Auto merge of #42527 - qnighy:unsized-tuple-coercions, r=arielb1

Unsized tuple coercions

Part of #18469. Fixes #32702.

#37685 and #34451 might also be related.

This PR does the following:

- Introduce explicit `Sized` constraints on tuple initializers, similar to that of record-struct initializers. Not much relevant to the main contribution but I noticed this when making tests for unsized tuple coercions.
- Implement `(.., T): Unsize<(.., U)>` where `T: Unsize<U>`.
- Assume `(.., T)` is MaybeUnsizedUnivariant.
- Modify `src/librustc/ty/util.rs` and `src/librustc_trans/glue.rs` so that tuples and structs are uniformly traversed when translating.

1 files changed, 6 insertions, 0 deletions

diff --git a/src/libsyntax/feature_gate.rs b/src/libsyntax/feature_gate.rs
index 74bf19b841e..df8ee189d21 100644
--- a/src/libsyntax/feature_gate.rs
+++ b/src/libsyntax/feature_gate.rs
@@ -357,6 +357,9 @@ declare_features! (
 
     // Allows a test to fail without failing the whole suite
     (active, allow_fail, "1.19.0", Some(42219)),
+
+    // Allows unsized tuple coercion.
+    (active, unsized_tuple_coercion, "1.20.0", Some(42877)),
 );
 
 declare_features! (
@@ -1041,6 +1044,9 @@ pub const EXPLAIN_VIS_MATCHER: &'static str =
 pub const EXPLAIN_PLACEMENT_IN: &'static str =
     "placement-in expression syntax is experimental and subject to change.";
 
+pub const EXPLAIN_UNSIZED_TUPLE_COERCION: &'static str =
+    "Unsized tuple coercion is not stable enough for use and is subject to change";
+
 struct PostExpansionVisitor<'a> {
     context: &'a Context<'a>,
 }