New lint `cast_enum_truncation`

10 files changed, 183 insertions, 52 deletions

diff --git a/CHANGELOG.md b/CHANGELOG.md
index f41dcce0e00..1b52a6fcd05 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -3068,6 +3068,7 @@ Released 2018-09-13
 [`bytes_nth`]: https://rust-lang.github.io/rust-clippy/master/index.html#bytes_nth
 [`cargo_common_metadata`]: https://rust-lang.github.io/rust-clippy/master/index.html#cargo_common_metadata
 [`case_sensitive_file_extension_comparisons`]: https://rust-lang.github.io/rust-clippy/master/index.html#case_sensitive_file_extension_comparisons
+[`cast_enum_truncation`]: https://rust-lang.github.io/rust-clippy/master/index.html#cast_enum_truncation
 [`cast_lossless`]: https://rust-lang.github.io/rust-clippy/master/index.html#cast_lossless
 [`cast_possible_truncation`]: https://rust-lang.github.io/rust-clippy/master/index.html#cast_possible_truncation
 [`cast_possible_wrap`]: https://rust-lang.github.io/rust-clippy/master/index.html#cast_possible_wrap
diff --git a/clippy_lints/src/casts/cast_possible_truncation.rs b/clippy_lints/src/casts/cast_possible_truncation.rs
index 2384e4a3748..6c938062c19 100644
--- a/clippy_lints/src/casts/cast_possible_truncation.rs
+++ b/clippy_lints/src/casts/cast_possible_truncation.rs
@@ -2,12 +2,14 @@ use clippy_utils::consts::{constant, Constant};
 use clippy_utils::diagnostics::span_lint;
 use clippy_utils::expr_or_init;
 use clippy_utils::ty::is_isize_or_usize;
+use rustc_ast::ast;
+use rustc_attr::IntType;
 use rustc_hir::def::{DefKind, Res};
 use rustc_hir::{BinOpKind, Expr, ExprKind};
 use rustc_lint::LateContext;
-use rustc_middle::ty::{self, FloatTy, Ty};
+use rustc_middle::ty::{self, FloatTy, Ty, VariantDiscr};
 
-use super::{utils, CAST_POSSIBLE_TRUNCATION};
+use super::{utils, CAST_ENUM_TRUNCATION, CAST_POSSIBLE_TRUNCATION};
 
 fn constant_int(cx: &LateContext<'_>, expr: &Expr<'_>) -> Option<u128> {
     if let Some((Constant::Int(c), _)) = constant(cx, cx.typeck_results(), expr) {
@@ -110,27 +112,54 @@ pub(super) fn check(cx: &LateContext<'_>, expr: &Expr<'_>, cast_expr: &Expr<'_>,
         },
 
         (ty::Adt(def, _), true) if def.is_enum() => {
-            if let ExprKind::Path(p) = &cast_expr.kind
-                && let Res::Def(DefKind::Ctor(..), _) = cx.qpath_res(p, cast_expr.hir_id)
+            let (from_nbits, variant) = if let ExprKind::Path(p) = &cast_expr.kind
+                && let Res::Def(DefKind::Ctor(..), id) = cx.qpath_res(p, cast_expr.hir_id)
             {
-                return
-            }
-
-            let from_nbits = utils::enum_ty_to_nbits(def, cx.tcx);
+                let i = def.variant_index_with_ctor_id(id);
+                let variant = &def.variants[i];
+                let nbits: u64 = match variant.discr {
+                    VariantDiscr::Explicit(id) => utils::read_explicit_enum_value(cx.tcx, id).unwrap().nbits(),
+                    VariantDiscr::Relative(x) => {
+                        match def.variants[(i.as_usize() - x as usize).into()].discr {
+                            VariantDiscr::Explicit(id) => {
+                                utils::read_explicit_enum_value(cx.tcx, id).unwrap().add(x).nbits()
+                            }
+                            VariantDiscr::Relative(_) => (32 - x.leading_zeros()).into(),
+                        }
+                    }
+                };
+                (nbits, Some(variant))
+            } else {
+                (utils::enum_ty_to_nbits(def, cx.tcx), None)
+            };
             let to_nbits = utils::int_ty_to_nbits(cast_to, cx.tcx);
 
