Auto merge of #48326 - RalfJung:generic-bounds, r=petrochenkov

Warn about ignored generic bounds in `for`

This adds a new lint to fix #42181. For consistency and to avoid code duplication, I also moved the existing "bounds in type aliases are ignored" here.

Questions to the reviewer:
* Is it okay to just remove a diagnostic error code like this? Should I instead keep the warning about type aliases where it is? The old code provided a detailed explanation of what's going on when asked, that information is now lost. On the other hand, `span_warn!` seems deprecated (after this patch, it has exactly one user left!).
* Did I miss any syntactic construct that can appear as `for` in the surface syntax? I covered function types (`for<'a> fn(...)`), generic traits (`for <'a> Fn(...)`, can appear both as bounds as as trait objects) and bounds (`for<'a> F: ...`).
* For the sake of backwards compatibility, this adds a warning, not an error. @nikomatsakis suggested an error in https://github.com/rust-lang/rust/issues/42181#issuecomment-306924389, but I feel that can only happen in a new epoch -- right?

Cc @eddyb

2 files changed, 22 insertions, 12 deletions

diff --git a/src/libsyntax/ast.rs b/src/libsyntax/ast.rs
index 10b8e121f83..8b3a7164ccc 100644
--- a/src/libsyntax/ast.rs
+++ b/src/libsyntax/ast.rs
@@ -294,6 +294,15 @@ pub enum TyParamBound {
     RegionTyParamBound(Lifetime)
 }
 
+impl TyParamBound {
+    pub fn span(&self) -> Span {
+        match self {
+            &TraitTyParamBound(ref t, ..) => t.span,
+            &RegionTyParamBound(ref l) => l.span,
+        }
+    }
+}
+
 /// A modifier on a bound, currently this is only used for `?Sized`, where the
 /// modifier is `Maybe`. Negative bounds should also be handled here.
 #[derive(Copy, Clone, PartialEq, Eq, RustcEncodable, RustcDecodable, Hash, Debug)]
@@ -404,6 +413,16 @@ pub enum WherePredicate {
     EqPredicate(WhereEqPredicate),
 }
 
+impl WherePredicate {
+    pub fn span(&self) -> Span {
+        match self {
+            &WherePredicate::BoundPredicate(ref p) => p.span,
+            &WherePredicate::RegionPredicate(ref p) => p.span,
+            &WherePredicate::EqPredicate(ref p) => p.span,
+        }
+    }
+}
+
 /// A type bound.
 ///
 /// E.g. `for<'c> Foo: Send+Clone+'c`
diff --git a/src/libsyntax/parse/parser.rs b/src/libsyntax/parse/parser.rs
index 847733e1e37..f5aa01fb034 100644
--- a/src/libsyntax/parse/parser.rs
+++ b/src/libsyntax/parse/parser.rs
@@ -4951,6 +4951,7 @@ impl<'a> Parser<'a> {
                         }
                     ));
                 // FIXME: Decide what should be used here, `=` or `==`.
+                // FIXME: We are just dropping the binders in lifetime_defs on the floor here.
                 } else if self.eat(&token::Eq) || self.eat(&token::EqEq) {
                     let rhs_ty = self.parse_ty()?;
                     where_clause.predicates.push(ast::WherePredicate::EqPredicate(
@@ -5608,18 +5609,8 @@ impl<'a> Parser<'a> {
             self.expect_lt()?;
             let params = self.parse_generic_params()?;
             self.expect_gt()?;
-
-            let first_non_lifetime_param_span = params.iter()
-                .filter_map(|param| match *param {
-                    ast::GenericParam::Lifetime(_) => None,
-                    ast::GenericParam::Type(ref t) => Some(t.span),
-                })
-                .next();
-
-            if let Some(span) = first_non_lifetime_param_span {
-                self.span_err(span, "only lifetime parameters can be used in this context");
-            }
-
+            // We rely on AST validation to rule out invalid cases: There must not be type
+            // parameters, and the lifetime parameters must not have bounds.
             Ok(params)
         } else {
             Ok(Vec::new())