Various improvements to the `incompatible_msrv` lint (#14433)

This PR supersedes #14328 following the [2025-03-18 Clippy meeting
discussion](https://rust-lang.zulipchat.com/#narrow/channel/257328-clippy/topic/Meeting.202025-03-18/with/506527762).
It uses a simpler approach than what was proposed initially in #14328
and does not add new options.

First, it documents how `cfg_attr` can be used to change the MSRV
considered by Clippy to trigger the lint. This allows the MSRV to be
feature gated, or to be raised in tests.

Also, the lint stops warning about items which have been explicitly
allowed through a rustc feature. This works even if the feature has been
stabilized since. It allows using an older compiler with some features
turned on, as is done in Rust for Linux. This fixes #14425.

Then, if the lint triggers, and it looks like the code is located below
a `cfg` or `cfg_attr` attribute, an additional note is issued, once, to
indicate that the `clippy::msrv` attribute can be controlled by an
attribute.

Finally, the lint is extended to cover any path, not just method and
function calls. For example, enumeration variants, or constants, were
not MSRV checked. This required replacing two `u32::MAX` by
`u32::max_value()` in MSRV-limited tests.

An extra commit adds a TODO for checking the const stability also, as
this is not done right now.

@Centri3 I'll assign this to you because you were assigned #14328 and
you were the one who nominated the issue for discussion (thanks!), but
of course feel free to reroll!

r? @Centri3

changelog: [`incompatible_msrv`]: better documentation, honor the
`features` attribute, and lint non-function entities as well

5 files changed, 99 insertions, 30 deletions

diff --git a/tests/ui/checked_conversions.fixed b/tests/ui/checked_conversions.fixed
index 279a5b6e1ff..6175275ef04 100644
--- a/tests/ui/checked_conversions.fixed
+++ b/tests/ui/checked_conversions.fixed
@@ -95,7 +95,7 @@ pub const fn issue_8898(i: u32) -> bool {
 #[clippy::msrv = "1.33"]
 fn msrv_1_33() {
     let value: i64 = 33;
-    let _ = value <= (u32::MAX as i64) && value >= 0;
+    let _ = value <= (u32::max_value() as i64) && value >= 0;
 }
 
 #[clippy::msrv = "1.34"]
diff --git a/tests/ui/checked_conversions.rs b/tests/ui/checked_conversions.rs
index c339bc674bb..9ed0e8f660d 100644
--- a/tests/ui/checked_conversions.rs
+++ b/tests/ui/checked_conversions.rs
@@ -95,13 +95,13 @@ pub const fn issue_8898(i: u32) -> bool {
 #[clippy::msrv = "1.33"]
 fn msrv_1_33() {
     let value: i64 = 33;
-    let _ = value <= (u32::MAX as i64) && value >= 0;
+    let _ = value <= (u32::max_value() as i64) && value >= 0;
 }
 
 #[clippy::msrv = "1.34"]
 fn msrv_1_34() {
     let value: i64 = 34;
-    let _ = value <= (u32::MAX as i64) && value >= 0;
+    let _ = value <= (u32::max_value() as i64) && value >= 0;
     //~^ checked_conversions
 }
 
diff --git a/tests/ui/checked_conversions.stderr b/tests/ui/checked_conversions.stderr
index 3841b9d5a4d..624876dacb2 100644
--- a/tests/ui/checked_conversions.stderr
+++ b/tests/ui/checked_conversions.stderr
@@ -100,8 +100,8 @@ LL |     let _ = value <= u16::MAX as u32 && value as i32 == 5;
 error: checked cast can be simplified
   --> tests/ui/checked_conversions.rs:104:13
    |
-LL |     let _ = value <= (u32::MAX as i64) && value >= 0;
-   |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: try: `u32::try_from(value).is_ok()`
+LL |     let _ = value <= (u32::max_value() as i64) && value >= 0;
+   |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: try: `u32::try_from(value).is_ok()`
 
 error: aborting due to 17 previous errors
 
diff --git a/tests/ui/incompatible_msrv.rs b/tests/ui/incompatible_msrv.rs
index 99101b2bb8f..37e55e9e18a 100644
--- a/tests/ui/incompatible_msrv.rs
+++ b/tests/ui/incompatible_msrv.rs
@@ -1,6 +1,7 @@
 #![warn(clippy::incompatible_msrv)]
 #![feature(custom_inner_attributes)]
-#![feature(panic_internals)]
+#![allow(stable_features)]
+#![feature(strict_provenance)] // For use in test
 #![clippy::msrv = "1.3.0"]
 
