Check MSRV before suggesting applying const to a function

If a function environment contains trait bounds other than `Sized`,
`const` cannot be used before Rust 1.61.

Author: samueltardieu

clippy_utils/src/msrvs.rs

@@ -42,6 +42,7 @@ msrv_aliases! {
     1,65,0 { LET_ELSE, POINTER_CAST_CONSTNESS }
     1,63,0 { CLONE_INTO, CONST_SLICE_FROM_REF }
     1,62,0 { BOOL_THEN_SOME, DEFAULT_ENUM_ATTRIBUTE, CONST_EXTERN_C_FN }
+    1,61,0 { CONST_FN_TRAIT_BOUND }
     1,60,0 { ABS_DIFF }
     1,59,0 { THREAD_LOCAL_CONST_INIT }
     1,58,0 { FORMAT_ARGS_CAPTURE, PATTERN_TRAIT_CHAR_ARRAY, CONST_RAW_PTR_DEREF }

clippy_utils/src/qualify_min_const_fn.rs

@@ -32,6 +32,19 @@ pub fn is_min_const_fn<'tcx>(cx: &LateContext<'tcx>, body: &Body<'tcx>, msrv: Ms
     for local in &body.local_decls {
         check_ty(cx, local.ty, local.source_info.span, msrv)?;
     }
+    if !msrv.meets(cx, msrvs::CONST_FN_TRAIT_BOUND)
+        && let Some(sized_did) = cx.tcx.lang_items().sized_trait()
+        && cx.tcx.param_env(def_id).caller_bounds().iter().any(|bound| {
+            bound
+                .as_trait_clause()
+                .is_some_and(|clause| clause.def_id() != sized_did)
+        })
+    {
+        return Err((
+            body.span,
+            "non-`Sized` trait clause before `const_fn_trait_bound` is stabilized".into(),
+        ));
+    }
     // impl trait is gone in MIR, so check the return type manually
     check_ty(
         cx,

tests/ui/missing_const_for_fn/could_be_const.fixed

@@ -221,3 +221,60 @@ const fn mut_add(x: &mut i32) {
     //~^ missing_const_for_fn
     *x += 1;
 }
+
+mod issue_15079 {
+    pub trait Trait {}
+
+    pub struct Struct<T: Trait> {
+        _t: Option<T>,
+    }
+
+    impl<T: Trait> Struct<T> {
+        #[clippy::msrv = "1.60"]
+        pub fn new_1_60() -> Self {
+            Self { _t: None }
+        }
+
+        #[clippy::msrv = "1.61"]
+        pub const fn new_1_61() -> Self {
+            //~^ missing_const_for_fn
+            Self { _t: None }
+        }
+    }
+
+    pub struct S2<T> {
+        _t: Option<T>,
+    }
+
+    impl<T> S2<T> {
+        #[clippy::msrv = "1.60"]
+        pub const fn new_1_60() -> Self {
+            //~^ missing_const_for_fn
+            Self { _t: None }
+        }
+
+        #[clippy::msrv = "1.61"]
+        pub const fn new_1_61() -> Self {
+            //~^ missing_const_for_fn
+            Self { _t: None }
+        }
+    }
+
+    pub struct S3<T: ?Sized + 'static> {
+        _t: Option<&'static T>,
+    }
+
+    impl<T: ?Sized + 'static> S3<T> {
+        #[clippy::msrv = "1.60"]
+        pub const fn new_1_60() -> Self {
+            //~^ missing_const_for_fn
+            Self { _t: None }
+        }
+
+        #[clippy::msrv = "1.61"]
+        pub const fn new_1_61() -> Self {
+            //~^ missing_const_for_fn
+            Self { _t: None }
+        }
+    }
+}

