Document no_std feature in README

Also clean up a few imports.

README.md

@@ -158,6 +158,18 @@ The API shouldn't know about the specifics of any of these requirements - it sho
 </details>
 <p></p>
 
+## no_std
+
+rectangle-pack supports `no_std` by disabling the `std` feature.
+
+```toml
+rectangle-pack = {version = "0.4", default-features = false}
+```
+
+Disabling the `std` feature does the following.
+
+- `BTreeMap`s are used internally in places where `HashMap`s would have been used.
+
 ## Features
 
 - Place any number of 2d / 3d rectangles into any number of 2d / 3d target bins.

src/bin_section.rs

@@ -3,7 +3,7 @@ use crate::{BoxSizeHeuristicFn, PackedLocation, RectToInsert, WidthHeightDepth};
 
 use core::{
     cmp::Ordering,
-    fmt::{Debug, Display, Error as ErrorFmt, Formatter},
+    fmt::{Debug, Display, Error as FmtError, Formatter},
 };
 
 mod overlaps;
@@ -60,7 +60,7 @@ pub enum BinSectionError {
 }
 
 impl Display for BinSectionError {
-    fn fmt(&self, f: &mut Formatter<'_>) -> Result<(), ErrorFmt> {
+    fn fmt(&self, f: &mut Formatter<'_>) -> Result<(), FmtError> {
         let err = match self {
             BinSectionError::PlacementWiderThanBinSection => {
                 "Can not place a rectangle inside of a bin that is wider than that rectangle."

src/grouped_rects_to_place.rs

@@ -1,16 +1,15 @@
 use crate::RectToInsert;
 
-#[cfg(std)]
-use crate::Vec;
 #[cfg(not(std))]
 use alloc::collections::BTreeMap as KeyValMap;
+#[cfg(std)]
+use std::collections::HashMap as KeyValMap;
+
 use alloc::{
     collections::{btree_map::Entry, BTreeMap},
     vec::Vec,
 };
 use core::{fmt::Debug, hash::Hash};
-#[cfg(std)]
-use std::collections::HashMap as KeyValMap;
 
 /// Groups of rectangles that need to be placed into bins.
 ///

src/lib.rs

@@ -8,13 +8,15 @@ extern crate alloc;
 
 #[cfg(not(std))]
 use alloc::collections::BTreeMap as KeyValMap;
+#[cfg(std)]
+use std::collections::HashMap as KeyValMap;
+
 use alloc::{collections::BTreeMap, vec::Vec};
+
 use core::{
-    fmt::{Debug, Display, Error as ErrorFmt, Formatter},
+    fmt::{Debug, Display, Error as FmtError, Formatter},
     hash::Hash,
 };
-#[cfg(std)]
-use std::collections::HashMap as KeyValMap;
 
 pub use crate::bin_section::contains_smallest_box;
 pub use crate::bin_section::BinSection;
@@ -284,17 +286,18 @@ pub enum RectanglePackError {
     NotEnoughBinSpace,
 }
 
+#[cfg(std)]
+impl std::error::Error for RectanglePackError {}
+
 impl Display for RectanglePackError {
-    fn fmt(&self, f: &mut Formatter<'_>) -> Result<(), ErrorFmt> {
+    fn fmt(&self, f: &mut Formatter<'_>) -> Result<(), FmtError> {
         match self {
             RectanglePackError::NotEnoughBinSpace => {
                 f.write_str("Not enough space to place all of the rectangles.")
             }
         }
     }
 }
-#[cfg(std)]
-impl std::error::Error for RectanglePackError {}
 
 fn sort_bins_smallest_to_largest<BinId>(
     bins: &mut Vec<(&BinId, &mut TargetBin)>,

src/target_bin/push_available_bin_section.rs

@@ -8,7 +8,7 @@
 
 use crate::bin_section::BinSection;
 use crate::TargetBin;
-use core::fmt::{Display, Formatter, Result as ResultFmt};
+use core::fmt::{Display, Formatter, Result as FmtResult};
 
 impl TargetBin {
     /// Push a [`BinSection`] to the list of remaining [`BinSection`]'s that rectangles can be
@@ -77,7 +77,7 @@ pub enum PushBinSectionError {
 }
 
 impl Display for PushBinSectionError {
-    fn fmt(&self, f: &mut Formatter<'_>) -> ResultFmt {
+    fn fmt(&self, f: &mut Formatter<'_>) -> FmtResult {
         match self {
             PushBinSectionError::OutOfBounds(oob) => {
                 f.debug_tuple("BinSection").field(oob).finish()