diff --git a/compiler/rustc_metadata/src/lib.rs b/compiler/rustc_metadata/src/lib.rs
index 3df18098a07..5e4c1284e24 100644
--- a/compiler/rustc_metadata/src/lib.rs
+++ b/compiler/rustc_metadata/src/lib.rs
@@ -10,6 +10,7 @@
 #![feature(macro_metavar_expr)]
 #![feature(min_specialization)]
 #![feature(slice_as_chunks)]
+#![feature(trusted_len)]
 #![feature(try_blocks)]
 #![feature(never_type)]
 #![recursion_limit = "256"]
diff --git a/compiler/rustc_metadata/src/rmeta/decoder.rs b/compiler/rustc_metadata/src/rmeta/decoder.rs
index 6042d0e49c3..72b7c08f04b 100644
--- a/compiler/rustc_metadata/src/rmeta/decoder.rs
+++ b/compiler/rustc_metadata/src/rmeta/decoder.rs
@@ -38,6 +38,7 @@ use rustc_span::{self, BytePos, ExpnId, Pos, Span, SyntaxContext, DUMMY_SP};
 
 use proc_macro::bridge::client::ProcMacro;
 use std::io;
+use std::iter::TrustedLen;
 use std::mem;
 use std::num::NonZeroUsize;
 use std::path::Path;
@@ -277,17 +278,25 @@ struct DecodeIterator<'a, 'tcx, T> {
 impl<'a, 'tcx, T: Decodable<DecodeContext<'a, 'tcx>>> Iterator for DecodeIterator<'a, 'tcx, T> {
     type Item = T;
 
+    #[inline(always)]
     fn next(&mut self) -> Option<Self::Item> {
         self.elem_counter.next().map(|_| T::decode(&mut self.dcx))
     }
+
+    #[inline(always)]
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        self.elem_counter.size_hint()
+    }
 }
 
 impl<'a, 'tcx, T: Decodable<DecodeContext<'a, 'tcx>>> ExactSizeIterator
     for DecodeIterator<'a, 'tcx, T>
 {
-    fn len(&self) -> usize {
-        self.elem_counter.len()
-    }
+}
+
+unsafe impl<'a, 'tcx, T: Decodable<DecodeContext<'a, 'tcx>>> TrustedLen
+    for DecodeIterator<'a, 'tcx, T>
+{
 }
 
 impl<'a: 'x, 'tcx: 'x, 'x, T: Decodable<DecodeContext<'a, 'tcx>>> LazyArray<T> {
@@ -321,6 +330,7 @@ impl<'a, 'tcx> DecodeContext<'a, 'tcx> {
         self.cdata().map_encoded_cnum_to_current(cnum)
     }
 
+    #[inline]
     fn read_lazy_offset_then<T>(&mut self, f: impl Fn(NonZeroUsize) -> T) -> T {
         let distance = self.read_usize();
         let position = match self.lazy_state {
diff --git a/compiler/rustc_metadata/src/rmeta/decoder/cstore_impl.rs b/compiler/rustc_metadata/src/rmeta/decoder/cstore_impl.rs
index da2ac41af92..e3581a7607f 100644
--- a/compiler/rustc_metadata/src/rmeta/decoder/cstore_impl.rs
+++ b/compiler/rustc_metadata/src/rmeta/decoder/cstore_impl.rs
@@ -32,24 +32,28 @@ trait ProcessQueryValue<'tcx, T> {
 }
 
 impl<T> ProcessQueryValue<'_, Option<T>> for Option<T> {
+    #[inline(always)]
     fn process_decoded(self, _tcx: TyCtxt<'_>, _err: impl Fn() -> !) -> Option<T> {
         self
     }
 }
 
 impl<T> ProcessQueryValue<'_, T> for Option<T> {
+    #[inline(always)]
     fn process_decoded(self, _tcx: TyCtxt<'_>, err: impl Fn() -> !) -> T {
         if let Some(value) = self { value } else { err() }
     }
 }
 
 impl<'tcx, T: ArenaAllocatable<'tcx>> ProcessQueryValue<'tcx, &'tcx T> for Option<T> {
+    #[inline(always)]
     fn process_decoded(self, tcx: TyCtxt<'tcx>, err: impl Fn() -> !) -> &'tcx T {
         if let Some(value) = self { tcx.arena.alloc(value) } else { err() }
     }
 }
 
 impl<T, E> ProcessQueryValue<'_, Result<Option<T>, E>> for Option<T> {
+    #[inline(always)]
     fn process_decoded(self, _tcx: TyCtxt<'_>, _err: impl Fn() -> !) -> Result<Option<T>, E> {
         Ok(self)
     }
@@ -58,12 +62,14 @@ impl<T, E> ProcessQueryValue<'_, Result<Option<T>, E>> for Option<T> {
 impl<'a, 'tcx, T: Copy + Decodable<DecodeContext<'a, 'tcx>>> ProcessQueryValue<'tcx, &'tcx [T]>
     for Option<DecodeIterator<'a, 'tcx, T>>
 {
+    #[inline(always)]
     fn process_decoded(self, tcx: TyCtxt<'tcx>, _err: impl Fn() -> !) -> &'tcx [T] {
         if let Some(iter) = self { tcx.arena.alloc_from_iter(iter) } else { &[] }
     }
 }
 
 impl ProcessQueryValue<'_, Option<DeprecationEntry>> for Option<Deprecation> {
+    #[inline(always)]
     fn process_decoded(self, _tcx: TyCtxt<'_>, _err: impl Fn() -> !) -> Option<DeprecationEntry> {
         self.map(DeprecationEntry::external)
     }