add `manual_abs_diff` lint

10 files changed, 465 insertions, 1 deletions

diff --git a/CHANGELOG.md b/CHANGELOG.md
index 3e6f00e857f..44407e04ba3 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -5784,6 +5784,7 @@ Released 2018-09-13
 [`macro_metavars_in_unsafe`]: https://rust-lang.github.io/rust-clippy/master/index.html#macro_metavars_in_unsafe
 [`macro_use_imports`]: https://rust-lang.github.io/rust-clippy/master/index.html#macro_use_imports
 [`main_recursion`]: https://rust-lang.github.io/rust-clippy/master/index.html#main_recursion
+[`manual_abs_diff`]: https://rust-lang.github.io/rust-clippy/master/index.html#manual_abs_diff
 [`manual_assert`]: https://rust-lang.github.io/rust-clippy/master/index.html#manual_assert
 [`manual_async_fn`]: https://rust-lang.github.io/rust-clippy/master/index.html#manual_async_fn
 [`manual_bits`]: https://rust-lang.github.io/rust-clippy/master/index.html#manual_bits
diff --git a/book/src/lint_configuration.md b/book/src/lint_configuration.md
index a677745379c..93506b81025 100644
--- a/book/src/lint_configuration.md
+++ b/book/src/lint_configuration.md
@@ -797,6 +797,7 @@ The minimum rust version that the project supports. Defaults to the `rust-versio
 * [`iter_kv_map`](https://rust-lang.github.io/rust-clippy/master/index.html#iter_kv_map)
 * [`legacy_numeric_constants`](https://rust-lang.github.io/rust-clippy/master/index.html#legacy_numeric_constants)
 * [`lines_filter_map_ok`](https://rust-lang.github.io/rust-clippy/master/index.html#lines_filter_map_ok)
+* [`manual_abs_diff`](https://rust-lang.github.io/rust-clippy/master/index.html#manual_abs_diff)
 * [`manual_bits`](https://rust-lang.github.io/rust-clippy/master/index.html#manual_bits)
 * [`manual_c_str_literals`](https://rust-lang.github.io/rust-clippy/master/index.html#manual_c_str_literals)
 * [`manual_clamp`](https://rust-lang.github.io/rust-clippy/master/index.html#manual_clamp)
diff --git a/clippy_config/src/conf.rs b/clippy_config/src/conf.rs
index a5c0ff8978d..07c5dad5b54 100644
--- a/clippy_config/src/conf.rs
+++ b/clippy_config/src/conf.rs
@@ -645,6 +645,7 @@ define_Conf! {
         iter_kv_map,
         legacy_numeric_constants,
         lines_filter_map_ok,
+        manual_abs_diff,
         manual_bits,
         manual_c_str_literals,
         manual_clamp,
diff --git a/clippy_lints/src/declared_lints.rs b/clippy_lints/src/declared_lints.rs
index c2274f0a619..48212e6bf70 100644
--- a/clippy_lints/src/declared_lints.rs
+++ b/clippy_lints/src/declared_lints.rs
@@ -313,6 +313,7 @@ pub static LINTS: &[&crate::LintInfo] = &[
     crate::macro_metavars_in_unsafe::MACRO_METAVARS_IN_UNSAFE_INFO,
     crate::macro_use::MACRO_USE_IMPORTS_INFO,
     crate::main_recursion::MAIN_RECURSION_INFO,
+    crate::manual_abs_diff::MANUAL_ABS_DIFF_INFO,
     crate::manual_assert::MANUAL_ASSERT_INFO,
     crate::manual_async_fn::MANUAL_ASYNC_FN_INFO,
     crate::manual_bits::MANUAL_BITS_INFO,
diff --git a/clippy_lints/src/lib.rs b/clippy_lints/src/lib.rs
index ba80e122448..9e1df7881ce 100644
--- a/clippy_lints/src/lib.rs
+++ b/clippy_lints/src/lib.rs
@@ -205,6 +205,7 @@ mod loops;
 mod macro_metavars_in_unsafe;
 mod macro_use;
 mod main_recursion;
+mod manual_abs_diff;
 mod manual_assert;
 mod manual_async_fn;
 mod manual_bits;
@@ -878,6 +879,7 @@ pub fn register_lints(store: &mut rustc_lint::LintStore, conf: &'static Conf) {
     store.register_late_pass(move |_| Box::new(std_instead_of_core::StdReexports::new(conf)));
     store.register_late_pass(move |_| Box::new(instant_subtraction::InstantSubtraction::new(conf)));
     store.register_late_pass(|_| Box::new(partialeq_to_none::PartialeqToNone));
+    store.register_late_pass(move |_| Box::new(manual_abs_diff::ManualAbsDiff::new(conf)));
