make `useless_vec` smarter

1 files changed, 97 insertions, 14 deletions

diff --git a/clippy_lints/src/vec.rs b/clippy_lints/src/vec.rs
index 7329e508106..ec21f3bb54f 100644
--- a/clippy_lints/src/vec.rs
+++ b/clippy_lints/src/vec.rs
@@ -1,16 +1,21 @@
+use std::ops::ControlFlow;
+
 use clippy_utils::consts::{constant, Constant};
 use clippy_utils::diagnostics::span_lint_and_sugg;
-use clippy_utils::higher;
 use clippy_utils::source::snippet_with_applicability;
 use clippy_utils::ty::is_copy;
+use clippy_utils::visitors::for_each_local_use_after_expr;
+use clippy_utils::{get_parent_expr, higher, is_trait_method};
 use if_chain::if_chain;
+use rustc_ast::BindingAnnotation;
 use rustc_errors::Applicability;
-use rustc_hir::{BorrowKind, Expr, ExprKind, Mutability};
+use rustc_hir::{BorrowKind, Expr, ExprKind, Mutability, Node, PatKind};
 use rustc_lint::{LateContext, LateLintPass};
 use rustc_middle::ty::layout::LayoutOf;
 use rustc_middle::ty::{self, Ty};
 use rustc_session::{declare_tool_lint, impl_lint_pass};
 use rustc_span::source_map::Span;
+use rustc_span::sym;
 
 #[expect(clippy::module_name_repetitions)]
 #[derive(Copy, Clone)]
@@ -20,7 +25,7 @@ pub struct UselessVec {
 
 declare_clippy_lint! {
     /// ### What it does
-    /// Checks for usage of `&vec![..]` when using `&[..]` would
+    /// Checks for usage of `vec![..]` when using `[..]` would
     /// be possible.
     ///
     /// ### Why is this bad?
@@ -46,16 +51,70 @@ declare_clippy_lint! {
 
 impl_lint_pass!(UselessVec => [USELESS_VEC]);
 
+fn adjusts_to_slice(cx: &LateContext<'_>, e: &Expr<'_>) -> bool {
+    if let ty::Ref(_, ty, _) = cx.typeck_results().expr_ty_adjusted(e).kind() {
+        ty.is_slice()
+    } else {
+        false
+    }
+}
+
+/// Checks if the given expression is a method call to a `Vec` method
+/// that also exists on slices. If this returns true, it means that
+/// this expression does not actually require a `Vec` and could just work with an array.
+fn is_allowed_vec_method(cx: &LateContext<'_>, e: &Expr<'_>) -> bool {
+    const ALLOWED_METHOD_NAMES: &[&str] = &["len", "as_ptr", "as_slice", "is_empty"];
+
+    if let ExprKind::MethodCall(path, ..) = e.kind {
+        ALLOWED_METHOD_NAMES.contains(&path.ident.name.as_str())
+    } else {
+        is_trait_method(cx, e, sym::IntoIterator)
+    }
+}
+
 impl<'tcx> LateLintPass<'tcx> for UselessVec {
     fn check_expr(&mut self, cx: &LateContext<'tcx>, expr: &'tcx Expr<'_>) {
         // search for `&vec![_]` expressions where the adjusted type is `&[_]`
         if_chain! {
-            if let ty::Ref(_, ty, _) = cx.typeck_results().expr_ty_adjusted(expr).kind();
-            if let ty::Slice(..) = ty.kind();
+            if adjusts_to_slice(cx, expr);
             if let ExprKind::AddrOf(BorrowKind::Ref, mutability, addressee) = expr.kind;
             if let Some(vec_args) = higher::VecArgs::hir(cx, addressee);
             then {
-                self.check_vec_macro(cx, &vec_args, mutability, expr.span);
+                self.check_vec_macro(cx, &vec_args, mutability, expr.span, SuggestSlice::Yes);
+            }
+        }
+
+        // search for `let foo = vec![_]` expressions where all uses of `foo`
+        // adjust to slices or call a method that exist on slices (e.g. len)
+        if let Some(vec_args) = higher::VecArgs::hir(cx, expr)
+            && let Node::Local(local) = cx.tcx.hir().get_parent(expr.hir_id)
+            // for now ignore locals with type annotations.
+            // this is to avoid compile errors when doing the suggestion here: let _: Vec<_> = vec![..];
+            && local.ty.is_none()
+            && let PatKind::Binding(BindingAnnotation(_, mutbl), id, ..) = local.pat.kind
+            && is_copy(cx, vec_type(cx.typeck_results().expr_ty_adjusted(expr)))
+        {
+            let only_slice_uses = for_each_local_use_after_expr(cx, id, expr.hir_id, |expr| {
+                // allow indexing into a vec and some set of allowed method calls that exist on slices, too
+                if let Some(parent) = get_parent_expr(cx, expr)
+                    && (adjusts_to_slice(cx, expr)
+                        || matches!(parent.kind, ExprKind::Index(..))
+                        || is_allowed_vec_method(cx, parent))
+                {
+                    ControlFlow::Continue(())
+                } else {
+                    ControlFlow::Break(())
+                }
+            }).is_continue();
+
+            if only_slice_uses {
+                self.check_vec_macro(
+                    cx,
+                    &vec_args,
+                    mutbl,
+                    expr.span.ctxt().outer_expn_data().call_site,
+                    SuggestSlice::No
+                );
             }
         }
 