-            let suffix = if is_isize_or_usize(cast_to) {
-                if from_nbits > 32 {
-                    " on targets with 32-bit wide pointers"
-                } else {
-                    return;
-                }
-            } else if to_nbits < from_nbits {
-                ""
-            } else {
-                return;
+            let cast_from_ptr_size = def.repr.int.map_or(true, |ty| {
+                matches!(
+                    ty,
+                    IntType::SignedInt(ast::IntTy::Isize) | IntType::UnsignedInt(ast::UintTy::Usize)
+                )
+            });
+            let suffix = match (cast_from_ptr_size, is_isize_or_usize(cast_to)) {
+                (false, false) if from_nbits > to_nbits => "",
+                (true, false) if from_nbits > to_nbits => "",
+                (false, true) if from_nbits > 64 => "",
+                (false, true) if from_nbits > 32 => " on targets with 32-bit wide pointers",
+                _ => return,
             };
 
+            if let Some(variant) = variant {
+                span_lint(
+                    cx,
+                    CAST_ENUM_TRUNCATION,
+                    expr.span,
+                    &format!(
+                        "casting `{}::{}` to `{}` will truncate the value{}",
+                        cast_from, variant.name, cast_to, suffix,
+                    ),
+                );
+                return;
+            }
             format!(
                 "casting `{}` to `{}` may truncate the value{}",
                 cast_from, cast_to, suffix,
diff --git a/clippy_lints/src/casts/mod.rs b/clippy_lints/src/casts/mod.rs
index b8c9f7db994..f2077c569c0 100644
--- a/clippy_lints/src/casts/mod.rs
+++ b/clippy_lints/src/casts/mod.rs
@@ -390,6 +390,25 @@ declare_clippy_lint! {
     "casting using `as` from and to raw pointers that doesn't change its mutability, where `pointer::cast` could take the place of `as`"
 }
 
+declare_clippy_lint! {
+    /// ### What it does
+    /// Checks for casts from an enum type to an integral type which will definitely truncate the
+    /// value.
+    ///
+    /// ### Why is this bad?
+    /// The resulting integral value will not match the value of the variant it came from.
+    ///
+    /// ### Example
+    /// ```rust
+    /// enum E { X = 256 };
+    /// let _ = E::X as u8;
+    /// ```
+    #[clippy::version = "1.60.0"]
+    pub CAST_ENUM_TRUNCATION,
+    suspicious,
+    "casts from an enum type to an integral type which will truncate the value"
+}
+
 pub struct Casts {
     msrv: Option<RustcVersion>,
 }
@@ -415,6 +434,7 @@ impl_lint_pass!(Casts => [
     FN_TO_NUMERIC_CAST_WITH_TRUNCATION,
     CHAR_LIT_AS_U8,
     PTR_AS_PTR,
+    CAST_ENUM_TRUNCATION,
 ]);
 
 impl<'tcx> LateLintPass<'tcx> for Casts {
diff --git a/clippy_lints/src/casts/utils.rs b/clippy_lints/src/casts/utils.rs
index d41e745d733..fab0c194b37 100644
--- a/clippy_lints/src/casts/utils.rs
+++ b/clippy_lints/src/casts/utils.rs
@@ -1,5 +1,6 @@
 use rustc_middle::mir::interpret::{ConstValue, Scalar};
 use rustc_middle::ty::{self, AdtDef, IntTy, Ty, TyCtxt, UintTy, VariantDiscr};
+use rustc_span::def_id::DefId;
 use rustc_target::abi::Size;
 
 /// Returns the size in bits of an integral type.
@@ -26,48 +27,83 @@ pub(super) fn int_ty_to_nbits(typ: Ty<'_>, tcx: TyCtxt<'_>) -> u64 {
     }
 }
 