     store.register_late_pass(move |_| Box::new(manual_clamp::ManualClamp::new(conf)));
     store.register_late_pass(|_| Box::new(manual_string_new::ManualStringNew));
     store.register_late_pass(|_| Box::new(unused_peekable::UnusedPeekable));
diff --git a/clippy_lints/src/manual_abs_diff.rs b/clippy_lints/src/manual_abs_diff.rs
new file mode 100644
index 00000000000..c515e41f242
--- /dev/null
+++ b/clippy_lints/src/manual_abs_diff.rs
@@ -0,0 +1,152 @@
+use clippy_config::Conf;
+use clippy_utils::diagnostics::span_lint_and_then;
+use clippy_utils::higher::If;
+use clippy_utils::msrvs::{self, Msrv};
+use clippy_utils::source::HasSession as _;
+use clippy_utils::sugg::Sugg;
+use clippy_utils::ty::is_type_diagnostic_item;
+use clippy_utils::{eq_expr_value, peel_blocks, span_contains_comment};
+use rustc_errors::Applicability;
+use rustc_hir::{BinOpKind, Expr, ExprKind};
+use rustc_lint::{LateContext, LateLintPass};
+use rustc_middle::ty::{self, Ty};
+use rustc_session::impl_lint_pass;
+use rustc_span::sym;
+
+declare_clippy_lint! {
+    /// ### What it does
+    /// Detects patterns like `if a > b { a - b } else { b - a }` and suggests using `a.abs_diff(b)`.
+    ///
+    /// ### Why is this bad?
+    /// Using `abs_diff` is shorter, more readable, and avoids control flow.
+    ///
+    /// ### Examples
+    /// ```no_run
+    /// # let (a, b) = (5_usize, 3_usize);
+    /// if a > b {
+    ///     a - b
+    /// } else {
+    ///     b - a
+    /// }
+    /// # ;
+    /// ```
+    /// Use instead:
+    /// ```no_run
+    /// # let (a, b) = (5_usize, 3_usize);
+    /// a.abs_diff(b)
+    /// # ;
+    /// ```
+    #[clippy::version = "1.86.0"]
+    pub MANUAL_ABS_DIFF,
+    complexity,
+    "using an if-else pattern instead of `abs_diff`"
+}
+
+impl_lint_pass!(ManualAbsDiff => [MANUAL_ABS_DIFF]);
+
+pub struct ManualAbsDiff {
+    msrv: Msrv,
+}
+
+impl ManualAbsDiff {
+    pub fn new(conf: &'static Conf) -> Self {
+        Self { msrv: conf.msrv }
+    }
+}
+
+impl<'tcx> LateLintPass<'tcx> for ManualAbsDiff {
+    fn check_expr(&mut self, cx: &LateContext<'tcx>, expr: &'tcx Expr<'tcx>) {
+        if !expr.span.from_expansion()
+            && let Some(if_expr) = If::hir(expr)
+            && let Some(r#else) = if_expr.r#else
+            && let ExprKind::Binary(op, rhs, lhs) = if_expr.cond.kind
+            && let (BinOpKind::Gt | BinOpKind::Ge, mut a, mut b) | (BinOpKind::Lt | BinOpKind::Le, mut b, mut a) =
+                (op.node, rhs, lhs)
+            && let Some(ty) = self.are_ty_eligible(cx, a, b)
+            && is_sub_expr(cx, if_expr.then, a, b, ty)
+            && is_sub_expr(cx, r#else, b, a, ty)
+        {
+            span_lint_and_then(
+                cx,
+                MANUAL_ABS_DIFF,
+                expr.span,
+                "manual absolute difference pattern without using `abs_diff`",
+                |diag| {
+                    if is_unsuffixed_numeral_lit(a) && !is_unsuffixed_numeral_lit(b) {
+                        (a, b) = (b, a);
+                    }
+                    let applicability = {
+                        let source_map = cx.sess().source_map();
+                        if span_contains_comment(source_map, if_expr.then.span)
+                            || span_contains_comment(source_map, r#else.span)
+                        {
+                            Applicability::MaybeIncorrect
+                        } else {
+                            Applicability::MachineApplicable
+                        }
+                    };
+                    let sugg = format!(
+                        "{}.abs_diff({})",
+                        Sugg::hir(cx, a, "..").maybe_paren(),
+                        Sugg::hir(cx, b, "..")