tests/ui/missing_const_for_fn/could_be_const.rs

@@ -221,3 +221,60 @@ fn mut_add(x: &mut i32) {
     //~^ missing_const_for_fn
     *x += 1;
 }
+
+mod issue_15079 {
+    pub trait Trait {}
+
+    pub struct Struct<T: Trait> {
+        _t: Option<T>,
+    }
+
+    impl<T: Trait> Struct<T> {
+        #[clippy::msrv = "1.60"]
+        pub fn new_1_60() -> Self {
+            Self { _t: None }
+        }
+
+        #[clippy::msrv = "1.61"]
+        pub fn new_1_61() -> Self {
+            //~^ missing_const_for_fn
+            Self { _t: None }
+        }
+    }
+
+    pub struct S2<T> {
+        _t: Option<T>,
+    }
+
+    impl<T> S2<T> {
+        #[clippy::msrv = "1.60"]
+        pub fn new_1_60() -> Self {
+            //~^ missing_const_for_fn
+            Self { _t: None }
+        }
+
+        #[clippy::msrv = "1.61"]
+        pub fn new_1_61() -> Self {
+            //~^ missing_const_for_fn
+            Self { _t: None }
+        }
+    }
+
+    pub struct S3<T: ?Sized + 'static> {
+        _t: Option<&'static T>,
+    }
+
+    impl<T: ?Sized + 'static> S3<T> {
+        #[clippy::msrv = "1.60"]
+        pub fn new_1_60() -> Self {
+            //~^ missing_const_for_fn
+            Self { _t: None }
+        }
+
+        #[clippy::msrv = "1.61"]
+        pub fn new_1_61() -> Self {
+            //~^ missing_const_for_fn
+            Self { _t: None }
+        }
+    }
+}

tests/ui/missing_const_for_fn/could_be_const.stderr

@@ -332,5 +332,75 @@ help: make the function `const`
 LL | const fn mut_add(x: &mut i32) {
    | +++++
 
-error: aborting due to 25 previous errors
+error: this could be a `const fn`
+  --> tests/ui/missing_const_for_fn/could_be_const.rs:239:9
+   |
+LL | /         pub fn new_1_61() -> Self {
+LL | |
+LL | |             Self { _t: None }
+LL | |         }
+   | |_________^
+   |
+help: make the function `const`
+   |
+LL |         pub const fn new_1_61() -> Self {
+   |             +++++
+
+error: this could be a `const fn`
+  --> tests/ui/missing_const_for_fn/could_be_const.rs:251:9
+   |
+LL | /         pub fn new_1_60() -> Self {
+LL | |
+LL | |             Self { _t: None }
+LL | |         }
+   | |_________^
+   |
+help: make the function `const`
+   |
+LL |         pub const fn new_1_60() -> Self {
+   |             +++++
+
+error: this could be a `const fn`
+  --> tests/ui/missing_const_for_fn/could_be_const.rs:257:9
+   |
+LL | /         pub fn new_1_61() -> Self {
+LL | |
+LL | |             Self { _t: None }
+LL | |         }
+   | |_________^
+   |
+help: make the function `const`
+   |
+LL |         pub const fn new_1_61() -> Self {
+   |             +++++
+
+error: this could be a `const fn`
+  --> tests/ui/missing_const_for_fn/could_be_const.rs:269:9
+   |
+LL | /         pub fn new_1_60() -> Self {
+LL | |
+LL | |             Self { _t: None }
+LL | |         }
+   | |_________^
+   |
+help: make the function `const`
+   |
+LL |         pub const fn new_1_60() -> Self {
+   |             +++++
+
+error: this could be a `const fn`
+  --> tests/ui/missing_const_for_fn/could_be_const.rs:275:9
+   |
+LL | /         pub fn new_1_61() -> Self {
+LL | |
+LL | |             Self { _t: None }
+LL | |         }
+   | |_________^
+   |
+help: make the function `const`
+   |
+LL |         pub const fn new_1_61() -> Self {
+   |             +++++
+
+error: aborting due to 30 previous errors