+pub(super) enum EnumValue {
+    Unsigned(u128),
+    Signed(i128),
+}
+impl EnumValue {
+    pub(super) fn add(self, n: u32) -> Self {
+        match self {
+            Self::Unsigned(x) => Self::Unsigned(x + u128::from(n)),
+            Self::Signed(x) => Self::Signed(x + i128::from(n)),
+        }
+    }
+
+    pub(super) fn nbits(self) -> u64 {
+        match self {
+            Self::Unsigned(x) => 128 - x.leading_zeros(),
+            Self::Signed(x) if x < 0 => 128 - (-(x + 1)).leading_zeros() + 1,
+            Self::Signed(x) => 128 - x.leading_zeros(),
+        }
+        .into()
+    }
+}
+
+#[allow(clippy::cast_possible_truncation, clippy::cast_possible_wrap)]
+pub(super) fn read_explicit_enum_value(tcx: TyCtxt<'_>, id: DefId) -> Option<EnumValue> {
+    if let Ok(ConstValue::Scalar(Scalar::Int(value))) = tcx.const_eval_poly(id) {
+        match tcx.type_of(id).kind() {
+            ty::Int(_) => Some(EnumValue::Signed(match value.size().bytes() {
+                1 => i128::from(value.assert_bits(Size::from_bytes(1)) as u8 as i8),
+                2 => i128::from(value.assert_bits(Size::from_bytes(2)) as u16 as i16),
+                4 => i128::from(value.assert_bits(Size::from_bytes(4)) as u32 as i32),
+                8 => i128::from(value.assert_bits(Size::from_bytes(8)) as u64 as i64),
+                16 => value.assert_bits(Size::from_bytes(16)) as i128,
+                _ => return None,
+            })),
+            ty::Uint(_) => Some(EnumValue::Unsigned(match value.size().bytes() {
+                1 => value.assert_bits(Size::from_bytes(1)),
+                2 => value.assert_bits(Size::from_bytes(2)),
+                4 => value.assert_bits(Size::from_bytes(4)),
+                8 => value.assert_bits(Size::from_bytes(8)),
+                16 => value.assert_bits(Size::from_bytes(16)),
+                _ => return None,
+            })),
+            _ => None,
+        }
+    } else {
+        None
+    }
+}
+
 pub(super) fn enum_ty_to_nbits(adt: &AdtDef, tcx: TyCtxt<'_>) -> u64 {
     let mut explicit = 0i128;
     let (start, end) = adt
         .variants
         .iter()
-        .fold((i128::MAX, i128::MIN), |(start, end), variant| match variant.discr {
+        .fold((0, i128::MIN), |(start, end), variant| match variant.discr {
             VariantDiscr::Relative(x) => match explicit.checked_add(i128::from(x)) {
                 Some(x) => (start, end.max(x)),
                 None => (i128::MIN, end),
             },
-            VariantDiscr::Explicit(id) => {
-                let ty = tcx.type_of(id);
-                if let Ok(ConstValue::Scalar(Scalar::Int(value))) = tcx.const_eval_poly(id) {
-                    #[allow(clippy::cast_possible_truncation, clippy::cast_possible_wrap)]
-                    let value = match (value.size().bytes(), ty.kind()) {
-                        (1, ty::Int(_)) => i128::from(value.assert_bits(Size::from_bytes(1)) as u8 as i8),
-                        (1, ty::Uint(_)) => i128::from(value.assert_bits(Size::from_bytes(1)) as u8),
-                        (2, ty::Int(_)) => i128::from(value.assert_bits(Size::from_bytes(2)) as u16 as i16),
-                        (2, ty::Uint(_)) => i128::from(value.assert_bits(Size::from_bytes(2)) as u16),
-                        (4, ty::Int(_)) => i128::from(value.assert_bits(Size::from_bytes(4)) as u32 as i32),
-                        (4, ty::Uint(_)) => i128::from(value.assert_bits(Size::from_bytes(4)) as u32),
-                        (8, ty::Int(_)) => i128::from(value.assert_bits(Size::from_bytes(8)) as u64 as i64),
-                        (8, ty::Uint(_)) => i128::from(value.assert_bits(Size::from_bytes(8)) as u64),
-                        (16, ty::Int(_)) => value.assert_bits(Size::from_bytes(16)) as i128,
-                        (16, ty::Uint(_)) => match i128::try_from(value.assert_bits(Size::from_bytes(16))) {
-                            Ok(x) => x,
-                            // Requires 128 bits
-                            Err(_) => return (i128::MIN, end),
-                        },
-                        // Shouldn't happen if compilation was successful
-                        _ => return (start, end),
-                    };
-                    explicit = value;
-                    (start.min(value), end.max(value))
-                } else {
-                    // Shouldn't happen if compilation was successful
-                    (start, end)
-                }
+            VariantDiscr::Explicit(id) => match read_explicit_enum_value(tcx, id) {
+                Some(EnumValue::Signed(x)) => {
+                    explicit = x;
+                    (start.min(x), end.max(x))
+                },
+                Some(EnumValue::Unsigned(x)) => match i128::try_from(x) {
+                    Ok(x) => {
+                        explicit = x;
+                        (start, end.max(x))
+                    },
+                    Err(_) => (i128::MIN, end),
+                },
+                None => (start, end),
             },
         });
 
