1 files changed, 174 insertions, 0 deletions
diff --git a/src/tools/clippy/clippy_lints/src/missing_const_for_fn.rs b/src/tools/clippy/clippy_lints/src/missing_const_for_fn.rs
new file mode 100644
index 000000000..16d65966c
--- /dev/null
+++ b/src/tools/clippy/clippy_lints/src/missing_const_for_fn.rs
@@ -0,0 +1,174 @@
+use clippy_utils::diagnostics::span_lint;
+use clippy_utils::qualify_min_const_fn::is_min_const_fn;
+use clippy_utils::ty::has_drop;
+use clippy_utils::{fn_has_unsatisfiable_preds, is_entrypoint_fn, meets_msrv, msrvs, trait_ref_of_method};
+use rustc_hir as hir;
+use rustc_hir::def_id::CRATE_DEF_ID;
+use rustc_hir::intravisit::FnKind;
+use rustc_hir::{Body, Constness, FnDecl, GenericParamKind, HirId};
+use rustc_lint::{LateContext, LateLintPass};
+use rustc_middle::lint::in_external_macro;
+use rustc_semver::RustcVersion;
+use rustc_session::{declare_tool_lint, impl_lint_pass};
+use rustc_span::Span;
+use rustc_typeck::hir_ty_to_ty;
+
+declare_clippy_lint! {
+    /// ### What it does
+    /// Suggests the use of `const` in functions and methods where possible.
+    ///
+    /// ### Why is this bad?
+    /// Not having the function const prevents callers of the function from being const as well.
+    ///
+    /// ### Known problems
+    /// Const functions are currently still being worked on, with some features only being available
+    /// on nightly. This lint does not consider all edge cases currently and the suggestions may be
+    /// incorrect if you are using this lint on stable.
+    ///
+    /// Also, the lint only runs one pass over the code. Consider these two non-const functions:
+    ///
+    /// ```rust
+    /// fn a() -> i32 {
+    ///     0
+    /// }
+    /// fn b() -> i32 {
+    ///     a()
+    /// }
+    /// ```
+    ///
+    /// When running Clippy, the lint will only suggest to make `a` const, because `b` at this time
+    /// can't be const as it calls a non-const function. Making `a` const and running Clippy again,
+    /// will suggest to make `b` const, too.
+    ///
+    /// ### Example
+    /// ```rust
+    /// # struct Foo {
+    /// #     random_number: usize,
+    /// # }
+    /// # impl Foo {
+    /// fn new() -> Self {
+    ///     Self { random_number: 42 }
+    /// }
+    /// # }
+    /// ```
+    ///
+    /// Could be a const fn:
+    ///
+    /// ```rust
+    /// # struct Foo {
+    /// #     random_number: usize,
+    /// # }
+    /// # impl Foo {
+    /// const fn new() -> Self {
+    ///     Self { random_number: 42 }
+    /// }
+    /// # }
+    /// ```
+    #[clippy::version = "1.34.0"]
+    pub MISSING_CONST_FOR_FN,
+    nursery,
+    "Lint functions definitions that could be made `const fn`"
+}
+
+impl_lint_pass!(MissingConstForFn => [MISSING_CONST_FOR_FN]);
+
+pub struct MissingConstForFn {
+    msrv: Option<RustcVersion>,
+}
+
+impl MissingConstForFn {
+    #[must_use]
+    pub fn new(msrv: Option<RustcVersion>) -> Self {
+        Self { msrv }
+    }
+}
+
+impl<'tcx> LateLintPass<'tcx> for MissingConstForFn {
+    fn check_fn(
+        &mut self,
+        cx: &LateContext<'_>,
+        kind: FnKind<'_>,
+        _: &FnDecl<'_>,
+        _: &Body<'_>,
+        span: Span,
+        hir_id: HirId,
+    ) {
+        if !meets_msrv(self.msrv, msrvs::CONST_IF_MATCH) {
+            return;
+        }
+
+        let def_id = cx.tcx.hir().local_def_id(hir_id);
+
+        if in_external_macro(cx.tcx.sess, span) || is_entrypoint_fn(cx, def_id.to_def_id()) {
+            return;
+        }
+
+        // Building MIR for `fn`s with unsatisfiable preds results in ICE.
+        if fn_has_unsatisfiable_preds(cx, def_id.to_def_id()) {
+            return;
+        }
+
+        // Perform some preliminary checks that rule out constness on the Clippy side. This way we
+        // can skip the actual const check and return early.
+        match kind {
+            FnKind::ItemFn(_, generics, header, ..) => {
+                let has_const_generic_params = generics
+                    .params
+                    .iter()
+                    .any(|param| matches!(param.kind, GenericParamKind::Const { .. }));
+
+                if already_const(header) || has_const_generic_params {
+                    return;
+                }
+            },
+            FnKind::Method(_, sig, ..) => {
+                if trait_ref_of_method(cx, def_id).is_some()
+                    || already_const(sig.header)
+                    || method_accepts_dropable(cx, sig.decl.inputs)
+                {
+                    return;
+                }
+            },
+            FnKind::Closure => return,
+        }
+
+        // Const fns are not allowed as methods in a trait.
+        {
+            let parent = cx.tcx.hir().get_parent_item(hir_id);
+            if parent != CRATE_DEF_ID {
+                if let hir::Node::Item(item) = cx.tcx.hir().get_by_def_id(parent) {
+                    if let hir::ItemKind::Trait(..) = &item.kind {
+                        return;
+                    }
+                }
+            }
+        }
+
+        let mir = cx.tcx.optimized_mir(def_id);
+
+        if let Err((span, err)) = is_min_const_fn(cx.tcx, mir, self.msrv) {
+            if cx.tcx.is_const_fn_raw(def_id.to_def_id()) {
+                cx.tcx.sess.span_err(span, err.as_ref());
+            }
+        } else {
+            span_lint(cx, MISSING_CONST_FOR_FN, span, "this could be a `const fn`");
+        }
+    }
+    extract_msrv_attr!(LateContext);
+}
+
+/// Returns true if any of the method parameters is a type that implements `Drop`. The method
+/// can't be made const then, because `drop` can't be const-evaluated.
+fn method_accepts_dropable(cx: &LateContext<'_>, param_tys: &[hir::Ty<'_>]) -> bool {
+    // If any of the params are droppable, return true
+    param_tys.iter().any(|hir_ty| {
+        let ty_ty = hir_ty_to_ty(cx.tcx, hir_ty);
+        has_drop(cx, ty_ty)
+    })
+}
+
+// We don't have to lint on something that's already `const`
+#[must_use]
+fn already_const(header: hir::FnHeader) -> bool {
+    header.constness == Constness::Const
+}