Auto merge of #10891 - Centri3:missing_const_for_fn, r=Jarcho

[`missing_const_for_fn`]: Ensure dropped locals are `~const Destruct`

this will check every local for `TerminatorKind::Drop` to ensure they can be evaluated at compile time, not sure if this is the best way to do this but MIR is confusing and it works so...

fixes #10617

changelog: [`missing_const_for_fn`]: Ensure dropped locals are `~const Destruct`

3 files changed, 63 insertions, 13 deletions

diff --git a/tests/ui/missing_const_for_fn/cant_be_const.rs b/tests/ui/missing_const_for_fn/cant_be_const.rs
index 5db73a7b8ea..286d208b25b 100644
--- a/tests/ui/missing_const_for_fn/cant_be_const.rs
+++ b/tests/ui/missing_const_for_fn/cant_be_const.rs
@@ -13,7 +13,7 @@ extern crate proc_macros;
 
 use proc_macros::with_span;
 
-struct Game;
+struct Game; // You just lost.
 
 // This should not be linted because it's already const
 const fn already_const() -> i32 {
@@ -126,3 +126,34 @@ with_span! {
     span
     fn dont_check_in_proc_macro() {}
 }
+
+// Do not lint `String` has `Vec<u8>`, which cannot be dropped in const contexts
+fn a(this: String) {}
+
+enum A {
+    F(String),
+    N,
+}
+
+// Same here.
+fn b(this: A) {}
+
+// Minimized version of `a`.
+fn c(this: Vec<u16>) {}
+
+struct F(A);
+
+// Do not lint
+fn f(this: F) {}
+
+// Do not lint
+fn g<T>(this: T) {}
+
+struct Issue10617(String);
+
+impl Issue10617 {
+    // Do not lint
+    pub fn name(self) -> String {
+        self.0
+    }
+}
diff --git a/tests/ui/missing_const_for_fn/could_be_const.rs b/tests/ui/missing_const_for_fn/could_be_const.rs
index 0246c8622ed..b1980b1b523 100644
--- a/tests/ui/missing_const_for_fn/could_be_const.rs
+++ b/tests/ui/missing_const_for_fn/could_be_const.rs
@@ -1,5 +1,7 @@
 #![warn(clippy::missing_const_for_fn)]
 #![allow(incomplete_features, clippy::let_and_return)]
+#![feature(const_mut_refs)]
+#![feature(const_trait_impl)]
 
 use std::mem::transmute;
 
@@ -87,3 +89,14 @@ fn msrv_1_46() -> i32 {
 
 // Should not be const
 fn main() {}
+
+struct D;
+
+impl const Drop for D {
+    fn drop(&mut self) {
+        todo!();
+    }
+}
+
+// Lint this, since it can be dropped in const contexts
+fn d(this: D) {}
diff --git a/tests/ui/missing_const_for_fn/could_be_const.stderr b/tests/ui/missing_const_for_fn/could_be_const.stderr
index 955e1ed2634..7be2cc0ca93 100644
--- a/tests/ui/missing_const_for_fn/could_be_const.stderr
+++ b/tests/ui/missing_const_for_fn/could_be_const.stderr
@@ -1,5 +1,5 @@
 error: this could be a `const fn`
-  --> $DIR/could_be_const.rs:12:5
+  --> $DIR/could_be_const.rs:14:5
    |
 LL | /     pub fn new() -> Self {
 LL | |         Self { guess: 42 }
@@ -9,7 +9,7 @@ LL | |     }
    = note: `-D clippy::missing-const-for-fn` implied by `-D warnings`
 
 error: this could be a `const fn`
-  --> $DIR/could_be_const.rs:16:5
+  --> $DIR/could_be_const.rs:18:5
    |
 LL | /     fn const_generic_params<'a, T, const N: usize>(&self, b: &'a [T; N]) -> &'a [T; N] {
 LL | |         b
@@ -17,7 +17,7 @@ LL | |     }
    | |_____^
 
 error: this could be a `const fn`
-  --> $DIR/could_be_const.rs:22:1
+  --> $DIR/could_be_const.rs:24:1
    |
 LL | / fn one() -> i32 {
 LL | |     1
@@ -25,7 +25,7 @@ LL | | }
    | |_^
 
 error: this could be a `const fn`
-  --> $DIR/could_be_const.rs:27:1
+  --> $DIR/could_be_const.rs:29:1
    |
 LL | / fn two() -> i32 {
 LL | |     let abc = 2;
@@ -34,7 +34,7 @@ LL | | }
    | |_^
 
 error: this could be a `const fn`
-  --> $DIR/could_be_const.rs:33:1
+  --> $DIR/could_be_const.rs:35:1
    |
 LL | / fn string() -> String {
 LL | |     String::new()
@@ -42,7 +42,7 @@ LL | | }
    | |_^
 
 error: this could be a `const fn`
-  --> $DIR/could_be_const.rs:38:1
+  --> $DIR/could_be_const.rs:40:1
    |
 LL | / unsafe fn four() -> i32 {
 LL | |     4
@@ -50,7 +50,7 @@ LL | | }
    | |_^
 
 error: this could be a `const fn`
-  --> $DIR/could_be_const.rs:43:1
+  --> $DIR/could_be_const.rs:45:1
    |
 LL | / fn generic<T>(t: T) -> T {
 LL | |     t
@@ -58,7 +58,7 @@ LL | | }
    | |_^
 
 error: this could be a `const fn`
-  --> $DIR/could_be_const.rs:51:1
+  --> $DIR/could_be_const.rs:53:1
    |
 LL | / fn generic_arr<T: Copy>(t: [T; 1]) -> T {
 LL | |     t[0]
@@ -66,7 +66,7 @@ LL | | }
    | |_^
 
 error: this could be a `const fn`
-  --> $DIR/could_be_const.rs:64:9
+  --> $DIR/could_be_const.rs:66:9
    |
 LL | /         pub fn b(self, a: &A) -> B {
 LL | |             B
@@ -74,7 +74,7 @@ LL | |         }
    | |_________^
 
 error: this could be a `const fn`
-  --> $DIR/could_be_const.rs:73:5
+  --> $DIR/could_be_const.rs:75:5
    |
 LL | /     fn const_fn_stabilized_before_msrv(byte: u8) {
 LL | |         byte.is_ascii_digit();
@@ -82,12 +82,18 @@ LL | |     }
    | |_____^
 
 error: this could be a `const fn`
-  --> $DIR/could_be_const.rs:84:1
+  --> $DIR/could_be_const.rs:86:1
    |
 LL | / fn msrv_1_46() -> i32 {
 LL | |     46
 LL | | }
    | |_^
 
-error: aborting due to 11 previous errors
+error: this could be a `const fn`
+  --> $DIR/could_be_const.rs:102:1
+   |
+LL | fn d(this: D) {}
+   | ^^^^^^^^^^^^^^^^
+
+error: aborting due to 12 previous errors