Auto merge of #11609 - y21:get_first_non_primitives, r=giraffate

[`get_first`]: lint on non-primitive slices

Fixes #11594

I left the issue open for a couple days before making the PR to see if anyone has something to say, but it looks like there aren't any objections to removing this check that prevented linting on non-primitive slices, so here's the PR now.
There's a couple of instances in clippy itself where we now emit the lint. The actual relevant change is in the first commit and fixing the `.get(0)` instances in clippy itself is in the 2nd commit.

changelog: [`get_first`]: lint on non-primitive slices

14 files changed, 45 insertions, 27 deletions

diff --git a/clippy_lints/src/loops/same_item_push.rs b/clippy_lints/src/loops/same_item_push.rs
index f7b3b2358a0..5fffb27cda2 100644
--- a/clippy_lints/src/loops/same_item_push.rs
+++ b/clippy_lints/src/loops/same_item_push.rs
@@ -185,7 +185,7 @@ fn get_vec_push<'tcx>(
             if let StmtKind::Semi(semi_stmt) = &stmt.kind;
             if let ExprKind::MethodCall(path, self_expr, args, _) = &semi_stmt.kind;
             // Figure out the parameters for the method call
-            if let Some(pushed_item) = args.get(0);
+            if let Some(pushed_item) = args.first();
             // Check that the method being called is push() on a Vec
             if is_type_diagnostic_item(cx, cx.typeck_results().expr_ty(self_expr), sym::Vec);
             if path.ident.name.as_str() == "push";
diff --git a/clippy_lints/src/manual_bits.rs b/clippy_lints/src/manual_bits.rs
index 6c7c57ba1d6..552c57d5e02 100644
--- a/clippy_lints/src/manual_bits.rs
+++ b/clippy_lints/src/manual_bits.rs
@@ -103,9 +103,9 @@ fn get_size_of_ty<'tcx>(cx: &LateContext<'tcx>, expr: &'tcx Expr<'_>) -> Option<
         if let ExprKind::Path(ref count_func_qpath) = count_func.kind;
 
         if let QPath::Resolved(_, count_func_path) = count_func_qpath;
-        if let Some(segment_zero) = count_func_path.segments.get(0);
+        if let Some(segment_zero) = count_func_path.segments.first();
         if let Some(args) = segment_zero.args;
-        if let Some(GenericArg::Type(real_ty)) = args.args.get(0);
+        if let Some(GenericArg::Type(real_ty)) = args.args.first();
 
