Early error for mixing order of const and type parameters

src/librustc_passes/ast_validation.rs

@@ -21,7 +21,7 @@ use syntax::visit::{self, Visitor};
 use syntax::{span_err, struct_span_err, walk_list};
 use syntax_ext::proc_macro_decls::is_proc_macro_attr;
 use syntax_pos::{Span, MultiSpan};
-use errors::Applicability;
+use errors::{Applicability, FatalError};
 use log::debug;
 
 #[derive(Copy, Clone, Debug)]
@@ -368,6 +368,8 @@ fn validate_generics_order<'a>(
     let mut max_param: Option<ParamKindOrd> = None;
     let mut out_of_order = FxHashMap::default();
     let mut param_idents = vec![];
+    let mut found_type = false;
+    let mut found_const = false;
 
     for (kind, bounds, span, ident) in generics {
         if let Some(ident) = ident {
@@ -381,6 +383,11 @@ fn validate_generics_order<'a>(
             }
             Some(_) | None => *max_param = Some(kind),
         };
+        match kind {
+            ParamKindOrd::Type => found_type = true,
+            ParamKindOrd::Const => found_const = true,
+            _ => {}
+        }
     }
 
     let mut ordered_params = "<".to_string();
@@ -408,8 +415,8 @@ fn validate_generics_order<'a>(
         GenericPosition::Arg => "argument",
     };
 
-    for (param_ord, (max_param, spans)) in out_of_order {
-        let mut err = handler.struct_span_err(spans,
+    for (param_ord, (max_param, spans)) in &out_of_order {
+        let mut err = handler.struct_span_err(spans.clone(),
             &format!(
                 "{} {pos}s must be declared prior to {} {pos}s",
                 param_ord,
@@ -430,6 +437,13 @@ fn validate_generics_order<'a>(
         }
         err.emit();
     }
+
+    // FIXME(const_generics): we shouldn't have to abort here at all, but we currently get ICEs
+    // if we don't. Const parameters and type parameters can currently conflict if they
+    // are out-of-order.
+    if !out_of_order.is_empty() && found_type && found_const {
+        FatalError.raise();
+    }
 }
 
 impl<'a> Visitor<'a> for AstValidator<'a> {

src/test/ui/const-generics/const-param-before-other-params.rs

@@ -1,12 +1,12 @@
 #![feature(const_generics)]
 //~^ WARN the feature `const_generics` is incomplete and may cause the compiler to crash
 
-fn foo<const X: (), T>(_: &T) {
-    //~^ ERROR type parameters must be declared prior to const parameters
-}
-
 fn bar<const X: (), 'a>(_: &'a ()) {
     //~^ ERROR lifetime parameters must be declared prior to const parameters
 }
 
+fn foo<const X: (), T>(_: &T) {
+    //~^ ERROR type parameters must be declared prior to const parameters
+}
+
 fn main() {}

src/test/ui/const-generics/const-param-before-other-params.stderr

@@ -4,17 +4,17 @@ warning: the feature `const_generics` is incomplete and may cause the compiler t
 LL | #![feature(const_generics)]
    |            ^^^^^^^^^^^^^^
 
-error: type parameters must be declared prior to const parameters
-  --> $DIR/const-param-before-other-params.rs:4:21
-   |
-LL | fn foo<const X: (), T>(_: &T) {
-   |       --------------^- help: reorder the parameters: lifetimes, then types, then consts: `<T, const X: ()>`
-
 error: lifetime parameters must be declared prior to const parameters
-  --> $DIR/const-param-before-other-params.rs:8:21
+  --> $DIR/const-param-before-other-params.rs:4:21
    |
 LL | fn bar<const X: (), 'a>(_: &'a ()) {
    |       --------------^^- help: reorder the parameters: lifetimes, then types, then consts: `<'a, const X: ()>`
 
+error: type parameters must be declared prior to const parameters
+  --> $DIR/const-param-before-other-params.rs:8:21
+   |
+LL | fn foo<const X: (), T>(_: &T) {
+   |       --------------^- help: reorder the parameters: lifetimes, then types, then consts: `<T, const X: ()>`
+
 error: aborting due to 2 previous errors