Remove (lots of) dead code

Found with https://github.com/est31/warnalyzer.

Dubious changes:
- Is anyone else using rustc_apfloat? I feel weird completely deleting
  x87 support.
- Maybe some of the dead code in rustc_data_structures, in case someone
  wants to use it in the future?
- Don't change rustc_serialize

  I plan to scrap most of the json module in the near future (see
  https://github.com/rust-lang/compiler-team/issues/418) and fixing the
  tests needed more work than I expected.

TODO: check if any of the comments on the deleted code should be kept.

compiler/rustc_span/src/def_id.rs

@@ -160,6 +160,8 @@ impl DefPathHash {
     }
 
     /// Returns the crate-local part of the [DefPathHash].
+    ///
+    /// Used for tests.
     #[inline]
     pub fn local_hash(&self) -> u64 {
         self.0.as_value().1

compiler/rustc_span/src/hygiene.rs

@@ -1176,11 +1176,7 @@ pub fn decode_syntax_context<
     Ok(new_ctxt)
 }
 
-pub fn num_syntax_ctxts() -> usize {
-    HygieneData::with(|data| data.syntax_context_data.len())
-}
-
-pub fn for_all_ctxts_in<E, F: FnMut((u32, SyntaxContext, &SyntaxContextData)) -> Result<(), E>>(
+fn for_all_ctxts_in<E, F: FnMut((u32, SyntaxContext, &SyntaxContextData)) -> Result<(), E>>(
     ctxts: impl Iterator<Item = SyntaxContext>,
     mut f: F,
 ) -> Result<(), E> {
@@ -1193,7 +1189,7 @@ pub fn for_all_ctxts_in<E, F: FnMut((u32, SyntaxContext, &SyntaxContextData)) ->
     Ok(())
 }
 
-pub fn for_all_expns_in<E, F: FnMut(u32, ExpnId, &ExpnData) -> Result<(), E>>(
+fn for_all_expns_in<E, F: FnMut(u32, ExpnId, &ExpnData) -> Result<(), E>>(
     expns: impl Iterator<Item = ExpnId>,
     mut f: F,
 ) -> Result<(), E> {
@@ -1206,16 +1202,6 @@ pub fn for_all_expns_in<E, F: FnMut(u32, ExpnId, &ExpnData) -> Result<(), E>>(
     Ok(())
 }
 
-pub fn for_all_data<E, F: FnMut((u32, SyntaxContext, &SyntaxContextData)) -> Result<(), E>>(
-    mut f: F,
-) -> Result<(), E> {
-    let all_data = HygieneData::with(|data| data.syntax_context_data.clone());
-    for (i, data) in all_data.into_iter().enumerate() {
-        f((i as u32, SyntaxContext(i as u32), &data))?;
-    }
-    Ok(())
-}
-
 impl<E: Encoder> Encodable<E> for ExpnId {
     default fn encode(&self, _: &mut E) -> Result<(), E::Error> {
         panic!("cannot encode `ExpnId` with `{}`", std::any::type_name::<E>());
@@ -1228,14 +1214,6 @@ impl<D: Decoder> Decodable<D> for ExpnId {
     }
 }
 
-pub fn for_all_expn_data<E, F: FnMut(u32, &ExpnData) -> Result<(), E>>(mut f: F) -> Result<(), E> {
-    let all_data = HygieneData::with(|data| data.expn_data.clone());
-    for (i, data) in all_data.into_iter().enumerate() {
-        f(i as u32, &data.unwrap_or_else(|| panic!("Missing ExpnData!")))?;
-    }
-    Ok(())
-}
-
 pub fn raw_encode_syntax_context<E: Encoder>(
     ctxt: SyntaxContext,
     context: &HygieneEncodeContext,

compiler/rustc_span/src/lib.rs

@@ -1037,10 +1037,6 @@ pub enum ExternalSourceKind {
 }
 
 impl ExternalSource {
-    pub fn is_absent(&self) -> bool {
-        !matches!(self, ExternalSource::Foreign { kind: ExternalSourceKind::Present(_), .. })
-    }
-
     pub fn get_source(&self) -> Option<&Lrc<String>> {
         match self {
             ExternalSource::Foreign { kind: ExternalSourceKind::Present(ref src), .. } => Some(src),
@@ -1433,9 +1429,6 @@ impl SourceFile {
         self.src.is_none()
     }
 
-    pub fn byte_length(&self) -> u32 {
-        self.end_pos.0 - self.start_pos.0
-    }
     pub fn count_lines(&self) -> usize {
         self.lines.len()
     }