+                    );
+                    diag.span_suggestion(expr.span, "replace with `abs_diff`", sugg, applicability);
+                },
+            );
+        }
+    }
+}
+
+impl ManualAbsDiff {
+    /// Returns a type if `a` and `b` are both of it, and this lint can be applied to that
+    /// type (currently, any primitive int, or a `Duration`)
+    fn are_ty_eligible<'tcx>(&self, cx: &LateContext<'tcx>, a: &Expr<'_>, b: &Expr<'_>) -> Option<Ty<'tcx>> {
+        let is_int = |ty: Ty<'_>| matches!(ty.kind(), ty::Uint(_) | ty::Int(_)) && self.msrv.meets(cx, msrvs::ABS_DIFF);
+        let is_duration =
+            |ty| is_type_diagnostic_item(cx, ty, sym::Duration) && self.msrv.meets(cx, msrvs::DURATION_ABS_DIFF);
+
+        let a_ty = cx.typeck_results().expr_ty(a).peel_refs();
+        (a_ty == cx.typeck_results().expr_ty(b).peel_refs() && (is_int(a_ty) || is_duration(a_ty))).then_some(a_ty)
+    }
+}
+
+/// Checks if the given expression is a subtraction operation between two expected expressions,
+/// i.e. if `expr` is `{expected_a} - {expected_b}`.
+///
+/// If `expected_ty` is a signed primitive integer, this function will only return `Some` if the
+/// subtraction expr is wrapped in a cast to the equivalent unsigned int.
+fn is_sub_expr(
+    cx: &LateContext<'_>,
+    expr: &Expr<'_>,
+    expected_a: &Expr<'_>,
+    expected_b: &Expr<'_>,
+    expected_ty: Ty<'_>,
+) -> bool {
+    let expr = peel_blocks(expr).kind;
+
+    if let ty::Int(ty) = expected_ty.kind() {
+        let unsigned = Ty::new_uint(cx.tcx, ty.to_unsigned());
+
+        return if let ExprKind::Cast(expr, cast_ty) = expr
+            && cx.typeck_results().node_type(cast_ty.hir_id) == unsigned
+        {
+            is_sub_expr(cx, expr, expected_a, expected_b, unsigned)
+        } else {
+            false
+        };
+    }
+
+    if let ExprKind::Binary(op, a, b) = expr
+        && let BinOpKind::Sub = op.node
+        && eq_expr_value(cx, a, expected_a)
+        && eq_expr_value(cx, b, expected_b)
+    {
+        true
+    } else {
+        false
+    }
+}
+
+fn is_unsuffixed_numeral_lit(expr: &Expr<'_>) -> bool {
+    matches!(expr.kind, ExprKind::Lit(lit) if lit.node.is_numeric() && lit.node.is_unsuffixed())
+}
diff --git a/clippy_utils/src/msrvs.rs b/clippy_utils/src/msrvs.rs
index 86f4f190b95..f709337da39 100644
--- a/clippy_utils/src/msrvs.rs
+++ b/clippy_utils/src/msrvs.rs
@@ -27,7 +27,7 @@ msrv_aliases! {
     1,84,0 { CONST_OPTION_AS_SLICE }
     1,83,0 { CONST_EXTERN_FN, CONST_FLOAT_BITS_CONV, CONST_FLOAT_CLASSIFY, CONST_MUT_REFS, CONST_UNWRAP }
     1,82,0 { IS_NONE_OR, REPEAT_N, RAW_REF_OP }
-    1,81,0 { LINT_REASONS_STABILIZATION, ERROR_IN_CORE, EXPLICIT_SELF_TYPE_ELISION }
+    1,81,0 { LINT_REASONS_STABILIZATION, ERROR_IN_CORE, EXPLICIT_SELF_TYPE_ELISION, DURATION_ABS_DIFF }
     1,80,0 { BOX_INTO_ITER, LAZY_CELL }
     1,77,0 { C_STR_LITERALS }
     1,76,0 { PTR_FROM_REF, OPTION_RESULT_INSPECT }
@@ -40,6 +40,7 @@ msrv_aliases! {
