auto merge of #14145 : pnkfelix/rust/fsk-better-svh-via-visitor, r=alexcrichton

Teach SVH computation to ignore more implementation artifacts.

In particular, this version of strict version hash (SVH) works much
like the deriving(Hash)-based implementation did, except that it
deliberately:

  1. skips over content known not affect the generated crates, and,

  2. uses a content-based hash for names instead of using the value of
     the `Name` index itself, which can differ depending on the order
     in which strings are interned (which in turn is affected by
     e.g. the presence of `--cfg` options on the command line).

Fix #14132.

1 files changed, 13 insertions, 3 deletions

diff --git a/src/libsyntax/visit.rs b/src/libsyntax/visit.rs
index f820fbd784a..ce10d0db3ba 100644
--- a/src/libsyntax/visit.rs
+++ b/src/libsyntax/visit.rs
@@ -61,7 +61,17 @@ pub fn generics_of_fn(fk: &FnKind) -> Generics {
     }
 }
 
+/// Each method of the Visitor trait is a hook to be potentially
+/// overriden.  Each method's default implementation recursively visits
+/// the substructure of the input via the corresponding `walk` method;
+/// e.g. the `visit_mod` method by default calls `visit::walk_mod`.
+///
+/// If you want to ensure that your code handles every variant
+/// explicitly, you need to override each method.  (And you also need
+/// to monitor future changes to `Visitor` in case a new method with a
+/// new default implementation gets introduced.)
 pub trait Visitor<E: Clone> {
+
     fn visit_ident(&mut self, _sp: Span, _ident: Ident, _e: E) {
         /*! Visit the idents */
     }
@@ -179,9 +189,9 @@ pub fn walk_local<E: Clone, V: Visitor<E>>(visitor: &mut V, local: &Local, env:
     }
 }
 
-fn walk_explicit_self<E: Clone, V: Visitor<E>>(visitor: &mut V,
-                                               explicit_self: &ExplicitSelf,
-                                               env: E) {
+pub fn walk_explicit_self<E: Clone, V: Visitor<E>>(visitor: &mut V,
+                                                   explicit_self: &ExplicitSelf,
+                                                   env: E) {
     match explicit_self.node {
         SelfStatic | SelfValue | SelfUniq => {}
         SelfRegion(ref lifetime, _) => {