Auto merge of #3700 - phansch:would_you_like_some_help_with_this_const_fn, r=oli-obk

Prevent incorrect cast_lossless suggestion in const_fn

`::from` is not a const fn, so applying the suggestion of
`cast_lossless` would fail to compile. The fix is to skip the lint if
the cast is found inside a const fn.

Fixes #3656

clippy_lints/src/utils/mod.rs

@@ -82,6 +82,10 @@ pub fn in_constant(cx: &LateContext<'_, '_>, id: NodeId) -> bool {
             node: ItemKind::Static(..),
             ..
         }) => true,
+        Node::Item(&Item {
+            node: ItemKind::Fn(_, header, ..),
+            ..
+        }) => header.constness == Constness::Const,
         _ => false,
     }
 }

tests/ui/cast_lossless_float.fixed

@@ -1,7 +1,8 @@
 // run-rustfix
 
-#[warn(clippy::cast_lossless)]
-#[allow(clippy::no_effect, clippy::unnecessary_operation)]
+#![allow(clippy::no_effect, clippy::unnecessary_operation, dead_code)]
+#![warn(clippy::cast_lossless)]
+
 fn main() {
     // Test clippy::cast_lossless with casts to floating-point types
     f32::from(1i8);
@@ -15,3 +16,10 @@ fn main() {
     f64::from(1i32);
     f64::from(1u32);
 }
+
+// The lint would suggest using `f64::from(input)` here but the `XX::from` function is not const,
+// so we skip the lint if the expression is in a const fn.
+// See #3656
+const fn abc(input: f32) -> f64 {
+    input as f64
+}

tests/ui/cast_lossless_float.rs

@@ -1,7 +1,8 @@
 // run-rustfix
 
-#[warn(clippy::cast_lossless)]
-#[allow(clippy::no_effect, clippy::unnecessary_operation)]
+#![allow(clippy::no_effect, clippy::unnecessary_operation, dead_code)]
+#![warn(clippy::cast_lossless)]
+
 fn main() {
     // Test clippy::cast_lossless with casts to floating-point types
     1i8 as f32;
@@ -15,3 +16,10 @@ fn main() {
     1i32 as f64;
     1u32 as f64;
 }
+
+// The lint would suggest using `f64::from(input)` here but the `XX::from` function is not const,
+// so we skip the lint if the expression is in a const fn.
+// See #3656
+const fn abc(input: f32) -> f64 {
+    input as f64
+}

tests/ui/cast_lossless_float.stderr

@@ -1,5 +1,5 @@
 error: casting i8 to f32 may become silently lossy if types change
-  --> $DIR/cast_lossless_float.rs:7:5
+  --> $DIR/cast_lossless_float.rs:8:5
    |
 LL |     1i8 as f32;
    |     ^^^^^^^^^^ help: try: `f32::from(1i8)`
@@ -7,55 +7,55 @@ LL |     1i8 as f32;
    = note: `-D clippy::cast-lossless` implied by `-D warnings`
 
 error: casting i8 to f64 may become silently lossy if types change
-  --> $DIR/cast_lossless_float.rs:8:5
+  --> $DIR/cast_lossless_float.rs:9:5
    |
 LL |     1i8 as f64;
    |     ^^^^^^^^^^ help: try: `f64::from(1i8)`
 
 error: casting u8 to f32 may become silently lossy if types change
-  --> $DIR/cast_lossless_float.rs:9:5
+  --> $DIR/cast_lossless_float.rs:10:5
    |
 LL |     1u8 as f32;
    |     ^^^^^^^^^^ help: try: `f32::from(1u8)`
 
 error: casting u8 to f64 may become silently lossy if types change
-  --> $DIR/cast_lossless_float.rs:10:5
+  --> $DIR/cast_lossless_float.rs:11:5
    |
 LL |     1u8 as f64;
    |     ^^^^^^^^^^ help: try: `f64::from(1u8)`
 
 error: casting i16 to f32 may become silently lossy if types change
-  --> $DIR/cast_lossless_float.rs:11:5
+  --> $DIR/cast_lossless_float.rs:12:5
    |
 LL |     1i16 as f32;
    |     ^^^^^^^^^^^ help: try: `f32::from(1i16)`
 
 error: casting i16 to f64 may become silently lossy if types change
-  --> $DIR/cast_lossless_float.rs:12:5
+  --> $DIR/cast_lossless_float.rs:13:5
    |
 LL |     1i16 as f64;
    |     ^^^^^^^^^^^ help: try: `f64::from(1i16)`
 