     1,65,0 { LET_ELSE, POINTER_CAST_CONSTNESS }
     1,63,0 { CLONE_INTO }
     1,62,0 { BOOL_THEN_SOME, DEFAULT_ENUM_ATTRIBUTE, CONST_EXTERN_C_FN }
+    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 }
     1,57,0 { MAP_WHILE }
diff --git a/tests/ui/manual_abs_diff.fixed b/tests/ui/manual_abs_diff.fixed
new file mode 100644
index 00000000000..f1b1278ea6d
--- /dev/null
+++ b/tests/ui/manual_abs_diff.fixed
@@ -0,0 +1,106 @@
+#![warn(clippy::manual_abs_diff)]
+
+use std::time::Duration;
+
+fn main() {
+    let a: usize = 5;
+    let b: usize = 3;
+    let c: usize = 8;
+    let d: usize = 11;
+
+    let _ = a.abs_diff(b);
+    //~^ manual_abs_diff
+    let _ = b.abs_diff(a);
+    //~^ manual_abs_diff
+
+    let _ = b.abs_diff(5);
+    //~^ manual_abs_diff
+    let _ = b.abs_diff(5);
+    //~^ manual_abs_diff
+
+    let _ = a.abs_diff(b);
+    //~^ manual_abs_diff
+    let _ = b.abs_diff(a);
+    //~^ manual_abs_diff
+
+    #[allow(arithmetic_overflow)]
+    {
+        let _ = if a > b { b - a } else { a - b };
+        let _ = if a < b { a - b } else { b - a };
+    }
+
+    let _ = (a + b).abs_diff(c + d);
+    let _ = (c + d).abs_diff(a + b);
+
+    const A: usize = 5;
+    const B: usize = 3;
+    // check const context
+    const _: usize = A.abs_diff(B);
+    //~^ manual_abs_diff
+
+    let a = Duration::from_secs(3);
+    let b = Duration::from_secs(5);
+    let _ = a.abs_diff(b);
+    //~^ manual_abs_diff
+
+    let a: i32 = 3;
+    let b: i32 = -5;
+    let _ = if a > b { a - b } else { b - a };
+    let _ = a.abs_diff(b);
+    //~^ manual_abs_diff
+}
+
+// FIXME: bunch of patterns that should be linted
+fn fixme() {
+    let a: usize = 5;
+    let b: usize = 3;
+    let c: usize = 8;
+    let d: usize = 11;
+
+    {
+        let out;
+        if a > b {
+            out = a - b;
+        } else {
+            out = b - a;
+        }
+    }
+
+    {
+        let mut out = 0;
+        if a > b {
+            out = a - b;
+        } else if a < b {
+            out = b - a;
+        }
+    }
+
+    #[allow(clippy::implicit_saturating_sub)]
+    let _ = if a > b {
+        a - b
+    } else if a < b {
+        b - a
+    } else {
+        0
+    };
+
+    let a: i32 = 3;
+    let b: i32 = 5;
+    let _: u32 = if a > b { a - b } else { b - a } as u32;
+}
+
+fn non_primitive_ty() {
+    #[derive(Eq, PartialEq, PartialOrd)]
+    struct S(i32);
+
+    impl std::ops::Sub for S {
+        type Output = S;
+
+        fn sub(self, rhs: Self) -> Self::Output {
+            Self(self.0 - rhs.0)
+        }
+    }
+
+    let (a, b) = (S(10), S(20));
+    let _ = if a < b { b - a } else { a - b };
+}
diff --git a/tests/ui/manual_abs_diff.rs b/tests/ui/manual_abs_diff.rs
new file mode 100644
index 00000000000..60ef819c12d
--- /dev/null
+++ b/tests/ui/manual_abs_diff.rs
@@ -0,0 +1,116 @@
+#![warn(clippy::manual_abs_diff)]
+
+use std::time::Duration;
+
+fn main() {
+    let a: usize = 5;
+    let b: usize = 3;
+    let c: usize = 8;
+    let d: usize = 11;
+
+    let _ = if a > b { a - b } else { b - a };