         if let Some(def_id) = cx.qpath_res(count_func_qpath, count_func.hir_id).opt_def_id();
         if cx.tcx.is_diagnostic_item(sym::mem_size_of, def_id);
diff --git a/clippy_lints/src/methods/get_first.rs b/clippy_lints/src/methods/get_first.rs
index ee063adac64..e7b4564c651 100644
--- a/clippy_lints/src/methods/get_first.rs
+++ b/clippy_lints/src/methods/get_first.rs
@@ -1,5 +1,4 @@
 use clippy_utils::diagnostics::span_lint_and_sugg;
-use clippy_utils::is_slice_of_primitives;
 use clippy_utils::source::snippet_with_applicability;
 use if_chain::if_chain;
 use rustc_ast::LitKind;
@@ -20,7 +19,6 @@ pub(super) fn check<'tcx>(
         if let Some(method_id) = cx.typeck_results().type_dependent_def_id(expr.hir_id);
         if let Some(impl_id) = cx.tcx.impl_of_method(method_id);
         if cx.tcx.type_of(impl_id).instantiate_identity().is_slice();
-        if let Some(_) = is_slice_of_primitives(cx, recv);
         if let hir::ExprKind::Lit(Spanned { node: LitKind::Int(0, _), .. }) = arg.kind;
         then {
             let mut app = Applicability::MachineApplicable;
diff --git a/clippy_lints/src/methods/search_is_some.rs b/clippy_lints/src/methods/search_is_some.rs
index afdb8ce94ac..04ddaaa2f46 100644
--- a/clippy_lints/src/methods/search_is_some.rs
+++ b/clippy_lints/src/methods/search_is_some.rs
@@ -39,7 +39,7 @@ pub(super) fn check<'tcx>(
                 if search_method == "find";
                 if let hir::ExprKind::Closure(&hir::Closure { body, .. }) = search_arg.kind;
                 let closure_body = cx.tcx.hir().body(body);
-                if let Some(closure_arg) = closure_body.params.get(0);
+                if let Some(closure_arg) = closure_body.params.first();
                 then {
                     if let hir::PatKind::Ref(..) = closure_arg.pat.kind {
                         Some(search_snippet.replacen('&', "", 1))
diff --git a/clippy_lints/src/missing_doc.rs b/clippy_lints/src/missing_doc.rs
index f2773cad400..0629dee4f72 100644
--- a/clippy_lints/src/missing_doc.rs
+++ b/clippy_lints/src/missing_doc.rs
@@ -67,7 +67,7 @@ impl MissingDoc {
         if_chain! {
             if let Some(meta) = meta;
             if let MetaItemKind::List(list) = meta.kind;
-            if let Some(meta) = list.get(0);
+            if let Some(meta) = list.first();
             if let Some(name) = meta.ident();
             then {
                 name.name == sym::include
diff --git a/clippy_lints/src/needless_continue.rs b/clippy_lints/src/needless_continue.rs
index 38a75034cd3..377cbef7b99 100644
--- a/clippy_lints/src/needless_continue.rs
+++ b/clippy_lints/src/needless_continue.rs
@@ -189,7 +189,7 @@ fn needless_continue_in_else(else_expr: &ast::Expr, label: Option<&ast::Label>)
 }
 
 fn is_first_block_stmt_continue(block: &ast::Block, label: Option<&ast::Label>) -> bool {
-    block.stmts.get(0).map_or(false, |stmt| match stmt.kind {
+    block.stmts.first().map_or(false, |stmt| match stmt.kind {
         ast::StmtKind::Semi(ref e) | ast::StmtKind::Expr(ref e) => {
             if let ast::ExprKind::Continue(ref l) = e.kind {
                 compare_labels(label, l.as_ref())
@@ -434,7 +434,7 @@ fn erode_from_back(s: &str) -> String {
 }
 
 fn span_of_first_expr_in_block(block: &ast::Block) -> Option<Span> {
-    block.stmts.get(0).map(|stmt| stmt.span)
+    block.stmts.first().map(|stmt| stmt.span)
 }
 
 #[cfg(test)]
diff --git a/clippy_lints/src/slow_vector_initialization.rs b/clippy_lints/src/slow_vector_initialization.rs
index 9db18c2976c..2278e41be37 100644
--- a/clippy_lints/src/slow_vector_initialization.rs
+++ b/clippy_lints/src/slow_vector_initialization.rs
@@ -335,7 +335,7 @@ impl<'a, 'tcx> Visitor<'tcx> for VectorInitializationVisitor<'a, 'tcx> {
 
     fn visit_block(&mut self, block: &'tcx Block<'_>) {
         if self.initialization_found {
-            if let Some(s) = block.stmts.get(0) {
+            if let Some(s) = block.stmts.first() {
                 self.visit_stmt(s);
             }
 
