Auto merge of #6873 - Y-Nak:refactor-casts-lint, r=flip1995

Refactor casts lint

Ref: #6724

Changes:
1. Separate the `casts` group from the `types` group.
2. Reorganize the lints of the `casts` group into their own modules.

Notes:
1. I didn't `fix` #6874 in order to maintain this PR as small as possible.

---
changelog: none

Author: bors

clippy_lints/src/casts/cast_lossless.rs

@@ -0,0 +1,87 @@
+use rustc_errors::Applicability;
+use rustc_hir::{Expr, ExprKind};
+use rustc_lint::LateContext;
+use rustc_middle::ty::{self, FloatTy, Ty};
+
+use crate::utils::{in_constant, is_isize_or_usize, snippet_opt, span_lint_and_sugg};
+
+use super::{utils, CAST_LOSSLESS};
+
+pub(super) fn check(cx: &LateContext<'_>, expr: &Expr<'_>, cast_op: &Expr<'_>, cast_from: Ty<'_>, cast_to: Ty<'_>) {
+    if !should_lint(cx, expr, cast_from, cast_to) {
+        return;
+    }
+
+    // The suggestion is to use a function call, so if the original expression
+    // has parens on the outside, they are no longer needed.
+    let mut applicability = Applicability::MachineApplicable;
+    let opt = snippet_opt(cx, cast_op.span);
+    let sugg = opt.as_ref().map_or_else(
+        || {
+            applicability = Applicability::HasPlaceholders;
+            ".."
+        },
+        |snip| {
+            if should_strip_parens(cast_op, snip) {
+                &snip[1..snip.len() - 1]
+            } else {
+                snip.as_str()
+            }
+        },
+    );
+
+    span_lint_and_sugg(
+        cx,
+        CAST_LOSSLESS,
+        expr.span,
+        &format!(
+            "casting `{}` to `{}` may become silently lossy if you later change the type",
+            cast_from, cast_to
+        ),
+        "try",
+        format!("{}::from({})", cast_to, sugg),
+        applicability,
+    );
+}
+
+fn should_lint(cx: &LateContext<'_>, expr: &Expr<'_>, cast_from: Ty<'_>, cast_to: Ty<'_>) -> bool {
+    // Do not suggest using From in consts/statics until it is valid to do so (see #2267).
+    if in_constant(cx, expr.hir_id) {
+        return false;
+    }
+
+    match (cast_from.is_integral(), cast_to.is_integral()) {
+        (true, true) => {
+            let cast_signed_to_unsigned = cast_from.is_signed() && !cast_to.is_signed();
+            let from_nbits = utils::int_ty_to_nbits(cast_from, cx.tcx);
+            let to_nbits = utils::int_ty_to_nbits(cast_to, cx.tcx);
+            !is_isize_or_usize(cast_from)
+                && !is_isize_or_usize(cast_to)
+                && from_nbits < to_nbits
+                && !cast_signed_to_unsigned
+        },
+
+        (true, false) => {
+            let from_nbits = utils::int_ty_to_nbits(cast_from, cx.tcx);
+            let to_nbits = if let ty::Float(FloatTy::F32) = cast_to.kind() {
+                32
+            } else {
+                64
+            };
+            from_nbits < to_nbits
+        },
+
+        (_, _) => {
+            matches!(cast_from.kind(), ty::Float(FloatTy::F32)) && matches!(cast_to.kind(), ty::Float(FloatTy::F64))
+        },
+    }
+}
+
+fn should_strip_parens(cast_expr: &Expr<'_>, snip: &str) -> bool {
+    if let ExprKind::Binary(_, _, _) = cast_expr.kind {
+        if snip.starts_with('(') && snip.ends_with(')') {
+            return true;
+        }
+    }
+    false
+}

clippy_lints/src/casts/cast_possible_truncation.rs

@@ -0,0 +1,54 @@
+use rustc_hir::Expr;
+use rustc_lint::LateContext;
+use rustc_middle::ty::{self, FloatTy, Ty};
+
+use crate::utils::{is_isize_or_usize, span_lint};
+
+use super::{utils, CAST_POSSIBLE_TRUNCATION};
+
+pub(super) fn check(cx: &LateContext<'_>, expr: &Expr<'_>, cast_from: Ty<'_>, cast_to: Ty<'_>) {
+    let msg = match (cast_from.is_integral(), cast_to.is_integral()) {
+        (true, true) => {
+            let from_nbits = utils::int_ty_to_nbits(cast_from, cx.tcx);
+            let to_nbits = utils::int_ty_to_nbits(cast_to, cx.tcx);
+
+            let (should_lint, suffix) = match (is_isize_or_usize(cast_from), is_isize_or_usize(cast_to)) {
+                (true, true) | (false, false) => (to_nbits < from_nbits, ""),
+                (true, false) => (
+                    to_nbits <= 32,
+                    if to_nbits == 32 {
+                        " on targets with 64-bit wide pointers"
+                    } else {
+                        ""
+                    },
+                ),
+                (false, true) => (from_nbits == 64, " on targets with 32-bit wide pointers"),
+            };
+
+            if !should_lint {
+                return;
+            }
+
+            format!(
+                "casting `{}` to `{}` may truncate the value{}",
+                cast_from, cast_to, suffix,
+            )
+        },
+
+        (false, true) => {
+            format!("casting `{}` to `{}` may truncate the value", cast_from, cast_to)
+        },
+
+        (_, _) => {
+            if matches!(cast_from.kind(), &ty::Float(FloatTy::F64))
+                && matches!(cast_to.kind(), &ty::Float(FloatTy::F32))
+            {
+                "casting `f64` to `f32` may truncate the value".to_string()
+            } else {
+                return;
+            }
+        },
+    };
+
+    span_lint(cx, CAST_POSSIBLE_TRUNCATION, expr.span, &msg);
+}

