Remove LazyMeta::min_size

It is extremely conservative and as such barely reduces the size of
encoded Lazy distances, but does increase complexity.

Author: bjorn3

compiler/rustc_metadata/src/rmeta/decoder.rs

@@ -304,18 +304,17 @@ impl<'a, 'tcx> DecodeContext<'a, 'tcx> {
         &mut self,
         meta: T::Meta,
     ) -> Result<Lazy<T>, <Self as Decoder>::Error> {
-        let min_size = T::min_size(meta);
         let distance = self.read_usize()?;
         let position = match self.lazy_state {
             LazyState::NoNode => bug!("read_lazy_with_meta: outside of a metadata node"),
             LazyState::NodeStart(start) => {
                 let start = start.get();
-                assert!(distance + min_size <= start);
-                start - distance - min_size
+                assert!(distance <= start);
+                start - distance
             }
-            LazyState::Previous(last_min_end) => last_min_end.get() + distance,
+            LazyState::Previous(last_pos) => last_pos.get() + distance,
         };
-        self.lazy_state = LazyState::Previous(NonZeroUsize::new(position + min_size).unwrap());
+        self.lazy_state = LazyState::Previous(NonZeroUsize::new(position).unwrap());
         Ok(Lazy::from_position_and_meta(NonZeroUsize::new(position).unwrap(), meta))
     }
 

compiler/rustc_metadata/src/rmeta/encoder.rs

@@ -404,24 +404,24 @@ impl<'a, 'tcx> EncodeContext<'a, 'tcx> {
         &mut self,
         lazy: Lazy<T>,
     ) -> Result<(), <Self as Encoder>::Error> {
-        let min_end = lazy.position.get() + T::min_size(lazy.meta);
+        let pos = lazy.position.get();
         let distance = match self.lazy_state {
             LazyState::NoNode => bug!("emit_lazy_distance: outside of a metadata node"),
             LazyState::NodeStart(start) => {
                 let start = start.get();
-                assert!(min_end <= start);
-                start - min_end
+                assert!(pos <= start);
+                start - pos
             }
-            LazyState::Previous(last_min_end) => {
+            LazyState::Previous(last_pos) => {
                 assert!(
-                    last_min_end <= lazy.position,
+                    last_pos <= lazy.position,
                     "make sure that the calls to `lazy*` \
                      are in the same order as the metadata fields",
                 );
-                lazy.position.get() - last_min_end.get()
+                lazy.position.get() - last_pos.get()
             }
         };
-        self.lazy_state = LazyState::Previous(NonZeroUsize::new(min_end).unwrap());
+        self.lazy_state = LazyState::Previous(NonZeroUsize::new(pos).unwrap());
         self.emit_usize(distance)
     }
 
@@ -436,7 +436,7 @@ impl<'a, 'tcx> EncodeContext<'a, 'tcx> {
         let meta = value.encode_contents_for_lazy(self);
         self.lazy_state = LazyState::NoNode;
 
-        assert!(pos.get() + <T>::min_size(meta) <= self.position());
+        assert!(pos.get() <= self.position());
 
         Lazy::from_position_and_meta(pos, meta)
     }

compiler/rustc_metadata/src/rmeta/mod.rs

@@ -62,27 +62,14 @@ pub const METADATA_HEADER: &[u8] = &[b'r', b'u', b's', b't', 0, 0, 0, METADATA_V
 /// e.g. for `Lazy<[T]>`, this is the length (count of `T` values).
 trait LazyMeta {
     type Meta: Copy + 'static;
-
-    /// Returns the minimum encoded size.
-    // FIXME(eddyb) Give better estimates for certain types.
-    fn min_size(meta: Self::Meta) -> usize;
 }
 
 impl<T> LazyMeta for T {
     type Meta = ();
-
-    fn min_size(_: ()) -> usize {
-        assert_ne!(std::mem::size_of::<T>(), 0);
-        1
-    }
 }
 
 impl<T> LazyMeta for [T] {
     type Meta = usize;
-
-    fn min_size(len: usize) -> usize {
-        len * T::min_size(())
-    }
 }
 
 /// A value of type T referred to by its absolute position
@@ -160,8 +147,7 @@ enum LazyState {
     NodeStart(NonZeroUsize),
 
     /// Inside a metadata node, with a previous `Lazy`.
-    /// The position is a conservative estimate of where that
-    /// previous `Lazy` would end (see their comments).
+    /// The position is where that previous `Lazy` would start.
     Previous(NonZeroUsize),
 }
 

compiler/rustc_metadata/src/rmeta/table.rs

@@ -183,10 +183,6 @@ where
     Option<T>: FixedSizeEncoding,
 {
     type Meta = usize;
-
-    fn min_size(len: usize) -> usize {
-        len
-    }
 }
 
 impl<I: Idx, T> Lazy<Table<I, T>>