cleanup spaghetti code

tests/ui/missing_const_for_fn/cant_be_const.rs

@@ -127,6 +127,7 @@ with_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 {
@@ -134,6 +135,25 @@ enum A {
     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
+    }
+}

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) {}

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