+    //~^ manual_abs_diff
+    let _ = if a < b { b - a } else { a - b };
+    //~^ manual_abs_diff
+
+    let _ = if 5 > b { 5 - b } else { b - 5 };
+    //~^ manual_abs_diff
+    let _ = if b > 5 { b - 5 } else { 5 - b };
+    //~^ manual_abs_diff
+
+    let _ = if a >= b { a - b } else { b - a };
+    //~^ manual_abs_diff
+    let _ = if a <= b { b - a } else { a - b };
+    //~^ manual_abs_diff
+
+    #[allow(arithmetic_overflow)]
+    {
+        let _ = if a > b { b - a } else { a - b };
+        let _ = if a < b { a - b } else { b - a };
+    }
+
+    let _ = if (a + b) > (c + d) {
+        //~^ manual_abs_diff
+        (a + b) - (c + d)
+    } else {
+        (c + d) - (a + b)
+    };
+    let _ = if (a + b) < (c + d) {
+        //~^ manual_abs_diff
+        (c + d) - (a + b)
+    } else {
+        (a + b) - (c + d)
+    };
+
+    const A: usize = 5;
+    const B: usize = 3;
+    // check const context
+    const _: usize = if A > B { A - B } else { B - A };
+    //~^ manual_abs_diff
+
+    let a = Duration::from_secs(3);
+    let b = Duration::from_secs(5);
+    let _ = if a > b { a - b } else { b - a };
+    //~^ manual_abs_diff
+
+    let a: i32 = 3;
+    let b: i32 = -5;
+    let _ = if a > b { a - b } else { b - a };
+    let _ = if a > b { (a - b) as u32 } else { (b - a) as u32 };
+    //~^ manual_abs_diff
+}
+
+// FIXME: bunch of patterns that should be linted
+fn fixme() {
+    let a: usize = 5;
+    let b: usize = 3;
+    let c: usize = 8;
+    let d: usize = 11;
+
+    {
+        let out;
+        if a > b {
+            out = a - b;
+        } else {
+            out = b - a;
+        }
+    }
+
+    {
+        let mut out = 0;
+        if a > b {
+            out = a - b;
+        } else if a < b {
+            out = b - a;
+        }
+    }
+
+    #[allow(clippy::implicit_saturating_sub)]
+    let _ = if a > b {
+        a - b
+    } else if a < b {
+        b - a
+    } else {
+        0
+    };
+
+    let a: i32 = 3;
+    let b: i32 = 5;
+    let _: u32 = if a > b { a - b } else { b - a } as u32;
+}
+
+fn non_primitive_ty() {
+    #[derive(Eq, PartialEq, PartialOrd)]
+    struct S(i32);
+
+    impl std::ops::Sub for S {
+        type Output = S;
+
+        fn sub(self, rhs: Self) -> Self::Output {
+            Self(self.0 - rhs.0)
+        }
+    }
+
+    let (a, b) = (S(10), S(20));
+    let _ = if a < b { b - a } else { a - b };
+}
diff --git a/tests/ui/manual_abs_diff.stderr b/tests/ui/manual_abs_diff.stderr
new file mode 100644
index 00000000000..c14c1dc830f
--- /dev/null
+++ b/tests/ui/manual_abs_diff.stderr
@@ -0,0 +1,83 @@
+error: manual absolute difference pattern without using `abs_diff`
+  --> tests/ui/manual_abs_diff.rs:11:13
+   |
+LL |     let _ = if a > b { a - b } else { b - a };
+   |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: replace with `abs_diff`: `a.abs_diff(b)`
+   |
+   = note: `-D clippy::manual-abs-diff` implied by `-D warnings`
+   = help: to override `-D warnings` add `#[allow(clippy::manual_abs_diff)]`