 error: casting u16 to f32 may become silently lossy if types change
-  --> $DIR/cast_lossless_float.rs:13:5
+  --> $DIR/cast_lossless_float.rs:14:5
    |
 LL |     1u16 as f32;
    |     ^^^^^^^^^^^ help: try: `f32::from(1u16)`
 
 error: casting u16 to f64 may become silently lossy if types change
-  --> $DIR/cast_lossless_float.rs:14:5
+  --> $DIR/cast_lossless_float.rs:15:5
    |
 LL |     1u16 as f64;
    |     ^^^^^^^^^^^ help: try: `f64::from(1u16)`
 
 error: casting i32 to f64 may become silently lossy if types change
-  --> $DIR/cast_lossless_float.rs:15:5
+  --> $DIR/cast_lossless_float.rs:16:5
    |
 LL |     1i32 as f64;
    |     ^^^^^^^^^^^ help: try: `f64::from(1i32)`
 
 error: casting u32 to f64 may become silently lossy if types change
-  --> $DIR/cast_lossless_float.rs:16:5
+  --> $DIR/cast_lossless_float.rs:17:5
    |
 LL |     1u32 as f64;
    |     ^^^^^^^^^^^ help: try: `f64::from(1u32)`

tests/ui/cast_lossless_integer.fixed

@@ -1,7 +1,8 @@
 // run-rustfix
 
-#[warn(clippy::cast_lossless)]
-#[allow(clippy::no_effect, clippy::unnecessary_operation)]
+#![allow(clippy::no_effect, clippy::unnecessary_operation, dead_code)]
+#![warn(clippy::cast_lossless)]
+
 fn main() {
     // Test clippy::cast_lossless with casts to integer types
     i16::from(1i8);
@@ -23,3 +24,10 @@ fn main() {
     i64::from(1u32);
     u64::from(1u32);
 }
+
+// The lint would suggest using `f64::from(input)` here but the `XX::from` function is not const,
+// so we skip the lint if the expression is in a const fn.
+// See #3656
+const fn abc(input: u16) -> u32 {
+    input as u32
+}

tests/ui/cast_lossless_integer.rs

@@ -1,7 +1,8 @@
 // run-rustfix
 
-#[warn(clippy::cast_lossless)]
-#[allow(clippy::no_effect, clippy::unnecessary_operation)]
+#![allow(clippy::no_effect, clippy::unnecessary_operation, dead_code)]
+#![warn(clippy::cast_lossless)]
+
 fn main() {
     // Test clippy::cast_lossless with casts to integer types
     1i8 as i16;
@@ -23,3 +24,10 @@ fn main() {
     1u32 as i64;
     1u32 as u64;
 }
+
+// The lint would suggest using `f64::from(input)` here but the `XX::from` function is not const,
+// so we skip the lint if the expression is in a const fn.
+// See #3656
+const fn abc(input: u16) -> u32 {
+    input as u32
+}

tests/ui/cast_lossless_integer.stderr

@@ -1,5 +1,5 @@
 error: casting i8 to i16 may become silently lossy if types change
-  --> $DIR/cast_lossless_integer.rs:7:5
+  --> $DIR/cast_lossless_integer.rs:8:5
    |
 LL |     1i8 as i16;
    |     ^^^^^^^^^^ help: try: `i16::from(1i8)`
@@ -7,103 +7,103 @@ LL |     1i8 as i16;
    = note: `-D clippy::cast-lossless` implied by `-D warnings`
 
 error: casting i8 to i32 may become silently lossy if types change
-  --> $DIR/cast_lossless_integer.rs:8:5
+  --> $DIR/cast_lossless_integer.rs:9:5
    |
 LL |     1i8 as i32;
    |     ^^^^^^^^^^ help: try: `i32::from(1i8)`
 
 error: casting i8 to i64 may become silently lossy if types change
-  --> $DIR/cast_lossless_integer.rs:9:5
+  --> $DIR/cast_lossless_integer.rs:10:5
    |
 LL |     1i8 as i64;
    |     ^^^^^^^^^^ help: try: `i64::from(1i8)`
 
 error: casting u8 to i16 may become silently lossy if types change
-  --> $DIR/cast_lossless_integer.rs:10:5
+  --> $DIR/cast_lossless_integer.rs:11:5
    |
 LL |     1u8 as i16;
    |     ^^^^^^^^^^ help: try: `i16::from(1u8)`
 
