New upstream version 1.52.1+dfsg1

[rustc.git] / src / tools / clippy / clippy_lints / src / zero_sized_map_values.rs

diff --git a/src/tools/clippy/clippy_lints/src/zero_sized_map_values.rs b/src/tools/clippy/clippy_lints/src/zero_sized_map_values.rs
new file mode 100644 (file)
index 0000000..adf7077
--- /dev/null
+++ b/src/tools/clippy/clippy_lints/src/zero_sized_map_values.rs
@@ -0,0 +1,85 @@
+use if_chain::if_chain;
+use rustc_hir::{self as hir, HirId, ItemKind, Node};
+use rustc_lint::{LateContext, LateLintPass};
+use rustc_middle::ty::{Adt, Ty};
+use rustc_session::{declare_lint_pass, declare_tool_lint};
+use rustc_span::sym;
+use rustc_target::abi::LayoutOf as _;
+use rustc_typeck::hir_ty_to_ty;
+
+use crate::utils::{is_normalizable, is_type_diagnostic_item, match_type, paths, span_lint_and_help};
+
+declare_clippy_lint! {
+    /// **What it does:** Checks for maps with zero-sized value types anywhere in the code.
+    ///
+    /// **Why is this bad?** Since there is only a single value for a zero-sized type, a map
+    /// containing zero sized values is effectively a set. Using a set in that case improves
+    /// readability and communicates intent more clearly.
+    ///
+    /// **Known problems:**
+    /// * A zero-sized type cannot be recovered later if it contains private fields.
+    /// * This lints the signature of public items
+    ///
+    /// **Example:**
+    ///
+    /// ```rust
+    /// # use std::collections::HashMap;
+    /// fn unique_words(text: &str) -> HashMap<&str, ()> {
+    ///     todo!();
+    /// }
+    /// ```
+    /// Use instead:
+    /// ```rust
+    /// # use std::collections::HashSet;
+    /// fn unique_words(text: &str) -> HashSet<&str> {
+    ///     todo!();
+    /// }
+    /// ```
+    pub ZERO_SIZED_MAP_VALUES,
+    pedantic,
+    "usage of map with zero-sized value type"
+}
+
+declare_lint_pass!(ZeroSizedMapValues => [ZERO_SIZED_MAP_VALUES]);
+
+impl LateLintPass<'_> for ZeroSizedMapValues {
+    fn check_ty(&mut self, cx: &LateContext<'_>, hir_ty: &hir::Ty<'_>) {
+        if_chain! {
+            if !hir_ty.span.from_expansion();
+            if !in_trait_impl(cx, hir_ty.hir_id);
+            let ty = ty_from_hir_ty(cx, hir_ty);
+            if is_type_diagnostic_item(cx, ty, sym::hashmap_type) || match_type(cx, ty, &paths::BTREEMAP);
+            if let Adt(_, ref substs) = ty.kind();
+            let ty = substs.type_at(1);
+            // Do this to prevent `layout_of` crashing, being unable to fully normalize `ty`.
+            if is_normalizable(cx, cx.param_env, ty);
+            if let Ok(layout) = cx.layout_of(ty);
+            if layout.is_zst();
+            then {
+                span_lint_and_help(cx, ZERO_SIZED_MAP_VALUES, hir_ty.span, "map with zero-sized value type", None, "consider using a set instead");
+            }
+        }
+    }
+}
+
+fn in_trait_impl(cx: &LateContext<'_>, hir_id: HirId) -> bool {
+    let parent_id = cx.tcx.hir().get_parent_item(hir_id);
+    if let Some(Node::Item(item)) = cx.tcx.hir().find(cx.tcx.hir().get_parent_item(parent_id)) {
+        if let ItemKind::Impl(hir::Impl { of_trait: Some(_), .. }) = item.kind {
+            return true;
+        }
+    }
+    false
+}
+
+fn ty_from_hir_ty<'tcx>(cx: &LateContext<'tcx>, hir_ty: &hir::Ty<'_>) -> Ty<'tcx> {
+    cx.maybe_typeck_results()
+        .and_then(|results| {
+            if results.hir_owner == hir_ty.hir_id.owner {
+                results.node_type_opt(hir_ty.hir_id)
+            } else {
+                None
+            }
+        })
+        .unwrap_or_else(|| hir_ty_to_ty(cx.tcx, hir_ty))
+}