clippy_lints/src/casts/cast_possible_wrap.rs

@@ -0,0 +1,44 @@
+use rustc_hir::Expr;
+use rustc_lint::LateContext;
+use rustc_middle::ty::Ty;
+
+use crate::utils::{is_isize_or_usize, span_lint};
+
+use super::{utils, CAST_POSSIBLE_WRAP};
+
+pub(super) fn check(cx: &LateContext<'_>, expr: &Expr<'_>, cast_from: Ty<'_>, cast_to: Ty<'_>) {
+    if !(cast_from.is_integral() && cast_to.is_integral()) {
+        return;
+    }
+
+    let arch_64_suffix = " on targets with 64-bit wide pointers";
+    let arch_32_suffix = " on targets with 32-bit wide pointers";
+    let cast_unsigned_to_signed = !cast_from.is_signed() && cast_to.is_signed();
+    let from_nbits = utils::int_ty_to_nbits(cast_from, cx.tcx);
+    let to_nbits = utils::int_ty_to_nbits(cast_to, cx.tcx);
+
+    let (should_lint, suffix) = match (is_isize_or_usize(cast_from), is_isize_or_usize(cast_to)) {
+        (true, true) | (false, false) => (to_nbits == from_nbits && cast_unsigned_to_signed, ""),
+        (true, false) => (to_nbits <= 32 && cast_unsigned_to_signed, arch_32_suffix),
+        (false, true) => (
+            cast_unsigned_to_signed,
+            if from_nbits == 64 {
+                arch_64_suffix
+            } else {
+                arch_32_suffix
+            },
+        ),
+    };
+
+    if should_lint {
+        span_lint(
+            cx,
+            CAST_POSSIBLE_WRAP,
+            expr.span,
+            &format!(
+                "casting `{}` to `{}` may wrap around the value{}",
+                cast_from, cast_to, suffix,
+            ),
+        );
+    }
+}

clippy_lints/src/casts/cast_precision_loss.rs

@@ -0,0 +1,51 @@
+use rustc_hir::Expr;
+use rustc_lint::LateContext;
+use rustc_middle::ty::{self, FloatTy, Ty};
+
+use crate::utils::{is_isize_or_usize, span_lint};
+
+use super::{utils, CAST_PRECISION_LOSS};
+
+pub(super) fn check(cx: &LateContext<'_>, expr: &Expr<'_>, cast_from: Ty<'_>, cast_to: Ty<'_>) {
+    if !cast_from.is_integral() || cast_to.is_integral() {
+        return;
+    }
+
+    let from_nbits = utils::int_ty_to_nbits(cast_from, cx.tcx);
+    let to_nbits = if let ty::Float(FloatTy::F32) = cast_to.kind() {
+        32
+    } else {
+        64
+    };
+
+    if !(is_isize_or_usize(cast_from) || from_nbits >= to_nbits) {
+        return;
+    }
+
+    let cast_to_f64 = to_nbits == 64;
+    let mantissa_nbits = if cast_to_f64 { 52 } else { 23 };
+    let arch_dependent = is_isize_or_usize(cast_from) && cast_to_f64;
+    let arch_dependent_str = "on targets with 64-bit wide pointers ";
+    let from_nbits_str = if arch_dependent {
+        "64".to_owned()
+    } else if is_isize_or_usize(cast_from) {
+        "32 or 64".to_owned()
+    } else {
+        utils::int_ty_to_nbits(cast_from, cx.tcx).to_string()
+    };
+
+    span_lint(
+        cx,
+        CAST_PRECISION_LOSS,
+        expr.span,
+        &format!(
+            "casting `{0}` to `{1}` causes a loss of precision {2}(`{0}` is {3} bits wide, \
+             but `{1}`'s mantissa is only {4} bits wide)",
+            cast_from,
+            if cast_to_f64 { "f64" } else { "f32" },
+            if arch_dependent { arch_dependent_str } else { "" },
+            from_nbits_str,
+            mantissa_nbits
+        ),
+    );
+}