 error: casting u8 to i32 may become silently lossy if types change
-  --> $DIR/cast_lossless_integer.rs:11:5
+  --> $DIR/cast_lossless_integer.rs:12:5
    |
 LL |     1u8 as i32;
    |     ^^^^^^^^^^ help: try: `i32::from(1u8)`
 
 error: casting u8 to i64 may become silently lossy if types change
-  --> $DIR/cast_lossless_integer.rs:12:5
+  --> $DIR/cast_lossless_integer.rs:13:5
    |
 LL |     1u8 as i64;
    |     ^^^^^^^^^^ help: try: `i64::from(1u8)`
 
 error: casting u8 to u16 may become silently lossy if types change
-  --> $DIR/cast_lossless_integer.rs:13:5
+  --> $DIR/cast_lossless_integer.rs:14:5
    |
 LL |     1u8 as u16;
    |     ^^^^^^^^^^ help: try: `u16::from(1u8)`
 
 error: casting u8 to u32 may become silently lossy if types change
-  --> $DIR/cast_lossless_integer.rs:14:5
+  --> $DIR/cast_lossless_integer.rs:15:5
    |
 LL |     1u8 as u32;
    |     ^^^^^^^^^^ help: try: `u32::from(1u8)`
 
 error: casting u8 to u64 may become silently lossy if types change
-  --> $DIR/cast_lossless_integer.rs:15:5
+  --> $DIR/cast_lossless_integer.rs:16:5
    |
 LL |     1u8 as u64;
    |     ^^^^^^^^^^ help: try: `u64::from(1u8)`
 
 error: casting i16 to i32 may become silently lossy if types change
-  --> $DIR/cast_lossless_integer.rs:16:5
+  --> $DIR/cast_lossless_integer.rs:17:5
    |
 LL |     1i16 as i32;
    |     ^^^^^^^^^^^ help: try: `i32::from(1i16)`
 
 error: casting i16 to i64 may become silently lossy if types change
-  --> $DIR/cast_lossless_integer.rs:17:5
+  --> $DIR/cast_lossless_integer.rs:18:5
    |
 LL |     1i16 as i64;
    |     ^^^^^^^^^^^ help: try: `i64::from(1i16)`
 
 error: casting u16 to i32 may become silently lossy if types change
-  --> $DIR/cast_lossless_integer.rs:18:5
+  --> $DIR/cast_lossless_integer.rs:19:5
    |
 LL |     1u16 as i32;
    |     ^^^^^^^^^^^ help: try: `i32::from(1u16)`
 
 error: casting u16 to i64 may become silently lossy if types change
-  --> $DIR/cast_lossless_integer.rs:19:5
+  --> $DIR/cast_lossless_integer.rs:20:5
    |
 LL |     1u16 as i64;
    |     ^^^^^^^^^^^ help: try: `i64::from(1u16)`
 
 error: casting u16 to u32 may become silently lossy if types change
-  --> $DIR/cast_lossless_integer.rs:20:5
+  --> $DIR/cast_lossless_integer.rs:21:5
    |
 LL |     1u16 as u32;
    |     ^^^^^^^^^^^ help: try: `u32::from(1u16)`
 
 error: casting u16 to u64 may become silently lossy if types change
-  --> $DIR/cast_lossless_integer.rs:21:5
+  --> $DIR/cast_lossless_integer.rs:22:5
    |
 LL |     1u16 as u64;
    |     ^^^^^^^^^^^ help: try: `u64::from(1u16)`
 
 error: casting i32 to i64 may become silently lossy if types change
-  --> $DIR/cast_lossless_integer.rs:22:5
+  --> $DIR/cast_lossless_integer.rs:23:5
    |
 LL |     1i32 as i64;
    |     ^^^^^^^^^^^ help: try: `i64::from(1i32)`
 
 error: casting u32 to i64 may become silently lossy if types change
-  --> $DIR/cast_lossless_integer.rs:23:5
+  --> $DIR/cast_lossless_integer.rs:24:5
    |
 LL |     1u32 as i64;
    |     ^^^^^^^^^^^ help: try: `i64::from(1u32)`
 
 error: casting u32 to u64 may become silently lossy if types change
-  --> $DIR/cast_lossless_integer.rs:24:5
+  --> $DIR/cast_lossless_integer.rs:25:5
    |
 LL |     1u32 as u64;
    |     ^^^^^^^^^^^ help: try: `u64::from(1u32)`