "mpmc_large", target_has_atomic = "ptr"),
+    all(not(feature = "mpmc_large"), target_has_atomic = "8")
+))]
 pub mod mpmc;
 #[cfg(any(arm_llsc, target_arch = "x86"))]
 pub mod pool;
 pub mod sorted_linked_list;
-#[cfg(has_atomics)]
+#[cfg(any(
+    // assume we have all atomics available if we're using portable-atomic
+    feature = "portable-atomic",
+    // target has native atomic CAS. Note this is too restrictive, spsc requires load/store only, not CAS.
+    // This should be `cfg(target_has_atomic_load_store)`, but that's not stable yet.
+    target_has_atomic = "ptr",
+    // or the current target is in a list in build.rs of targets known to have load/store but no CAS.
+    has_atomic_load_store
+))]
 pub mod spsc;
 
-#[cfg(feature = "ufmt-impl")]
+#[cfg(feature = "ufmt")]
 mod ufmt;
 
 mod sealed;
diff --git a/src/mpmc.rs b/src/mpmc.rs
index 53c515ea09..db38934b66 100644
--- a/src/mpmc.rs
+++ b/src/mpmc.rs
@@ -88,9 +88,9 @@
 
 use core::{cell::UnsafeCell, mem::MaybeUninit};
 
-#[cfg(not(use_portable_atomic_cas))]
+#[cfg(not(feature = "portable-atomic"))]
 use core::sync::atomic;
-#[cfg(use_portable_atomic_cas)]
+#[cfg(feature = "portable-atomic")]
 use portable_atomic as atomic;
 
 use atomic::Ordering;
diff --git a/src/spsc.rs b/src/spsc.rs
index 8cf4493df1..c8085720e7 100644
--- a/src/spsc.rs
+++ b/src/spsc.rs
@@ -96,9 +96,9 @@
 
 use core::{cell::UnsafeCell, fmt, hash, mem::MaybeUninit, ptr};
 
-#[cfg(not(use_portable_atomic))]
+#[cfg(not(feature = "portable-atomic"))]
 use core::sync::atomic;
-#[cfg(use_portable_atomic)]
+#[cfg(feature = "portable-atomic")]
 use portable_atomic as atomic;
 
 use atomic::{AtomicUsize, Ordering};