feat: add an alloc feature for no-std builds with an allocator (#214)

Name::to_string and the gvar-alloc heap spill required std purely for the allocator. Breaking: gvar-alloc now implies alloc rather than std, so it no longer transitively satisfies the std/no-std-float guard.

Author: iriswebb

Cargo.toml

@@ -22,11 +22,13 @@ exclude = ["benches/", "tests/", "testing-tools/", "examples/", "meson.build"]
 core_maths = { version = "0.1.1", optional = true } # only for no_std builds
 
 [features]
-default = ["std", "opentype-layout", "apple-layout", "variable-fonts", "glyph-names"]
+default = ["std", "alloc", "opentype-layout", "apple-layout", "variable-fonts", "glyph-names"]
 # Enables the use of the standard library.
 # When disabled, the `no-std-float` feature must be enabled instead.
-std = []
+std = ["alloc"]
 no-std-float = ["core_maths"]
+# Enables the use of the allocator without using std
+alloc = []
 # Enables variable fonts support. Increases binary size almost twice.
 # Includes avar, CFF2, fvar, gvar, HVAR, MVAR and VVAR tables.
 variable-fonts = []
@@ -45,7 +47,7 @@ glyph-names = []
 # while the spec allows up to 4095. Most variable fonts use 10-20 tuples,
 # so our limit is suitable for most of the cases. But if you need full support, you have to
 # enable this feature.
-gvar-alloc = ["std"]
+gvar-alloc = ["alloc"]
 
 [dev-dependencies]
 base64 = "0.23"

src/lib.rs

@@ -49,6 +49,9 @@ Font parsing starts with a [`Face`].
 #[macro_use]
 extern crate std;
 
+#[cfg(feature = "alloc")]
+extern crate alloc;
+
 #[cfg(not(any(feature = "std", feature = "no-std-float")))]
 compile_error!("You have to activate either the `std` or the `no-std-float` feature.");
 
@@ -795,8 +798,7 @@ impl core::fmt::Display for FaceParsingError {
     }
 }
 
-#[cfg(feature = "std")]
-impl std::error::Error for FaceParsingError {}
+impl core::error::Error for FaceParsingError {}
 
 /// A raw font face.
 ///

src/tables/gvar.rs

@@ -66,7 +66,7 @@ enum VariationTuples<'a> {
     },
     #[cfg(feature = "gvar-alloc")]
     Heap {
-        vec: std::vec::Vec<VariationTuple<'a>>,
+        vec: alloc::vec::Vec<VariationTuple<'a>>,
     },
 }
 
@@ -87,7 +87,7 @@ impl<'a> VariationTuples<'a> {
         if capacity > MAX_STACK_TUPLES_LEN {
             // ... and we're currently on the stack, move to the heap.
             if let Self::Stack { headers, len } = self {
-                let mut vec = std::vec::Vec::with_capacity(capacity as usize);
+                let mut vec = alloc::vec::Vec::with_capacity(capacity as usize);
                 for header in headers.iter_mut().take(*len as usize) {
                     let header = core::mem::take(header);
                     vec.push(header);

src/tables/name.rs

@@ -1,10 +1,10 @@
 //! A [Naming Table](
 //! https://docs.microsoft.com/en-us/typography/opentype/spec/name) implementation.
 
-#[cfg(feature = "std")]
-use std::string::String;
-#[cfg(feature = "std")]
-use std::vec::Vec;
+#[cfg(feature = "alloc")]
+use alloc::string::String;
+#[cfg(feature = "alloc")]
+use alloc::vec::Vec;
 
 use crate::parser::{FromData, LazyArray16, Offset, Offset16, Stream};
 use crate::Language;
@@ -140,7 +140,7 @@ impl<'a> Name<'a> {
     /// - Unicode Platform ID
     /// - Windows Platform ID + Symbol
     /// - Windows Platform ID + Unicode BMP
-    #[cfg(feature = "std")]
+    #[cfg(feature = "alloc")]
     #[inline(never)]
     pub fn to_string(&self) -> Option<String> {
         if self.is_unicode() {
@@ -156,7 +156,7 @@ impl<'a> Name<'a> {
         is_unicode_encoding(self.platform_id, self.encoding_id)
     }
 
-    #[cfg(feature = "std")]
+    #[cfg(feature = "alloc")]
     #[inline(never)]
     fn name_from_utf16_be(&self) -> Option<String> {
         let mut name: Vec<u16> = Vec::new();
@@ -182,7 +182,7 @@ impl<'a> Name<'a> {
     }
 }
 
-#[cfg(feature = "std")]
+#[cfg(feature = "alloc")]
 impl<'a> core::fmt::Debug for Name<'a> {
     fn fmt(&self, f: &mut core::fmt::Formatter) -> core::fmt::Result {
         let name = self.to_string();
@@ -197,7 +197,7 @@ impl<'a> core::fmt::Debug for Name<'a> {
     }
 }
 
-#[cfg(not(feature = "std"))]
+#[cfg(not(feature = "alloc"))]
 impl<'a> core::fmt::Debug for Name<'a> {
     fn fmt(&self, f: &mut core::fmt::Formatter) -> core::fmt::Result {
         f.debug_struct("Name")