refine comments, disambiguate len for array and tables

compiler/rustc_metadata/src/rmeta/decoder.rs

@@ -269,7 +269,7 @@ impl<'a, 'tcx, T: Decodable<DecodeContext<'a, 'tcx>>> LazyValue<T> {
 }
 
 struct DecodeIterator<'a, 'tcx, T> {
-    range: std::ops::Range<usize>,
+    elem_counter: std::ops::Range<usize>,
     dcx: DecodeContext<'a, 'tcx>,
     _phantom: PhantomData<fn() -> T>,
 }
@@ -278,7 +278,7 @@ impl<'a, 'tcx, T: Decodable<DecodeContext<'a, 'tcx>>> Iterator for DecodeIterato
     type Item = T;
 
     fn next(&mut self) -> Option<Self::Item> {
-        self.range.next().map(|_| T::decode(&mut self.dcx))
+        self.elem_counter.next().map(|_| T::decode(&mut self.dcx))
     }
 }
 
@@ -286,7 +286,7 @@ impl<'a, 'tcx, T: Decodable<DecodeContext<'a, 'tcx>>> ExactSizeIterator
     for DecodeIterator<'a, 'tcx, T>
 {
     fn len(&self) -> usize {
-        self.range.len()
+        self.elem_counter.len()
     }
 }
 
@@ -294,7 +294,7 @@ impl<'a: 'x, 'tcx: 'x, 'x, T: Decodable<DecodeContext<'a, 'tcx>>> LazyArray<T> {
     fn decode<M: Metadata<'a, 'tcx>>(self, metadata: M) -> DecodeIterator<'a, 'tcx, T> {
         let mut dcx = metadata.decoder(self.position.get());
         dcx.lazy_state = LazyState::NodeStart(self.position);
-        DecodeIterator { range: (0..self.len), dcx, _phantom: PhantomData }
+        DecodeIterator { elem_counter: (0..self.num_elems), dcx, _phantom: PhantomData }
     }
 }
 
@@ -342,11 +342,11 @@ impl<'a, 'tcx> DecodeContext<'a, 'tcx> {
     }
 
     fn read_lazy_array<T>(&mut self, len: usize) -> LazyArray<T> {
-        self.read_lazy_offset_then(|pos| LazyArray::from_position_and_len(pos, len))
+        self.read_lazy_offset_then(|pos| LazyArray::from_position_and_num_elems(pos, len))
     }
 
     fn read_lazy_table<I, T>(&mut self, len: usize) -> LazyTable<I, T> {
-        self.read_lazy_offset_then(|pos| LazyTable::from_position_and_len(pos, len))
+        self.read_lazy_offset_then(|pos| LazyTable::from_position_and_encoded_size(pos, len))
     }
 
     #[inline]

compiler/rustc_metadata/src/rmeta/encoder.rs

@@ -132,8 +132,8 @@ impl<'a, 'tcx, T> Encodable<EncodeContext<'a, 'tcx>> for LazyValue<T> {
 
 impl<'a, 'tcx, T> Encodable<EncodeContext<'a, 'tcx>> for LazyArray<T> {
     fn encode(&self, e: &mut EncodeContext<'a, 'tcx>) -> opaque::EncodeResult {
-        e.emit_usize(self.len)?;
-        if self.len == 0 {
+        e.emit_usize(self.num_elems)?;
+        if self.num_elems == 0 {
             return Ok(());
         }
         e.emit_lazy_distance(self.position)
@@ -142,7 +142,7 @@ impl<'a, 'tcx, T> Encodable<EncodeContext<'a, 'tcx>> for LazyArray<T> {
 
 impl<'a, 'tcx, I, T> Encodable<EncodeContext<'a, 'tcx>> for LazyTable<I, T> {
     fn encode(&self, e: &mut EncodeContext<'a, 'tcx>) -> opaque::EncodeResult {
-        e.emit_usize(self.len)?;
+        e.emit_usize(self.encoded_size)?;
         e.emit_lazy_distance(self.position)
     }
 }
@@ -421,7 +421,7 @@ impl<'a, 'tcx> EncodeContext<'a, 'tcx> {
 
         assert!(pos.get() <= self.position());
 
-        LazyArray::from_position_and_len(pos, len)
+        LazyArray::from_position_and_num_elems(pos, len)
     }
 
     fn encode_info_for_items(&mut self) {

compiler/rustc_metadata/src/rmeta/mod.rs

@@ -77,20 +77,7 @@ pub const METADATA_HEADER: &[u8] = &[b'r', b'u', b's', b't', 0, 0, 0, METADATA_V
 /// Distances start at 1, as 0-byte nodes are invalid.
 /// Also invalid are nodes being referred in a different
 /// order than they were encoded in.
-///
-/// # Sequences (`LazyArray<T>`)
-///
-/// Unlike `Lazy<Vec<T>>`, the length is encoded next to the
-/// position, not at the position, which means that the length
-/// doesn't need to be known before encoding all the elements.
-///
-/// If the length is 0, no position is encoded, but otherwise,
-/// the encoding is that of `Lazy`, with the distinction that
-/// the minimal distance the length of the sequence, i.e.
-/// it's assumed there's no 0-byte element in the sequence.
 #[must_use]
-// FIXME(#59875) the `Meta` parameter only exists to dodge
-// invariance wrt `T` (coming from the `meta: T::Meta` field).
 struct LazyValue<T> {
     position: NonZeroUsize,
     _marker: PhantomData<fn() -> T>,
@@ -102,34 +89,49 @@ impl<T> LazyValue<T> {
     }
 }
 