@@ -67,12 +126,20 @@ impl<'tcx> LateLintPass<'tcx> for UselessVec {
             then {
                 // report the error around the `vec!` not inside `<std macros>:`
                 let span = arg.span.ctxt().outer_expn_data().call_site;
-                self.check_vec_macro(cx, &vec_args, Mutability::Not, span);
+                self.check_vec_macro(cx, &vec_args, Mutability::Not, span, SuggestSlice::Yes);
             }
         }
     }
 }
 
+#[derive(Copy, Clone)]
+enum SuggestSlice {
+    /// Suggest using a slice `&[..]` / `&mut [..]`
+    Yes,
+    /// Suggest using an array: `[..]`
+    No,
+}
+
 impl UselessVec {
     fn check_vec_macro<'tcx>(
         self,
@@ -80,8 +147,15 @@ impl UselessVec {
         vec_args: &higher::VecArgs<'tcx>,
         mutability: Mutability,
         span: Span,
+        suggest_slice: SuggestSlice,
     ) {
         let mut applicability = Applicability::MachineApplicable;
+
+        let (borrow_prefix_mut, borrow_prefix) = match suggest_slice {
+            SuggestSlice::Yes => ("&mut ", "&"),
+            SuggestSlice::No => ("", ""),
+        };
+
         let snippet = match *vec_args {
             higher::VecArgs::Repeat(elem, len) => {
                 if let Some(Constant::Int(len_constant)) = constant(cx, cx.typeck_results(), len) {
@@ -93,14 +167,14 @@ impl UselessVec {
                     match mutability {
                         Mutability::Mut => {
                             format!(
-                                "&mut [{}; {}]",
+                                "{borrow_prefix_mut}[{}; {}]",
                                 snippet_with_applicability(cx, elem.span, "elem", &mut applicability),
                                 snippet_with_applicability(cx, len.span, "len", &mut applicability)
                             )
                         },
                         Mutability::Not => {
                             format!(
-                                "&[{}; {}]",
+                                "{borrow_prefix}[{}; {}]",
                                 snippet_with_applicability(cx, elem.span, "elem", &mut applicability),
                                 snippet_with_applicability(cx, len.span, "len", &mut applicability)
                             )
@@ -120,18 +194,21 @@ impl UselessVec {
                     match mutability {
                         Mutability::Mut => {
                             format!(
-                                "&mut [{}]",
+                                "{borrow_prefix_mut}[{}]",
                                 snippet_with_applicability(cx, span, "..", &mut applicability)
                             )
                         },
                         Mutability::Not => {
-                            format!("&[{}]", snippet_with_applicability(cx, span, "..", &mut applicability))
+                            format!(
+                                "{borrow_prefix}[{}]",
+                                snippet_with_applicability(cx, span, "..", &mut applicability)
+                            )
                         },
                     }
                 } else {
                     match mutability {
-                        Mutability::Mut => "&mut []".into(),
-                        Mutability::Not => "&[]".into(),
+                        Mutability::Mut => format!("{borrow_prefix_mut}[]"),
+                        Mutability::Not => format!("{borrow_prefix}[]"),
                     }
                 }
             },
@@ -142,7 +219,13 @@ impl UselessVec {
             USELESS_VEC,
             span,
             "useless use of `vec!`",
-            "you can use a slice directly",
+            &format!(
+                "you can use {} directly",
+                match suggest_slice {
+                    SuggestSlice::Yes => "a slice",
+                    SuggestSlice::No => "an array",
+                }
+            ),
             snippet,
             applicability,
         );