Remove E0245; improve E0404 explanation

src/librustc_resolve/diagnostics.rs

@@ -542,7 +542,7 @@ fn foo<T, T>(s: T, u: T) {} // error: the name `T` is already used for a type
                             //        parameter in this type parameter list
 ```
 
-Please verify that none of the type parameterss are misspelled, and rename any
+Please verify that none of the type parameters are misspelled, and rename any
 clashing parameters. Example:
 
 ```
@@ -551,7 +551,8 @@ fn foo<T, Y>(s: T, u: Y) {} // ok!
 "##,
 
 E0404: r##"
-You tried to implement something which was not a trait on an object.
+You tried to use something which is not a trait in a trait position, such as
+a bound or `impl`.
 
 Erroneous code example:
 
@@ -562,6 +563,14 @@ struct Bar;
 impl Foo for Bar {} // error: `Foo` is not a trait
 ```
 
+Another erroneous code example:
+
+```compile_fail,E0404
+struct Foo;
+
+fn bar<T: Foo>(t: T) {} // error: `Foo` is not a trait
+```
+
 Please verify that you didn't misspell the trait's name or otherwise use the
 wrong identifier. Example:
 
@@ -575,6 +584,17 @@ impl Foo for Bar { // ok!
     // functions implementation
 }
 ```
+
+or
+
+```
+trait Foo {
+    // some functions
+}
+
+fn bar<T: Foo>(t: T) {} // ok!
+```
+
 "##,
 
 E0405: r##"

src/librustc_resolve/lib.rs

@@ -2163,6 +2163,7 @@ impl<'a> Resolver<'a> {
         result
     }
 
+    /// This is called to resolve a trait reference from an `impl` (i.e. `impl Trait for Foo`)
     fn with_optional_trait_ref<T, F>(&mut self, opt_trait_ref: Option<&TraitRef>, f: F) -> T
         where F: FnOnce(&mut Resolver, Option<DefId>) -> T
     {
@@ -2172,13 +2173,14 @@ impl<'a> Resolver<'a> {
             let path: Vec<_> = trait_ref.path.segments.iter()
                 .map(|seg| respan(seg.span, seg.identifier))
                 .collect();
-            let def = self.smart_resolve_path_fragment(trait_ref.ref_id,
-                                                       None,
-                                                       &path,
-                                                       trait_ref.path.span,
-                                                       trait_ref.path.segments.last().unwrap().span,
-                                                       PathSource::Trait(AliasPossibility::No))
-                .base_def();
+            let def = self.smart_resolve_path_fragment(
+                trait_ref.ref_id,
+                None,
+                &path,
+                trait_ref.path.span,
+                trait_ref.path.segments.last().unwrap().span,
+                PathSource::Trait(AliasPossibility::No)
+            ).base_def();
             if def != Def::Err {
                 new_id = Some(def.def_id());
                 let span = trait_ref.path.span;

src/librustc_typeck/astconv.rs

@@ -313,7 +313,7 @@ impl<'o, 'gcx: 'tcx, 'tcx> AstConv<'gcx, 'tcx>+'o {
 
     /// Instantiates the path for the given trait reference, assuming that it's
     /// bound to a valid trait type. Returns the def_id for the defining trait.
-    /// Fails if the type is a type other than a trait type.
+    /// The type _cannot_ be a type other than a trait type.
     ///
     /// If the `projections` argument is `None`, then assoc type bindings like `Foo<T=X>`
     /// are disallowed. Otherwise, they are pushed onto the vector given.
@@ -331,6 +331,7 @@ impl<'o, 'gcx: 'tcx, 'tcx> AstConv<'gcx, 'tcx>+'o {
                                         trait_ref.path.segments.last().unwrap())
     }
 
+    /// Get the DefId of the given trait ref. It _must_ actually be a trait.
     fn trait_def_id(&self, trait_ref: &hir::TraitRef) -> DefId {
         let path = &trait_ref.path;
         match path.def {
@@ -339,13 +340,11 @@ impl<'o, 'gcx: 'tcx, 'tcx> AstConv<'gcx, 'tcx>+'o {
             Def::Err => {
                 self.tcx().sess.fatal("cannot continue compilation due to previous error");
             }
-            _ => {
-                span_fatal!(self.tcx().sess, path.span, E0245, "`{}` is not a trait",
-                            self.tcx().hir.node_to_pretty_string(trait_ref.ref_id));
-            }
+            _ => unreachable!(),
         }
     }
 
+    /// The given `trait_ref` must actually be trait.
     pub(super) fn instantiate_poly_trait_ref_inner(&self,
         trait_ref: &hir::TraitRef,
         self_ty: Ty<'tcx>,

src/librustc_typeck/diagnostics.rs

@@ -4802,7 +4802,7 @@ register_diagnostics! {
 //  E0240,
 //  E0241,
 //  E0242,
-    E0245, // not a trait
+//  E0245, // not a trait
 //  E0246, // invalid recursive type
 //  E0247,
 //  E0248, // value used as a type, now reported earlier during resolution as E0412

src/test/ui/error-codes/E0404.rs

@@ -13,5 +13,6 @@ struct Bar;
 
 impl Foo for Bar {} //~ ERROR E0404
 
-fn main() {
-}
+fn main() {}
+
+fn baz<T: Foo>(_: T) {} //~ ERROR E0404

src/test/ui/error-codes/E0404.stderr

@@ -4,6 +4,12 @@ error[E0404]: expected trait, found struct `Foo`
 LL | impl Foo for Bar {} //~ ERROR E0404
    |      ^^^ not a trait
 
+error[E0404]: expected trait, found struct `Foo`
+  --> $DIR/E0404.rs:18:11
+   |
+LL | fn baz<T: Foo>(_: T) {} //~ ERROR E0404
+   |           ^^^ not a trait
+
 error: cannot continue compilation due to previous error
 
 If you want more information on this error, try using "rustc --explain E0404"