 use std::collections::HashMap;
@@ -13,6 +14,8 @@ fn foo() {
     let mut map: HashMap<&str, u32> = HashMap::new();
     assert_eq!(map.entry("poneyland").key(), &"poneyland");
     //~^ incompatible_msrv
+    //~| NOTE: `-D clippy::incompatible-msrv` implied by `-D warnings`
+    //~| HELP: to override `-D warnings` add `#[allow(clippy::incompatible_msrv)]`
 
     if let Entry::Vacant(v) = map.entry("poneyland") {
         v.into_key();
@@ -43,21 +46,22 @@ fn core_special_treatment(p: bool) {
 
     // But still lint code calling `core` functions directly
     if p {
-        core::panicking::panic("foo");
-        //~^ ERROR: is `1.3.0` but this item is stable since `1.6.0`
+        let _ = core::iter::once_with(|| 0);
+        //~^ incompatible_msrv
     }
 
     // Lint code calling `core` from non-`core` macros
     macro_rules! my_panic {
         ($msg:expr) => {
-            core::panicking::panic($msg)
-        }; //~^ ERROR: is `1.3.0` but this item is stable since `1.6.0`
+            let _ = core::iter::once_with(|| $msg);
+            //~^ incompatible_msrv
+        };
     }
     my_panic!("foo");
 
     // Lint even when the macro comes from `core` and calls `core` functions
-    assert!(core::panicking::panic("out of luck"));
-    //~^ ERROR: is `1.3.0` but this item is stable since `1.6.0`
+    assert!(core::iter::once_with(|| 0).next().is_some());
+    //~^ incompatible_msrv
 }
 
 #[clippy::msrv = "1.26.0"]
@@ -70,7 +74,40 @@ fn lang_items() {
 #[clippy::msrv = "1.80.0"]
 fn issue14212() {
     let _ = std::iter::repeat_n((), 5);
-    //~^ ERROR: is `1.80.0` but this item is stable since `1.82.0`
+    //~^ incompatible_msrv
+}
+
+fn local_msrv_change_suggestion() {
+    let _ = std::iter::repeat_n((), 5);
+    //~^ incompatible_msrv
+
+    #[cfg(any(test, not(test)))]
+    {
+        let _ = std::iter::repeat_n((), 5);
+        //~^ incompatible_msrv
+        //~| NOTE: you may want to conditionally increase the MSRV
+
+        // Emit the additional note only once
+        let _ = std::iter::repeat_n((), 5);
+        //~^ incompatible_msrv
+    }
+}
+
+#[clippy::msrv = "1.78.0"]
+fn feature_enable_14425(ptr: *const u8) -> usize {
+    // Do not warn, because it is enabled through a feature even though
+    // it is stabilized only since Rust 1.84.0.
+    let r = ptr.addr();
+
+    // Warn about this which has been introduced in the same Rust version
+    // but is not allowed through a feature.
+    r.isqrt()
+    //~^ incompatible_msrv
+}
+
+fn non_fn_items() {
+    let _ = std::io::ErrorKind::CrossesDevices;
+    //~^ incompatible_msrv
 }
 
 fn main() {}
diff --git a/tests/ui/incompatible_msrv.stderr b/tests/ui/incompatible_msrv.stderr
index 5ea2bb9cc58..ed5ad5e1660 100644
--- a/tests/ui/incompatible_msrv.stderr
+++ b/tests/ui/incompatible_msrv.stderr
@@ -1,5 +1,5 @@
 error: current MSRV (Minimum Supported Rust Version) is `1.3.0` but this item is stable since `1.10.0`
-  --> tests/ui/incompatible_msrv.rs:14:39
+  --> tests/ui/incompatible_msrv.rs:15:39
    |
 LL |     assert_eq!(map.entry("poneyland").key(), &"poneyland");
    |                                       ^^^^^
@@ -8,45 +8,77 @@ LL |     assert_eq!(map.entry("poneyland").key(), &"poneyland");
    = help: to override `-D warnings` add `#[allow(clippy::incompatible_msrv)]`
 
 error: current MSRV (Minimum Supported Rust Version) is `1.3.0` but this item is stable since `1.12.0`
-  --> tests/ui/incompatible_msrv.rs:18:11
+  --> tests/ui/incompatible_msrv.rs:21:11
    |
 LL |         v.into_key();
    |           ^^^^^^^^^^
 
 error: current MSRV (Minimum Supported Rust Version) is `1.3.0` but this item is stable since `1.4.0`
-  --> tests/ui/incompatible_msrv.rs:22:5
+  --> tests/ui/incompatible_msrv.rs:25:5
    |
 LL |     sleep(Duration::new(1, 0));
    |     ^^^^^
 