-    if start >= end {
+    if start > end {
+        // No variants.
         0
     } else {
         let neg_bits = if start < 0 {
diff --git a/clippy_lints/src/lib.register_all.rs b/clippy_lints/src/lib.register_all.rs
index a94f6b528b4..c6f8470cd7d 100644
--- a/clippy_lints/src/lib.register_all.rs
+++ b/clippy_lints/src/lib.register_all.rs
@@ -23,6 +23,7 @@ store.register_group(true, "clippy::all", Some("clippy_all"), vec![
     LintId::of(bool_assert_comparison::BOOL_ASSERT_COMPARISON),
     LintId::of(booleans::LOGIC_BUG),
     LintId::of(booleans::NONMINIMAL_BOOL),
+    LintId::of(casts::CAST_ENUM_TRUNCATION),
     LintId::of(casts::CAST_REF_TO_MUT),
     LintId::of(casts::CHAR_LIT_AS_U8),
     LintId::of(casts::FN_TO_NUMERIC_CAST),
diff --git a/clippy_lints/src/lib.register_lints.rs b/clippy_lints/src/lib.register_lints.rs
index df40ca46314..75ef1b0a9d5 100644
--- a/clippy_lints/src/lib.register_lints.rs
+++ b/clippy_lints/src/lib.register_lints.rs
@@ -67,6 +67,7 @@ store.register_lints(&[
     cargo::REDUNDANT_FEATURE_NAMES,
     cargo::WILDCARD_DEPENDENCIES,
     case_sensitive_file_extension_comparisons::CASE_SENSITIVE_FILE_EXTENSION_COMPARISONS,
+    casts::CAST_ENUM_TRUNCATION,
     casts::CAST_LOSSLESS,
     casts::CAST_POSSIBLE_TRUNCATION,
     casts::CAST_POSSIBLE_WRAP,
diff --git a/clippy_lints/src/lib.register_suspicious.rs b/clippy_lints/src/lib.register_suspicious.rs
index da56f800804..6a8859e19d7 100644
--- a/clippy_lints/src/lib.register_suspicious.rs
+++ b/clippy_lints/src/lib.register_suspicious.rs
@@ -7,6 +7,7 @@ store.register_group(true, "clippy::suspicious", Some("clippy_suspicious"), vec!
     LintId::of(attrs::BLANKET_CLIPPY_RESTRICTION_LINTS),
     LintId::of(await_holding_invalid::AWAIT_HOLDING_LOCK),
     LintId::of(await_holding_invalid::AWAIT_HOLDING_REFCELL_REF),
+    LintId::of(casts::CAST_ENUM_TRUNCATION),
     LintId::of(eval_order_dependence::EVAL_ORDER_DEPENDENCE),
     LintId::of(float_equality_without_abs::FLOAT_EQUALITY_WITHOUT_ABS),
     LintId::of(formatting::SUSPICIOUS_ASSIGNMENT_FORMATTING),
diff --git a/clippy_lints/src/lib.rs b/clippy_lints/src/lib.rs
index 93b1d178ac2..a21a87899aa 100644
--- a/clippy_lints/src/lib.rs
+++ b/clippy_lints/src/lib.rs
@@ -25,6 +25,7 @@
 // (Currently there is no way to opt into sysroot crates without `extern crate`.)
 extern crate rustc_ast;
 extern crate rustc_ast_pretty;
+extern crate rustc_attr;
 extern crate rustc_data_structures;
 extern crate rustc_driver;
 extern crate rustc_errors;
diff --git a/tests/ui/cast.rs b/tests/ui/cast.rs
index 371bf292f74..2e31ad3172e 100644
--- a/tests/ui/cast.rs
+++ b/tests/ui/cast.rs
@@ -139,7 +139,9 @@ fn main() {
     impl E2 {
         fn test(self) {
             let _ = self as u8;
+            let _ = Self::B as u8;
             let _ = self as i16; // Don't lint. `255..=256` fits in i16
+            let _ = Self::A as u8; // Don't lint.
         }
     }
 