diff --git a/clippy_lints/src/uninit_vec.rs b/clippy_lints/src/uninit_vec.rs
index 6756df8e716..72569e10f05 100644
--- a/clippy_lints/src/uninit_vec.rs
+++ b/clippy_lints/src/uninit_vec.rs
@@ -201,7 +201,7 @@ fn extract_set_len_self<'tcx>(cx: &LateContext<'_>, expr: &'tcx Expr<'_>) -> Opt
     let expr = peel_hir_expr_while(expr, |e| {
         if let ExprKind::Block(block, _) = e.kind {
             // Extract the first statement/expression
-            match (block.stmts.get(0).map(|stmt| &stmt.kind), block.expr) {
+            match (block.stmts.first().map(|stmt| &stmt.kind), block.expr) {
                 (None, Some(expr)) => Some(expr),
                 (Some(StmtKind::Expr(expr) | StmtKind::Semi(expr)), _) => Some(expr),
                 _ => None,
diff --git a/clippy_lints/src/unnecessary_map_on_constructor.rs b/clippy_lints/src/unnecessary_map_on_constructor.rs
index 5aa057580e9..894de0d85c1 100644
--- a/clippy_lints/src/unnecessary_map_on_constructor.rs
+++ b/clippy_lints/src/unnecessary_map_on_constructor.rs
@@ -40,7 +40,7 @@ impl<'tcx> LateLintPass<'tcx> for UnnecessaryMapOnConstructor {
             let (constructor_path, constructor_item) =
                 if let hir::ExprKind::Call(constructor, constructor_args) = recv.kind
                     && let hir::ExprKind::Path(constructor_path) = constructor.kind
-                    && let Some(arg) = constructor_args.get(0)
+                    && let Some(arg) = constructor_args.first()
                 {
                     if constructor.span.from_expansion() || arg.span.from_expansion() {
                         return;
@@ -66,7 +66,7 @@ impl<'tcx> LateLintPass<'tcx> for UnnecessaryMapOnConstructor {
                 _ => return,
             }
 
-            if let Some(map_arg) = args.get(0)
+            if let Some(map_arg) = args.first()
                 && let hir::ExprKind::Path(fun) = map_arg.kind
             {
                 if map_arg.span.from_expansion() {
diff --git a/clippy_utils/src/consts.rs b/clippy_utils/src/consts.rs
index 0bae7056c4f..79c04c7c7f4 100644
--- a/clippy_utils/src/consts.rs
+++ b/clippy_utils/src/consts.rs
@@ -504,7 +504,7 @@ impl<'a, 'tcx> ConstEvalLateContext<'a, 'tcx> {
             },
             (Some(Constant::Vec(vec)), _) => {
                 if !vec.is_empty() && vec.iter().all(|x| *x == vec[0]) {
-                    match vec.get(0) {
+                    match vec.first() {
                         Some(Constant::F32(x)) => Some(Constant::F32(*x)),
                         Some(Constant::F64(x)) => Some(Constant::F64(*x)),
                         _ => None,
diff --git a/clippy_utils/src/higher.rs b/clippy_utils/src/higher.rs
index 802adbd4d2d..7c1be625bd3 100644
--- a/clippy_utils/src/higher.rs
+++ b/clippy_utils/src/higher.rs
@@ -449,7 +449,7 @@ pub fn get_vec_init_kind<'tcx>(cx: &LateContext<'tcx>, expr: &'tcx Expr<'tcx>) -
                 } else if name.ident.name == symbol::kw::Default {
                     return Some(VecInitKind::Default);
                 } else if name.ident.name.as_str() == "with_capacity" {
-                    let arg = args.get(0)?;
+                    let arg = args.first()?;
                     return match constant_simple(cx, cx.typeck_results(), arg) {
                         Some(Constant::Int(num)) => Some(VecInitKind::WithConstCapacity(num)),
                         _ => Some(VecInitKind::WithExprCapacity(arg.hir_id)),
diff --git a/tests/ui/get_first.fixed b/tests/ui/get_first.fixed
index b1a597fc4dd..a7cdd2a93ba 100644
--- a/tests/ui/get_first.fixed
+++ b/tests/ui/get_first.fixed
@@ -14,17 +14,20 @@ impl Bar {
 
 fn main() {
     let x = vec![2, 3, 5];
-    let _ = x.first(); // Use x.first()
+    let _ = x.first();
+    //~^ ERROR: accessing first element with `x.get(0)`
     let _ = x.get(1);
     let _ = x[0];
 
     let y = [2, 3, 5];
-    let _ = y.first(); // Use y.first()
+    let _ = y.first();
+    //~^ ERROR: accessing first element with `y.get(0)`
     let _ = y.get(1);
     let _ = y[0];
 
     let z = &[2, 3, 5];
-    let _ = z.first(); // Use z.first()
+    let _ = z.first();
+    //~^ ERROR: accessing first element with `z.get(0)`
     let _ = z.get(1);
     let _ = z[0];
 