-error: current MSRV (Minimum Supported Rust Version) is `1.3.0` but this item is stable since `1.6.0`
-  --> tests/ui/incompatible_msrv.rs:46:9
+error: current MSRV (Minimum Supported Rust Version) is `1.3.0` but this item is stable since `1.43.0`
+  --> tests/ui/incompatible_msrv.rs:49:17
    |
-LL |         core::panicking::panic("foo");
-   |         ^^^^^^^^^^^^^^^^^^^^^^
+LL |         let _ = core::iter::once_with(|| 0);
+   |                 ^^^^^^^^^^^^^^^^^^^^^
 
-error: current MSRV (Minimum Supported Rust Version) is `1.3.0` but this item is stable since `1.6.0`
-  --> tests/ui/incompatible_msrv.rs:53:13
+error: current MSRV (Minimum Supported Rust Version) is `1.3.0` but this item is stable since `1.43.0`
+  --> tests/ui/incompatible_msrv.rs:56:21
    |
-LL |             core::panicking::panic($msg)
-   |             ^^^^^^^^^^^^^^^^^^^^^^
+LL |             let _ = core::iter::once_with(|| $msg);
+   |                     ^^^^^^^^^^^^^^^^^^^^^
 ...
 LL |     my_panic!("foo");
    |     ---------------- in this macro invocation
    |
    = note: this error originates in the macro `my_panic` (in Nightly builds, run with -Z macro-backtrace for more info)
 
-error: current MSRV (Minimum Supported Rust Version) is `1.3.0` but this item is stable since `1.6.0`
-  --> tests/ui/incompatible_msrv.rs:59:13
+error: current MSRV (Minimum Supported Rust Version) is `1.3.0` but this item is stable since `1.43.0`
+  --> tests/ui/incompatible_msrv.rs:63:13
    |
-LL |     assert!(core::panicking::panic("out of luck"));
-   |             ^^^^^^^^^^^^^^^^^^^^^^
+LL |     assert!(core::iter::once_with(|| 0).next().is_some());
+   |             ^^^^^^^^^^^^^^^^^^^^^
 
 error: current MSRV (Minimum Supported Rust Version) is `1.80.0` but this item is stable since `1.82.0`
-  --> tests/ui/incompatible_msrv.rs:72:13
+  --> tests/ui/incompatible_msrv.rs:76:13
    |
 LL |     let _ = std::iter::repeat_n((), 5);
    |             ^^^^^^^^^^^^^^^^^^^
 
-error: aborting due to 7 previous errors
+error: current MSRV (Minimum Supported Rust Version) is `1.3.0` but this item is stable since `1.82.0`
+  --> tests/ui/incompatible_msrv.rs:81:13
+   |
+LL |     let _ = std::iter::repeat_n((), 5);
+   |             ^^^^^^^^^^^^^^^^^^^
+
+error: current MSRV (Minimum Supported Rust Version) is `1.3.0` but this item is stable since `1.82.0`
+  --> tests/ui/incompatible_msrv.rs:86:17
+   |
+LL |         let _ = std::iter::repeat_n((), 5);
+   |                 ^^^^^^^^^^^^^^^^^^^
+   |
+   = note: you may want to conditionally increase the MSRV considered by Clippy using the `clippy::msrv` attribute
+
+error: current MSRV (Minimum Supported Rust Version) is `1.3.0` but this item is stable since `1.82.0`
+  --> tests/ui/incompatible_msrv.rs:91:17
+   |
+LL |         let _ = std::iter::repeat_n((), 5);
+   |                 ^^^^^^^^^^^^^^^^^^^
+
+error: current MSRV (Minimum Supported Rust Version) is `1.78.0` but this item is stable since `1.84.0`
+  --> tests/ui/incompatible_msrv.rs:104:7
+   |
+LL |     r.isqrt()
+   |       ^^^^^^^
+
+error: current MSRV (Minimum Supported Rust Version) is `1.3.0` but this item is stable since `1.85.0`
+  --> tests/ui/incompatible_msrv.rs:109:13
+   |
+LL |     let _ = std::io::ErrorKind::CrossesDevices;
+   |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: aborting due to 12 previous errors