@@ -174,7 +176,9 @@ fn main() {
     impl E5 {
         fn test(self) {
             let _ = self as i8;
+            let _ = Self::A as i8;
             let _ = self as i16; // Don't lint. `-129..=127` fits in i16
+            let _ = Self::B as u8; // Don't lint.
         }
     }
 
@@ -189,6 +193,7 @@ fn main() {
             let _ = self as i16;
             let _ = Self::A as u16; // Don't lint. `2^16-1` fits in u16
             let _ = self as u32; // Don't lint. `2^16-1..=2^16` fits in u32
+            let _ = Self::A as u16; // Don't lint.
         }
     }
 
@@ -201,6 +206,7 @@ fn main() {
     impl E7 {
         fn test(self) {
             let _ = self as usize;
+            let _ = Self::A as usize; // Don't lint.
             let _ = self as u64; // Don't lint. `2^32-1..=2^32` fits in u64
         }
     }
@@ -227,7 +233,22 @@ fn main() {
     }
     impl E9 {
         fn test(self) {
+            let _ = Self::A as u8; // Don't lint.
             let _ = self as u128; // Don't lint. `0..=2^128-1` fits in u128
         }
     }
+
+    #[derive(Clone, Copy)]
+    #[repr(usize)]
+    enum E10 {
+        A,
+        B = u32::MAX as usize,
+    }
+    impl E10 {
+        fn test(self) {
+            let _ = self as u16;
+            let _ = Self::B as u32; // Don't lint.
+            let _ = self as u64; // Don't lint.
+        }
+    }
 }
diff --git a/tests/ui/cast.stderr b/tests/ui/cast.stderr
index 0fc66f73981..7a68c0984f1 100644
--- a/tests/ui/cast.stderr
+++ b/tests/ui/cast.stderr
@@ -156,23 +156,43 @@ error: casting `main::E2` to `u8` may truncate the value
 LL |             let _ = self as u8;
    |                     ^^^^^^^^^^
 
+error: casting `main::E2::B` to `u8` will truncate the value
+  --> $DIR/cast.rs:142:21
+   |
+LL |             let _ = Self::B as u8;
+   |                     ^^^^^^^^^^^^^
+   |
+   = note: `-D clippy::cast-enum-truncation` implied by `-D warnings`
+
 error: casting `main::E5` to `i8` may truncate the value
-  --> $DIR/cast.rs:176:21
+  --> $DIR/cast.rs:178:21
    |
 LL |             let _ = self as i8;
    |                     ^^^^^^^^^^
 
+error: casting `main::E5::A` to `i8` will truncate the value
+  --> $DIR/cast.rs:179:21
+   |
+LL |             let _ = Self::A as i8;
+   |                     ^^^^^^^^^^^^^
+
 error: casting `main::E6` to `i16` may truncate the value
-  --> $DIR/cast.rs:189:21
+  --> $DIR/cast.rs:193:21
    |
 LL |             let _ = self as i16;
    |                     ^^^^^^^^^^^
 
 error: casting `main::E7` to `usize` may truncate the value on targets with 32-bit wide pointers
-  --> $DIR/cast.rs:203:21
+  --> $DIR/cast.rs:208:21
    |
 LL |             let _ = self as usize;
    |                     ^^^^^^^^^^^^^
 
-error: aborting due to 28 previous errors
+error: casting `main::E10` to `u16` may truncate the value
+  --> $DIR/cast.rs:249:21
+   |
+LL |             let _ = self as u16;
+   |                     ^^^^^^^^^^^
+
+error: aborting due to 31 previous errors