+
+error: manual absolute difference pattern without using `abs_diff`
+  --> tests/ui/manual_abs_diff.rs:13:13
+   |
+LL |     let _ = if a < b { b - a } else { a - b };
+   |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: replace with `abs_diff`: `b.abs_diff(a)`
+
+error: manual absolute difference pattern without using `abs_diff`
+  --> tests/ui/manual_abs_diff.rs:16:13
+   |
+LL |     let _ = if 5 > b { 5 - b } else { b - 5 };
+   |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: replace with `abs_diff`: `b.abs_diff(5)`
+
+error: manual absolute difference pattern without using `abs_diff`
+  --> tests/ui/manual_abs_diff.rs:18:13
+   |
+LL |     let _ = if b > 5 { b - 5 } else { 5 - b };
+   |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: replace with `abs_diff`: `b.abs_diff(5)`
+
+error: manual absolute difference pattern without using `abs_diff`
+  --> tests/ui/manual_abs_diff.rs:21:13
+   |
+LL |     let _ = if a >= b { a - b } else { b - a };
+   |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: replace with `abs_diff`: `a.abs_diff(b)`
+
+error: manual absolute difference pattern without using `abs_diff`
+  --> tests/ui/manual_abs_diff.rs:23:13
+   |
+LL |     let _ = if a <= b { b - a } else { a - b };
+   |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: replace with `abs_diff`: `b.abs_diff(a)`
+
+error: manual absolute difference pattern without using `abs_diff`
+  --> tests/ui/manual_abs_diff.rs:32:13
+   |
+LL |       let _ = if (a + b) > (c + d) {
+   |  _____________^
+LL | |
+LL | |         (a + b) - (c + d)
+LL | |     } else {
+LL | |         (c + d) - (a + b)
+LL | |     };
+   | |_____^ help: replace with `abs_diff`: `(a + b).abs_diff(c + d)`
+
+error: manual absolute difference pattern without using `abs_diff`
+  --> tests/ui/manual_abs_diff.rs:38:13
+   |
+LL |       let _ = if (a + b) < (c + d) {
+   |  _____________^
+LL | |
+LL | |         (c + d) - (a + b)
+LL | |     } else {
+LL | |         (a + b) - (c + d)
+LL | |     };
+   | |_____^ help: replace with `abs_diff`: `(c + d).abs_diff(a + b)`
+
+error: manual absolute difference pattern without using `abs_diff`
+  --> tests/ui/manual_abs_diff.rs:48:22
+   |
+LL |     const _: usize = if A > B { A - B } else { B - A };
+   |                      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: replace with `abs_diff`: `A.abs_diff(B)`
+
+error: manual absolute difference pattern without using `abs_diff`
+  --> tests/ui/manual_abs_diff.rs:53:13
+   |
+LL |     let _ = if a > b { a - b } else { b - a };
+   |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: replace with `abs_diff`: `a.abs_diff(b)`
+
+error: manual absolute difference pattern without using `abs_diff`
+  --> tests/ui/manual_abs_diff.rs:59:13
+   |
+LL |     let _ = if a > b { (a - b) as u32 } else { (b - a) as u32 };
+   |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: replace with `abs_diff`: `a.abs_diff(b)`
+
+error: aborting due to 11 previous errors
+