Add new `confusing_method_to_numeric_cast` lint (#13979)

Fixes https://github.com/rust-lang/rust-clippy/issues/13973.

I don't think we can make `fn_to_numeric_cast_any` to be emitted in some
special cases. Its category cannot be changed at runtime.

I think in this case, the best might be a specialized new lint so we can
target exactly what we want.

----

changelog: Add new `confusing_method_to_numeric_cast` lint

3 files changed, 128 insertions, 0 deletions

diff --git a/tests/ui/confusing_method_to_numeric_cast.fixed b/tests/ui/confusing_method_to_numeric_cast.fixed
new file mode 100644
index 00000000000..e698b99edd5
--- /dev/null
+++ b/tests/ui/confusing_method_to_numeric_cast.fixed
@@ -0,0 +1,14 @@
+#![feature(float_minimum_maximum)]
+#![warn(clippy::confusing_method_to_numeric_cast)]
+
+fn main() {
+    let _ = u16::MAX as usize; //~ confusing_method_to_numeric_cast
+    let _ = u16::MIN as usize; //~ confusing_method_to_numeric_cast
+    let _ = u16::MAX as usize; //~ confusing_method_to_numeric_cast
+    let _ = u16::MIN as usize; //~ confusing_method_to_numeric_cast
+
+    let _ = f32::MAX as usize; //~ confusing_method_to_numeric_cast
+    let _ = f32::MAX as usize; //~ confusing_method_to_numeric_cast
+    let _ = f32::MIN as usize; //~ confusing_method_to_numeric_cast
+    let _ = f32::MIN as usize; //~ confusing_method_to_numeric_cast
+}
diff --git a/tests/ui/confusing_method_to_numeric_cast.rs b/tests/ui/confusing_method_to_numeric_cast.rs
new file mode 100644
index 00000000000..ef65c21563d
--- /dev/null
+++ b/tests/ui/confusing_method_to_numeric_cast.rs
@@ -0,0 +1,14 @@
+#![feature(float_minimum_maximum)]
+#![warn(clippy::confusing_method_to_numeric_cast)]
+
+fn main() {
+    let _ = u16::max as usize; //~ confusing_method_to_numeric_cast
+    let _ = u16::min as usize; //~ confusing_method_to_numeric_cast
+    let _ = u16::max_value as usize; //~ confusing_method_to_numeric_cast
+    let _ = u16::min_value as usize; //~ confusing_method_to_numeric_cast
+
+    let _ = f32::maximum as usize; //~ confusing_method_to_numeric_cast
+    let _ = f32::max as usize; //~ confusing_method_to_numeric_cast
+    let _ = f32::minimum as usize; //~ confusing_method_to_numeric_cast
+    let _ = f32::min as usize; //~ confusing_method_to_numeric_cast
+}
diff --git a/tests/ui/confusing_method_to_numeric_cast.stderr b/tests/ui/confusing_method_to_numeric_cast.stderr
new file mode 100644
index 00000000000..ba90df2059a
--- /dev/null
+++ b/tests/ui/confusing_method_to_numeric_cast.stderr
@@ -0,0 +1,100 @@
+error: casting function pointer `u16::max` to `usize`
+  --> tests/ui/confusing_method_to_numeric_cast.rs:5:13
+   |
+LL |     let _ = u16::max as usize;
+   |             ^^^^^^^^^^^^^^^^^
+   |
+   = note: `-D clippy::confusing-method-to-numeric-cast` implied by `-D warnings`
+   = help: to override `-D warnings` add `#[allow(clippy::confusing_method_to_numeric_cast)]`
+help: did you mean to use the associated constant?
+   |
+LL -     let _ = u16::max as usize;
+LL +     let _ = u16::MAX as usize;
+   |
+
+error: casting function pointer `u16::min` to `usize`
+  --> tests/ui/confusing_method_to_numeric_cast.rs:6:13
+   |
+LL |     let _ = u16::min as usize;
+   |             ^^^^^^^^^^^^^^^^^
+   |
+help: did you mean to use the associated constant?
+   |
+LL -     let _ = u16::min as usize;
+LL +     let _ = u16::MIN as usize;
+   |
+
+error: casting function pointer `u16::max_value` to `usize`
+  --> tests/ui/confusing_method_to_numeric_cast.rs:7:13
+   |
+LL |     let _ = u16::max_value as usize;
+   |             ^^^^^^^^^^^^^^^^^^^^^^^
+   |
+help: did you mean to use the associated constant?
+   |
+LL -     let _ = u16::max_value as usize;
+LL +     let _ = u16::MAX as usize;
+   |
+
+error: casting function pointer `u16::min_value` to `usize`
+  --> tests/ui/confusing_method_to_numeric_cast.rs:8:13
+   |
+LL |     let _ = u16::min_value as usize;
+   |             ^^^^^^^^^^^^^^^^^^^^^^^
+   |
+help: did you mean to use the associated constant?
+   |
+LL -     let _ = u16::min_value as usize;
+LL +     let _ = u16::MIN as usize;
+   |
+
+error: casting function pointer `f32::maximum` to `usize`
+  --> tests/ui/confusing_method_to_numeric_cast.rs:10:13
+   |
+LL |     let _ = f32::maximum as usize;
+   |             ^^^^^^^^^^^^^^^^^^^^^
+   |
+help: did you mean to use the associated constant?
+   |
+LL -     let _ = f32::maximum as usize;
+LL +     let _ = f32::MAX as usize;
+   |
+
+error: casting function pointer `f32::max` to `usize`
+  --> tests/ui/confusing_method_to_numeric_cast.rs:11:13
+   |
+LL |     let _ = f32::max as usize;
+   |             ^^^^^^^^^^^^^^^^^
+   |
+help: did you mean to use the associated constant?
+   |
+LL -     let _ = f32::max as usize;
+LL +     let _ = f32::MAX as usize;
+   |
+
+error: casting function pointer `f32::minimum` to `usize`
+  --> tests/ui/confusing_method_to_numeric_cast.rs:12:13
+   |
+LL |     let _ = f32::minimum as usize;
+   |             ^^^^^^^^^^^^^^^^^^^^^
+   |
+help: did you mean to use the associated constant?
+   |
+LL -     let _ = f32::minimum as usize;
+LL +     let _ = f32::MIN as usize;
+   |
+
+error: casting function pointer `f32::min` to `usize`
+  --> tests/ui/confusing_method_to_numeric_cast.rs:13:13
+   |
+LL |     let _ = f32::min as usize;
+   |             ^^^^^^^^^^^^^^^^^
+   |
+help: did you mean to use the associated constant?
+   |
+LL -     let _ = f32::min as usize;
+LL +     let _ = f32::MIN as usize;
+   |
+
+error: aborting due to 8 previous errors
+