Various clippy fixes

src/decode.rs

@@ -318,12 +318,13 @@ impl<R: Read> Decoder<R> {
 
     /// Returns an immutable reference to the underlying reader.
     #[inline]
-    pub fn reader(&self) -> &R {
+    pub const fn reader(&self) -> &R {
         &self.reader
     }
 
     /// Consumes the decoder and returns the underlying reader back.
     #[inline]
+    #[allow(clippy::missing_const_for_fn)]
     pub fn into_reader(self) -> R {
         self.reader
     }
@@ -343,7 +344,7 @@ impl<R: Reader> Decoder<R> {
     /// to decode RGB into RGBA (in which case the alpha channel will be set
     /// to 255), and vice versa (in which case the alpha channel will be ignored).
     #[inline]
-    pub fn with_channels(mut self, channels: Channels) -> Self {
+    pub const fn with_channels(mut self, channels: Channels) -> Self {
         self.channels = channels;
         self
     }
@@ -352,13 +353,13 @@ impl<R: Reader> Decoder<R> {
     ///
     /// Note: this may differ from the number of channels specified in the header.
     #[inline]
-    pub fn channels(&self) -> Channels {
+    pub const fn channels(&self) -> Channels {
         self.channels
     }
 
     /// Returns the decoded image header.
     #[inline]
-    pub fn header(&self) -> &Header {
+    pub const fn header(&self) -> &Header {
         &self.header
     }
 
@@ -366,7 +367,7 @@ impl<R: Reader> Decoder<R> {
     ///
     /// Can be used to pre-allocate the buffer to decode the image into.
     #[inline]
-    pub fn required_buf_len(&self) -> usize {
+    pub const fn required_buf_len(&self) -> usize {
         self.header.n_pixels().saturating_mul(self.channels.as_u8() as usize)
     }
 

src/lib.rs

@@ -62,7 +62,8 @@
     clippy::must_use_candidate,
     clippy::module_name_repetitions,
     clippy::cargo_common_metadata,
-    clippy::doc_markdown
+    clippy::doc_markdown,
+    clippy::return_self_not_must_use,
 )]
 #![cfg_attr(not(any(feature = "std", test)), no_std)]
 #[cfg(all(feature = "alloc", not(any(feature = "std", test))))]

src/utils.rs

@@ -84,7 +84,7 @@ pub struct GenericWriter<W> {
 
 #[cfg(feature = "std")]
 impl<W: Write> GenericWriter<W> {
-    pub fn new(writer: W) -> Self {
+    pub const fn new(writer: W) -> Self {
         Self { writer, n_written: 0 }
     }
 }