Add future incompatibility lint for array.into_iter()

As we might want to add `IntoIterator` impls for arrays in the future,
and since that introduces a breaking change, this lint warns and
suggests using `iter()` instead (which is shorter and more explicit).

Author: LukasKalbertodt

src/libcore/iter/traits/collect.rs

@@ -205,6 +205,7 @@ pub trait FromIterator<A>: Sized {
 ///         .collect()
 /// }
 /// ```
+#[rustc_diagnostic_item = "IntoIterator"]
 #[stable(feature = "rust1", since = "1.0.0")]
 pub trait IntoIterator {
     /// The type of the elements being iterated over.

src/librustc_lint/array_into_iter.rs

@@ -0,0 +1,91 @@
+use crate::lint::{LateContext, LateLintPass, LintArray, LintContext, LintPass};
+use rustc::{
+    lint::FutureIncompatibleInfo,
+    hir,
+    ty::{
+        self,
+        adjustment::{Adjust, Adjustment},
+    },
+};
+use syntax::{
+    errors::Applicability,
+    symbol::sym,
+};
+
+
+declare_lint! {
+    pub ARRAY_INTO_ITER,
+    Warn,
+    "detects calling `into_iter` on arrays",
+    @future_incompatible = FutureIncompatibleInfo {
+        reference: "issue #66145 <https://github.com/rust-lang/rust/issues/66145>",
+        edition: None,
+    };
+}
+
+declare_lint_pass!(
+    /// Checks for instances of calling `into_iter` on arrays.
+    ArrayIntoIter => [ARRAY_INTO_ITER]
+);
+
+impl<'a, 'tcx> LateLintPass<'a, 'tcx> for ArrayIntoIter {
+    fn check_expr(&mut self, cx: &LateContext<'a, 'tcx>, expr: &'tcx hir::Expr) {
+        // We only care about method call expressions.
+        if let hir::ExprKind::MethodCall(call, span, args) = &expr.kind {
+            if call.ident.name != sym::into_iter {
+                return;
+            }
+
+            // Check if the method call actually calls the libcore
+            // `IntoIterator::into_iter`.
+            let def_id = cx.tables.type_dependent_def_id(expr.hir_id).unwrap();
+            match cx.tcx.trait_of_item(def_id) {
+                Some(trait_id) if cx.tcx.is_diagnostic_item(sym::IntoIterator, trait_id) => {},
+                _ => return,
+            };
+
+            // As this is a method call expression, we have at least one
+            // argument.
+            let receiver_arg = &args[0];
+
+            // Test if the original `self` type is an array type.
+            match cx.tables.expr_ty(receiver_arg).kind {
+                ty::Array(..) => {}
+                _ => return,
+            }
+
+            // Make sure that the first adjustment is an autoref coercion.
+            match cx.tables.expr_adjustments(receiver_arg).get(0) {
+                Some(Adjustment { kind: Adjust::Borrow(_), .. }) => {}
+                _ => return,
+            }
+
+            // Emit lint diagnostic.
+            let target = match cx.tables.expr_ty_adjusted(receiver_arg).kind {
+                ty::Ref(_, ty::TyS { kind: ty::Array(..), ..}, _) => "[T; N]",
+                ty::Ref(_, ty::TyS { kind: ty::Slice(..), ..}, _) => "[T]",
+
+                // We know the original first argument type is an array type,
+                // we know that the first adjustment was an autoref coercion
+                // and we know that `IntoIterator` is the trait involved. The
+                // array cannot be coerced to something other than a reference
+                // to an array or to a slice.
+                _ => bug!("array type coerced to something other than array or slice"),
+            };
+            let msg = format!(
+                "this method call currently resolves to `<&{} as IntoIterator>::into_iter` (due \
+                    to autoref coercions), but that might change in the future when \
+                    `IntoIterator` impls for arrays are added.",
+                target,
+            );
+            cx.struct_span_lint(ARRAY_INTO_ITER, *span, &msg)
+                .span_suggestion(
+                    call.ident.span,
+                    "use `.iter()` instead of `.into_iter()` to avoid ambiguity",
+                    "iter".into(),
+                    Applicability::MachineApplicable,
+                )
+                .emit();
+        }
+    }
+}

src/librustc_lint/lib.rs

@@ -21,6 +21,7 @@
 #[macro_use]
 extern crate rustc;
 
+mod array_into_iter;
 mod error_codes;
 mod nonstandard_style;
 mod redundant_semicolon;
@@ -56,6 +57,7 @@ use types::*;
 use unused::*;
 use non_ascii_idents::*;
 use rustc::lint::internal::*;
+use array_into_iter::ArrayIntoIter;
 
 /// Useful for other parts of the compiler.
 pub use builtin::SoftLints;
@@ -130,6 +132,8 @@ macro_rules! late_lint_passes {
             // FIXME: Turn the computation of types which implement Debug into a query
             // and change this to a module lint pass
             MissingDebugImplementations: MissingDebugImplementations::default(),
+
+            ArrayIntoIter: ArrayIntoIter,
         ]);
     )
 }