feat(no_std)!: option to disable layout cache for no_std compatibility (#1795)

Resolves #1780 

BREAKING CHANGE: Disabling `default-features` will now disable layout
cache, which can have a negative impact on performance.
`Layout::init_cache` and `Layout::DEFAULT_CACHE_SIZE` are now only
available if `layout-cache` feature is enabled.

BREAKING-CHANGES.md

@@ -20,6 +20,8 @@ This is a quick summary of the sections below:
   - `Backend` now uses `Self::Error` for error handling instead of `std::io::Error`
   - `Terminal<B>` now uses `B::Error` for error handling instead of `std::io::Error`
   - `TestBackend` now uses `core::convert::Infallible` for error handling instead of `std::io::Error`
+  - Disabling `default-features` will now disable layout cache, which can have a negative impact on performance
+  - `Layout::init_cache` and `Layout::DEFAULT_CACHE_SIZE` are now only available if `layout-cache` feature is enabled
 - [v0.29.0](#v0290)
   - `Sparkline::data` takes `IntoIterator<Item = SparklineBar>` instead of `&[u64]` and is no longer const
   - Removed public fields from `Rect` iterators
@@ -84,6 +86,26 @@ This is a quick summary of the sections below:
 
 ## Unreleased (0.30.0)
 
+### `Layout::init_cache` and `Layout::DEFAULT_CACHE_SIZE` are now only available if `layout-cache` feature is enabled ([#1795])
+
+[#1795]: https://github.com/ratatui/ratatui/pull/1795
+
+Previously, `Layout::init_cache` and `Layout::DEFAULT_CACHE_SIZE` were available independently of
+enabled feature flags.
+
+### Disabling `default-features` will now disable layout cache, which can have a negative impact on performance ([#1795])
+
+[#1795]: https://github.com/ratatui/ratatui/pull/1795
+
+Layout cache is now opt-in in `ratatui-core` and enabled by default in `ratatui`. If app doesn't
+make use of `no_std`-compatibility, and disables `default-feature`, it is recommended to explicitly
+re-enable layout cache. Not doing so may impact performance.
+
+```diff
+- ratatui = { version = "0.29.0", default-features = false }
++ ratatui = { version = "0.30.0", default-features = false, features = ["layout-cache"] }
+```
+
 ### `TestBackend` now uses `core::convert::Infallible` for error handling instead of `std::io::Error` ([#1823])
 
 [#1823]: https://github.com/ratatui/ratatui/pull/1823

ratatui-core/Cargo.toml

@@ -24,6 +24,12 @@ rustdoc-args = ["--cfg", "docsrs"]
 [features]
 default = []
 
+## enables std
+std = []
+
+## enables layout cache
+layout-cache = ["std"]
+
 ## enables conversions to / from colors, modifiers, and styles in the ['anstyle'] crate
 anstyle = ["dep:anstyle"]
 

ratatui-core/src/layout/layout.rs

@@ -1,10 +1,9 @@
 use alloc::format;
 use alloc::rc::Rc;
 use alloc::vec::Vec;
-use core::cell::RefCell;
 use core::iter;
 use core::num::NonZeroUsize;
-use std::{dbg, thread_local};
+use std::dbg;
 
 use hashbrown::HashMap;
 use itertools::Itertools;
@@ -39,8 +38,9 @@ type Cache = LruCache<(Rect, Layout), (Segments, Spacers)>;
 // calculations.
 const FLOAT_PRECISION_MULTIPLIER: f64 = 100.0;
 
-thread_local! {
-    static LAYOUT_CACHE: RefCell<Cache> = RefCell::new(Cache::new(
+#[cfg(feature = "layout-cache")]
+std::thread_local! {
+    static LAYOUT_CACHE: core::cell::RefCell<Cache> = core::cell::RefCell::new(Cache::new(
         NonZeroUsize::new(Layout::DEFAULT_CACHE_SIZE).unwrap(),
     ));
 }
@@ -188,6 +188,8 @@ impl Layout {
     /// bit more to make it a round number. This gives enough entries to store a layout for every
     /// row and every column, twice over, which should be enough for most apps. For those that need
     /// more, the cache size can be set with [`Layout::init_cache()`].
+    /// This const is unused if layout cache is disabled.
+    #[cfg(feature = "layout-cache")]
     pub const DEFAULT_CACHE_SIZE: usize = 500;
 