clippy_lints/src/casts/cast_ptr_alignment.rs

@@ -0,0 +1,81 @@
+use rustc_hir::{Expr, ExprKind, GenericArg};
+use rustc_lint::LateContext;
+use rustc_middle::ty::{self, Ty};
+use rustc_span::symbol::sym;
+use rustc_target::abi::LayoutOf;
+
+use if_chain::if_chain;
+
+use crate::utils::{is_hir_ty_cfg_dependant, span_lint};
+
+use super::CAST_PTR_ALIGNMENT;
+
+pub(super) fn check(cx: &LateContext<'tcx>, expr: &'tcx Expr<'_>) {
+    if let ExprKind::Cast(ref cast_expr, cast_to) = expr.kind {
+        if is_hir_ty_cfg_dependant(cx, cast_to) {
+            return;
+        }
+        let (cast_from, cast_to) = (
+            cx.typeck_results().expr_ty(cast_expr),
+            cx.typeck_results().expr_ty(expr),
+        );
+        lint_cast_ptr_alignment(cx, expr, cast_from, cast_to);
+    } else if let ExprKind::MethodCall(method_path, _, args, _) = expr.kind {
+        if_chain! {
+            if method_path.ident.name == sym!(cast);
+            if let Some(generic_args) = method_path.args;
+            if let [GenericArg::Type(cast_to)] = generic_args.args;
+            // There probably is no obvious reason to do this, just to be consistent with `as` cases.
+            if !is_hir_ty_cfg_dependant(cx, cast_to);
+            then {
+                let (cast_from, cast_to) =
+                    (cx.typeck_results().expr_ty(&args[0]), cx.typeck_results().expr_ty(expr));
+                lint_cast_ptr_alignment(cx, expr, cast_from, cast_to);
+            }
+        }
+    }
+}
+
+fn lint_cast_ptr_alignment<'tcx>(cx: &LateContext<'tcx>, expr: &Expr<'_>, cast_from: Ty<'tcx>, cast_to: Ty<'tcx>) {
+    if_chain! {
+        if let ty::RawPtr(from_ptr_ty) = &cast_from.kind();
+        if let ty::RawPtr(to_ptr_ty) = &cast_to.kind();
+        if let Ok(from_layout) = cx.layout_of(from_ptr_ty.ty);
+        if let Ok(to_layout) = cx.layout_of(to_ptr_ty.ty);
+        if from_layout.align.abi < to_layout.align.abi;
+        // with c_void, we inherently need to trust the user
+        if !is_c_void(cx, from_ptr_ty.ty);
+        // when casting from a ZST, we don't know enough to properly lint
+        if !from_layout.is_zst();
+        then {
+            span_lint(
+                cx,
+                CAST_PTR_ALIGNMENT,
+                expr.span,
+                &format!(
+                    "casting from `{}` to a more-strictly-aligned pointer (`{}`) ({} < {} bytes)",
+                    cast_from,
+                    cast_to,
+                    from_layout.align.abi.bytes(),
+                    to_layout.align.abi.bytes(),
+                ),
+            );
+        }
+    }
+}
+
+/// Check if the given type is either `core::ffi::c_void` or
+/// one of the platform specific `libc::<platform>::c_void` of libc.
+fn is_c_void(cx: &LateContext<'_>, ty: Ty<'_>) -> bool {
+    if let ty::Adt(adt, _) = ty.kind() {
+        let names = cx.get_def_path(adt.did);
+
+        if names.is_empty() {
+            return false;
+        }
+        if names[0] == sym::libc || names[0] == sym::core && *names.last().unwrap() == sym!(c_void) {
+            return true;
+        }
+    }
+    false
+}

clippy_lints/src/casts/cast_ref_to_mut.rs

@@ -0,0 +1,28 @@
+use rustc_hir::{Expr, ExprKind, MutTy, Mutability, TyKind, UnOp};
+use rustc_lint::LateContext;
+use rustc_middle::ty;
+
+use if_chain::if_chain;
+
+use crate::utils::span_lint;
+
+use super::CAST_REF_TO_MUT;
+
+pub(super) fn check(cx: &LateContext<'tcx>, expr: &'tcx Expr<'_>) {
+    if_chain! {
+        if let ExprKind::Unary(UnOp::Deref, e) = &expr.kind;
+        if let ExprKind::Cast(e, t) = &e.kind;
+        if let TyKind::Ptr(MutTy { mutbl: Mutability::Mut, .. }) = t.kind;
+        if let ExprKind::Cast(e, t) = &e.kind;
+        if let TyKind::Ptr(MutTy { mutbl: Mutability::Not, .. }) = t.kind;
+        if let ty::Ref(..) = cx.typeck_results().node_type(e.hir_id).kind();
+        then {
+            span_lint(
+                cx,
+                CAST_REF_TO_MUT,
+                expr.span,
+                "casting `&T` to `&mut T` may cause undefined behavior, consider instead using an `UnsafeCell`",
+            );
+        }
+    }
+}