Auto merge of #110634 - saethlin:pointy-decoder, r=cjgillot

Rewrite MemDecoder around pointers not a slice

This is basically https://github.com/rust-lang/rust/pull/109910 but I'm being a lot more aggressive. The pointer-based structure means that it makes a lot more sense to absorb more complexity into `MemDecoder`, most of the diff is just complexity moving from one place to another.

The primary argument for this structure is that we only incur a single bounds check when doing multi-byte reads from a `MemDecoder`. With the slice-based implementation we need to do those with `data[position..position + len]` , which needs to account for `position + len` wrapping. It would be possible to dodge the first bounds check if we stored a slice that starts at `position`, but that would require updating the pointer and length on every read.

This PR also embeds the failure path in a separate function, which means that this PR should subsume all the perf wins observed in https://github.com/rust-lang/rust/pull/109867.

compiler/rustc_middle/src/ty/codec.rs

@@ -530,6 +530,16 @@ macro_rules! implement_ty_decoder {
                 fn read_raw_bytes(&mut self, len: usize) -> &[u8] {
                     self.opaque.read_raw_bytes(len)
                 }
+
+                #[inline]
+                fn position(&self) -> usize {
+                    self.opaque.position()
+                }
+
+                #[inline]
+                fn peek_byte(&self) -> u8 {
+                    self.opaque.peek_byte()
+                }
             }
         }
     }