     /// Creates a new layout with default values.
@@ -280,8 +282,9 @@ impl Layout {
     /// grows until `cache_size` is reached.
     ///
     /// By default, the cache size is [`Self::DEFAULT_CACHE_SIZE`].
+    #[cfg(feature = "layout-cache")]
     pub fn init_cache(cache_size: NonZeroUsize) {
-        LAYOUT_CACHE.with_borrow_mut(|c| c.resize(cache_size));
+        LAYOUT_CACHE.with_borrow_mut(|cache| cache.resize(cache_size));
     }
 
     /// Set the direction of the layout.
@@ -653,11 +656,18 @@ impl Layout {
     /// );
     /// ```
     pub fn split_with_spacers(&self, area: Rect) -> (Segments, Spacers) {
-        LAYOUT_CACHE.with_borrow_mut(|c| {
-            let key = (area, self.clone());
-            c.get_or_insert(key, || self.try_split(area).expect("failed to split"))
-                .clone()
-        })
+        let split = || self.try_split(area).expect("failed to split");
+
+        #[cfg(feature = "layout-cache")]
+        {
+            LAYOUT_CACHE.with_borrow_mut(|cache| {
+                let key = (area, self.clone());
+                cache.get_or_insert(key, split).clone()
+            })
+        }
+
+        #[cfg(not(feature = "layout-cache"))]
+        split()
     }
 
     fn try_split(&self, area: Rect) -> Result<(Segments, Spacers), AddConstraintError> {
@@ -1200,14 +1210,15 @@ mod tests {
     }
 
     #[test]
+    #[cfg(feature = "layout-cache")]
     fn cache_size() {
-        LAYOUT_CACHE.with_borrow(|c| {
-            assert_eq!(c.cap().get(), Layout::DEFAULT_CACHE_SIZE);
+        LAYOUT_CACHE.with_borrow(|cache| {
+            assert_eq!(cache.cap().get(), Layout::DEFAULT_CACHE_SIZE);
         });
 
         Layout::init_cache(NonZeroUsize::new(10).unwrap());
-        LAYOUT_CACHE.with_borrow(|c| {
-            assert_eq!(c.cap().get(), 10);
+        LAYOUT_CACHE.with_borrow(|cache| {
+            assert_eq!(cache.cap().get(), 10);
         });
     }
 

ratatui/Cargo.toml

@@ -25,9 +25,10 @@ rustdoc-args = ["--cfg", "docsrs"]
 #!
 ## By default, we enable the crossterm backend as this is a reasonable choice for most applications
 ## as it is supported on Linux/Mac/Windows systems. We also enable the `underline-color` feature
-## which allows you to set the underline color of text and the `macros` feature which provides
-## some useful macros.
-default = ["crossterm", "underline-color", "all-widgets", "macros"]
+## which allows you to set the underline color of text, the `macros` feature which provides
+## some useful macros and `layout-cache` which speeds up layout cache calculations
+## in `std`-enabled environments.
+default = ["crossterm", "underline-color", "all-widgets", "macros", "layout-cache"]
 #! Generally an application will only use one backend, so you should only enable one of the following features:
 ## enables the [`CrosstermBackend`](backend::CrosstermBackend) backend and adds a dependency on [`crossterm`].
 crossterm = ["dep:ratatui-crossterm"]
@@ -48,6 +49,9 @@ serde = [
   "ratatui-termwiz?/serde",
 ]
 
+## enables layout cache
+layout-cache = ["ratatui-core/layout-cache"]
+
 ## enables conversions from colors in the [`palette`] crate to [`Color`](crate::style::Color).
 palette = ["ratatui-core/palette", "dep:palette"]
 

xtask/src/commands/backend.rs

@@ -54,6 +54,15 @@ impl Run for TestBackend {
             Backend::Termion => "termion",
             Backend::Termwiz => "termwiz",
         };
+        // This is a temporary hack to run tests both with and without layout cache.
+        // https://github.com/ratatui/ratatui/issues/1820
+        run_cargo(vec![
+            "test",
+            "--all-targets",
+            "--no-default-features",
+            "--features",
+            format!("{backend},layout-cache").as_str(),
+        ])?;
         run_cargo(vec![
             "test",
             "--all-targets",