gccrs: constant evaluation like these are coercion sites

The code here was wrongly assuming the decl type from the folding of the
expression would be the type of the constant decl. This is not the case for
unsized coercions for slices, where the expression here is a reference to
an array then we require the coercion to fix the result up to the expected
type.

Fixes Rust-GCC#1525

gcc/rust/ChangeLog:

	* backend/rust-compile-base.cc: apply coercion site to result
	* backend/rust-compile-base.h: update prototype
	* backend/rust-compile-implitem.cc (CompileTraitItem::visit): send in coercion info
	* backend/rust-compile-item.cc (CompileItem::visit): likewise

gcc/testsuite/ChangeLog:

	* rust/compile/issue-1525.rs: New test.

Signed-off-by: Philip Herron <herron.philip@googlemail.com>

1 files changed, 4 insertions, 0 deletions

diff --git a/gcc/testsuite/rust/compile/issue-1525.rs b/gcc/testsuite/rust/compile/issue-1525.rs
new file mode 100644
index 0000000..b2247cd
--- /dev/null
+++ b/gcc/testsuite/rust/compile/issue-1525.rs
@@ -0,0 +1,4 @@
+fn main() {
+    const slice: &[i32] = &[1, 2, 3];
+    let _slice2: &[i32] = slice;
+}