From 11379f0494b6a8c81b3fb9e0411651bc749aaa06 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Esteban=20K=C3=BCber?= <esteban@kuber.com.ar>
Date: Tue, 4 May 2021 08:41:40 -0700
Subject: [PATCH] Do not ICE on invalid const param

When encountering a path that can't have generics, do not call
`generics_of`. This would happen when writing something like
`path::this_is_a_mod<const_val>`.

Fix #84831.
---
 compiler/rustc_typeck/src/collect/type_of.rs | 20 ++++++++++++++-
 src/test/ui/typeck/issue-84831.rs            |  9 +++++++
 src/test/ui/typeck/issue-84831.stderr        | 26 ++++++++++++++++++++
 3 files changed, 54 insertions(+), 1 deletion(-)
 create mode 100644 src/test/ui/typeck/issue-84831.rs
 create mode 100644 src/test/ui/typeck/issue-84831.stderr

diff --git a/compiler/rustc_typeck/src/collect/type_of.rs b/compiler/rustc_typeck/src/collect/type_of.rs
index 51d5f4ebe2bd2..97b6f5cf41211 100644
--- a/compiler/rustc_typeck/src/collect/type_of.rs
+++ b/compiler/rustc_typeck/src/collect/type_of.rs
@@ -191,7 +191,25 @@ pub(super) fn opt_const_param_of(tcx: TyCtxt<'_>, def_id: LocalDefId) -> Option<
                     Res::Def(DefKind::Ctor(..), def_id) => {
                         tcx.generics_of(tcx.parent(def_id).unwrap())
                     }
-                    Res::Def(_, def_id) => tcx.generics_of(def_id),
+                    // Other `DefKind`s don't have generics and would ICE when calling
+                    // `generics_of`.
+                    Res::Def(
+                        DefKind::Struct
+                        | DefKind::Union
+                        | DefKind::Enum
+                        | DefKind::Variant
+                        | DefKind::Trait
+                        | DefKind::OpaqueTy
+                        | DefKind::TyAlias
+                        | DefKind::ForeignTy
+                        | DefKind::TraitAlias
+                        | DefKind::AssocTy
+                        | DefKind::Fn
+                        | DefKind::AssocFn
+                        | DefKind::AssocConst
+                        | DefKind::Impl,
+                        def_id,
+                    ) => tcx.generics_of(def_id),
                     Res::Err => {
                         tcx.sess.delay_span_bug(tcx.def_span(def_id), "anon const with Res::Err");
                         return None;
diff --git a/src/test/ui/typeck/issue-84831.rs b/src/test/ui/typeck/issue-84831.rs
new file mode 100644
index 0000000000000..c646f71072532
--- /dev/null
+++ b/src/test/ui/typeck/issue-84831.rs
@@ -0,0 +1,9 @@
+fn f() {
+    std::<0>; //~ ERROR expected value
+}
+fn j() {
+    std::<_ as _>; //~ ERROR expected value
+    //~^ ERROR expected one of `,` or `>`, found keyword `as`
+}
+
+fn main () {}
diff --git a/src/test/ui/typeck/issue-84831.stderr b/src/test/ui/typeck/issue-84831.stderr
new file mode 100644
index 0000000000000..e3cce10a00fd1
--- /dev/null
+++ b/src/test/ui/typeck/issue-84831.stderr
@@ -0,0 +1,26 @@
+error: expected one of `,` or `>`, found keyword `as`
+  --> $DIR/issue-84831.rs:5:13
+   |
+LL |     std::<_ as _>;
+   |             ^^ expected one of `,` or `>`
+   |
+help: expressions must be enclosed in braces to be used as const generic arguments
+   |
+LL |     std::<{ _ as _ }>;
+   |           ^        ^
+
+error[E0423]: expected value, found crate `std`
+  --> $DIR/issue-84831.rs:2:5
+   |
+LL |     std::<0>;
+   |     ^^^^^^^^ not a value
+
+error[E0423]: expected value, found crate `std`
+  --> $DIR/issue-84831.rs:5:5
+   |
+LL |     std::<_ as _>;
+   |     ^^^^^^^^^^^^^ not a value
+
+error: aborting due to 3 previous errors
+
+For more information about this error, try `rustc --explain E0423`.