+/// A list of lazily-decoded values.
+///
+/// Unlike `Lazy<Vec<T>>`, the length is encoded next to the
+/// position, not at the position, which means that the length
+/// doesn't need to be known before encoding all the elements.
+///
+/// If the length is 0, no position is encoded, but otherwise,
+/// the encoding is that of `Lazy`, with the distinction that
+/// the minimal distance the length of the sequence, i.e.
+/// it's assumed there's no 0-byte element in the sequence.
 struct LazyArray<T> {
     position: NonZeroUsize,
-    len: usize,
+    num_elems: usize,
     _marker: PhantomData<fn() -> T>,
 }
 
 impl<T> LazyArray<T> {
-    fn from_position_and_len(position: NonZeroUsize, len: usize) -> LazyArray<T> {
-        LazyArray { position, len, _marker: PhantomData }
+    fn from_position_and_num_elems(position: NonZeroUsize, num_elems: usize) -> LazyArray<T> {
+        LazyArray { position, num_elems, _marker: PhantomData }
     }
 
     fn empty() -> LazyArray<T> {
-        LazyArray::from_position_and_len(NonZeroUsize::new(1).unwrap(), 0)
+        LazyArray::from_position_and_num_elems(NonZeroUsize::new(1).unwrap(), 0)
     }
 }
 
+/// A list of lazily-decoded values, with the added capability of random access.
+///
 /// Random-access table (i.e. offering constant-time `get`/`set`), similar to
 /// `LazyArray<T>`, but without requiring encoding or decoding all the values
 /// eagerly and in-order.
 struct LazyTable<I, T> {
     position: NonZeroUsize,
-    len: usize,
+    encoded_size: usize,
     _marker: PhantomData<fn(I) -> T>,
 }
 
 impl<I, T> LazyTable<I, T> {
-    fn from_position_and_len(position: NonZeroUsize, len: usize) -> LazyTable<I, T> {
-        LazyTable { position, len, _marker: PhantomData }
+    fn from_position_and_encoded_size(
+        position: NonZeroUsize,
+        encoded_size: usize,
+    ) -> LazyTable<I, T> {
+        LazyTable { position, encoded_size, _marker: PhantomData }
     }
 }
 

compiler/rustc_metadata/src/rmeta/table.rs

@@ -228,7 +228,7 @@ impl<T> FixedSizeEncoding for Option<LazyArray<T>> {
         let ([ref position_bytes, ref meta_bytes],[])= b.as_chunks::<4>() else { panic!() };
         let position = NonZeroUsize::new(u32::from_bytes(position_bytes) as usize)?;
         let len = u32::from_bytes(meta_bytes) as usize;
-        Some(LazyArray::from_position_and_len(position, len))
+        Some(LazyArray::from_position_and_num_elems(position, len))
     }
 
     #[inline]
@@ -239,7 +239,7 @@ impl<T> FixedSizeEncoding for Option<LazyArray<T>> {
         let position: u32 = position.try_into().unwrap();
         position.write_to_bytes(position_bytes);
 
-        let len = self.map_or(0, |lazy| lazy.len);
+        let len = self.map_or(0, |lazy| lazy.num_elems);
         let len: u32 = len.try_into().unwrap();
         len.write_to_bytes(meta_bytes);
     }
@@ -289,7 +289,10 @@ where
             buf.emit_raw_bytes(block).unwrap();
         }
         let num_bytes = self.blocks.len() * N;
-        LazyTable::from_position_and_len(NonZeroUsize::new(pos as usize).unwrap(), num_bytes)
+        LazyTable::from_position_and_encoded_size(
+            NonZeroUsize::new(pos as usize).unwrap(),
+            num_bytes,
+        )
     }
 }
 
@@ -307,10 +310,10 @@ where
     where
         Option<T>: FixedSizeEncoding<ByteArray = [u8; N]>,
     {
-        debug!("LazyTable::lookup: index={:?} len={:?}", i, self.len);
+        debug!("LazyTable::lookup: index={:?} len={:?}", i, self.encoded_size);
 
         let start = self.position.get();
-        let bytes = &metadata.blob()[start..start + self.len];
+        let bytes = &metadata.blob()[start..start + self.encoded_size];
         let (bytes, []) = bytes.as_chunks::<N>() else { panic!() };
         let bytes = bytes.get(i.index())?;
         FixedSizeEncoding::from_bytes(bytes)
@@ -321,6 +324,6 @@ where
     where
         Option<T>: FixedSizeEncoding<ByteArray = [u8; N]>,
     {
-        self.len / N
+        self.encoded_size / N
     }
 }