`missing_const_for_fn`: consider constness of instance (#14759)

When determining when a function or method can be called from a `const`
context, the determination must be made on the instance, not on the
declaration. This makes a difference, for example, with `const_trait`
traits whose implementations may or may not be `const`.

changelog: [`missing_const_for_fn`]: when checking if a function or
method can be called from a `const` context, look at the concrete
implementation rather than at the trait definition

Fixes rust-lang/rust-clippy#14658

r? @Jarcho

3 files changed, 89 insertions, 0 deletions

diff --git a/tests/ui/missing_const_for_fn/const_trait.fixed b/tests/ui/missing_const_for_fn/const_trait.fixed
new file mode 100644
index 00000000000..7e0d4fccaae
--- /dev/null
+++ b/tests/ui/missing_const_for_fn/const_trait.fixed
@@ -0,0 +1,36 @@
+#![feature(const_trait_impl)]
+#![warn(clippy::missing_const_for_fn)]
+
+// Reduced test case from https://github.com/rust-lang/rust-clippy/issues/14658
+
+#[const_trait]
+trait ConstTrait {
+    fn method(self);
+}
+
+impl ConstTrait for u32 {
+    fn method(self) {}
+}
+
+impl const ConstTrait for u64 {
+    fn method(self) {}
+}
+
+fn cannot_be_const() {
+    0u32.method();
+}
+
+//~v missing_const_for_fn
+const fn can_be_const() {
+    0u64.method();
+}
+
+// False negative, see FIXME comment in `clipy_utils::qualify_min_const`
+fn could_be_const_but_does_not_trigger<T>(t: T)
+where
+    T: const ConstTrait,
+{
+    t.method();
+}
+
+fn main() {}
diff --git a/tests/ui/missing_const_for_fn/const_trait.rs b/tests/ui/missing_const_for_fn/const_trait.rs
new file mode 100644
index 00000000000..439da4622d7
--- /dev/null
+++ b/tests/ui/missing_const_for_fn/const_trait.rs
@@ -0,0 +1,36 @@
+#![feature(const_trait_impl)]
+#![warn(clippy::missing_const_for_fn)]
+
+// Reduced test case from https://github.com/rust-lang/rust-clippy/issues/14658
+
+#[const_trait]
+trait ConstTrait {
+    fn method(self);
+}
+
+impl ConstTrait for u32 {
+    fn method(self) {}
+}
+
+impl const ConstTrait for u64 {
+    fn method(self) {}
+}
+
+fn cannot_be_const() {
+    0u32.method();
+}
+
+//~v missing_const_for_fn
+fn can_be_const() {
+    0u64.method();
+}
+
+// False negative, see FIXME comment in `clipy_utils::qualify_min_const`
+fn could_be_const_but_does_not_trigger<T>(t: T)
+where
+    T: const ConstTrait,
+{
+    t.method();
+}
+
+fn main() {}
diff --git a/tests/ui/missing_const_for_fn/const_trait.stderr b/tests/ui/missing_const_for_fn/const_trait.stderr
new file mode 100644
index 00000000000..b994b88fac6
--- /dev/null
+++ b/tests/ui/missing_const_for_fn/const_trait.stderr
@@ -0,0 +1,17 @@
+error: this could be a `const fn`
+  --> tests/ui/missing_const_for_fn/const_trait.rs:24:1
+   |
+LL | / fn can_be_const() {
+LL | |     0u64.method();
+LL | | }
+   | |_^
+   |
+   = note: `-D clippy::missing-const-for-fn` implied by `-D warnings`
+   = help: to override `-D warnings` add `#[allow(clippy::missing_const_for_fn)]`
+help: make the function `const`
+   |
+LL | const fn can_be_const() {
+   | +++++
+
+error: aborting due to 1 previous error
+