@@ -37,4 +40,8 @@ fn main() {
 
     let bar = Bar { arr: [0, 1, 2] };
     let _ = bar.get(0); // Do not lint, because Bar is struct.
+
+    let non_primitives = [vec![1, 2], vec![3, 4]];
+    let _ = non_primitives.first();
+    //~^ ERROR: accessing first element with `non_primitives.get(0)`
 }
diff --git a/tests/ui/get_first.rs b/tests/ui/get_first.rs
index e27ee4be8c0..cca743c4bf5 100644
--- a/tests/ui/get_first.rs
+++ b/tests/ui/get_first.rs
@@ -14,17 +14,20 @@ impl Bar {
 
 fn main() {
     let x = vec![2, 3, 5];
-    let _ = x.get(0); // Use x.first()
+    let _ = x.get(0);
+    //~^ ERROR: accessing first element with `x.get(0)`
     let _ = x.get(1);
     let _ = x[0];
 
     let y = [2, 3, 5];
-    let _ = y.get(0); // Use y.first()
+    let _ = y.get(0);
+    //~^ ERROR: accessing first element with `y.get(0)`
     let _ = y.get(1);
     let _ = y[0];
 
     let z = &[2, 3, 5];
-    let _ = z.get(0); // Use z.first()
+    let _ = z.get(0);
+    //~^ ERROR: accessing first element with `z.get(0)`
     let _ = z.get(1);
     let _ = z[0];
 
@@ -37,4 +40,8 @@ fn main() {
 
     let bar = Bar { arr: [0, 1, 2] };
     let _ = bar.get(0); // Do not lint, because Bar is struct.
+
+    let non_primitives = [vec![1, 2], vec![3, 4]];
+    let _ = non_primitives.get(0);
+    //~^ ERROR: accessing first element with `non_primitives.get(0)`
 }
diff --git a/tests/ui/get_first.stderr b/tests/ui/get_first.stderr
index 56b4c29a313..8ee66e33cc8 100644
--- a/tests/ui/get_first.stderr
+++ b/tests/ui/get_first.stderr
@@ -1,23 +1,29 @@
 error: accessing first element with `x.get(0)`
   --> $DIR/get_first.rs:17:13
    |
-LL |     let _ = x.get(0); // Use x.first()
+LL |     let _ = x.get(0);
    |             ^^^^^^^^ help: try: `x.first()`
    |
    = note: `-D clippy::get-first` implied by `-D warnings`
    = help: to override `-D warnings` add `#[allow(clippy::get_first)]`
 
 error: accessing first element with `y.get(0)`
-  --> $DIR/get_first.rs:22:13
+  --> $DIR/get_first.rs:23:13
    |
-LL |     let _ = y.get(0); // Use y.first()
+LL |     let _ = y.get(0);
    |             ^^^^^^^^ help: try: `y.first()`
 
 error: accessing first element with `z.get(0)`
-  --> $DIR/get_first.rs:27:13
+  --> $DIR/get_first.rs:29:13
    |
-LL |     let _ = z.get(0); // Use z.first()
+LL |     let _ = z.get(0);
    |             ^^^^^^^^ help: try: `z.first()`
 
-error: aborting due to 3 previous errors
+error: accessing first element with `non_primitives.get(0)`
+  --> $DIR/get_first.rs:45:13
+   |
+LL |     let _ = non_primitives.get(0);
+   |             ^^^^^^^^^^^^^^^^^^^^^ help: try: `non_primitives.first()`
+
+error: aborting due to 4 previous errors