From bddb266089deb0cb879caeff1bb9aa860746e062 Mon Sep 17 00:00:00 2001
From: David Wood <david@davidtw.co>
Date: Thu, 2 Jul 2020 14:13:39 +0100
Subject: [PATCH] typeck: check for infer before type impls trait

This commit checks that the target type of the cast (an error related
to which is being reported) does not have types to be inferred before
checking if it implements the `From` trait.

Signed-off-by: David Wood <david@davidtw.co>
---
 src/librustc_typeck/check/cast.rs     |  1 +
 src/test/ui/issues/issue-73886.rs     |  6 ++++++
 src/test/ui/issues/issue-73886.stderr | 15 +++++++++++++++
 3 files changed, 22 insertions(+)
 create mode 100644 src/test/ui/issues/issue-73886.rs
 create mode 100644 src/test/ui/issues/issue-73886.stderr

diff --git a/src/librustc_typeck/check/cast.rs b/src/librustc_typeck/check/cast.rs
index 1ea7bf25ef2ed..8948e5a3e00db 100644
--- a/src/librustc_typeck/check/cast.rs
+++ b/src/librustc_typeck/check/cast.rs
@@ -387,6 +387,7 @@ impl<'a, 'tcx> CastCheck<'tcx> {
                             // Check for infer types because cases like `Option<{integer}>` would
                             // panic otherwise.
                             if !expr_ty.has_infer_types()
+                                && !ty.has_infer_types()
                                 && fcx.tcx.type_implements_trait((
                                     from_trait,
                                     ty,
diff --git a/src/test/ui/issues/issue-73886.rs b/src/test/ui/issues/issue-73886.rs
new file mode 100644
index 0000000000000..2f1ec8c6d6227
--- /dev/null
+++ b/src/test/ui/issues/issue-73886.rs
@@ -0,0 +1,6 @@
+fn main() {
+    let _ = &&[0] as &[_];
+    //~^ ERROR non-primitive cast: `&&[i32; 1]` as `&[_]`
+    let _ = 7u32 as Option<_>;
+    //~^ ERROR non-primitive cast: `u32` as `std::option::Option<_>`
+}
diff --git a/src/test/ui/issues/issue-73886.stderr b/src/test/ui/issues/issue-73886.stderr
new file mode 100644
index 0000000000000..e8ab7db6b82c2
--- /dev/null
+++ b/src/test/ui/issues/issue-73886.stderr
@@ -0,0 +1,15 @@
+error[E0605]: non-primitive cast: `&&[i32; 1]` as `&[_]`
+  --> $DIR/issue-73886.rs:2:13
+   |
+LL |     let _ = &&[0] as &[_];
+   |             ^^^^^^^^^^^^^ an `as` expression can only be used to convert between primitive types or to coerce to a specific trait object
+
+error[E0605]: non-primitive cast: `u32` as `std::option::Option<_>`
+  --> $DIR/issue-73886.rs:4:13
+   |
+LL |     let _ = 7u32 as Option<_>;
+   |             ^^^^^^^^^^^^^^^^^ an `as` expression can only be used to convert between primitive types or to coerce to a specific trait object
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0605`.