Auto merge of rust-lang#5427 - pmk21:implicit-sat-sub, r=flip1995

Add lint named implicit_saturating_sub

Fixes: rust-lang#5399
I've made a basic skeleton of the lint, would love more feedback on how to make it better.
changelog: Add lint [`implicit_saturating_sub`]

CHANGELOG.md

@@ -1293,6 +1293,7 @@ Released 2018-09-13
 [`ifs_same_cond`]: https://rust-lang.github.io/rust-clippy/master/index.html#ifs_same_cond
 [`implicit_hasher`]: https://rust-lang.github.io/rust-clippy/master/index.html#implicit_hasher
 [`implicit_return`]: https://rust-lang.github.io/rust-clippy/master/index.html#implicit_return
+[`implicit_saturating_sub`]: https://rust-lang.github.io/rust-clippy/master/index.html#implicit_saturating_sub
 [`imprecise_flops`]: https://rust-lang.github.io/rust-clippy/master/index.html#imprecise_flops
 [`inconsistent_digit_grouping`]: https://rust-lang.github.io/rust-clippy/master/index.html#inconsistent_digit_grouping
 [`indexing_slicing`]: https://rust-lang.github.io/rust-clippy/master/index.html#indexing_slicing

clippy_lints/src/implicit_saturating_sub.rs

@@ -0,0 +1,173 @@
+use crate::utils::{higher, in_macro, match_qpath, span_lint_and_sugg, SpanlessEq};
+use if_chain::if_chain;
+use rustc_ast::ast::LitKind;
+use rustc_errors::Applicability;
+use rustc_hir::{BinOpKind, Expr, ExprKind, QPath, StmtKind};
+use rustc_lint::{LateContext, LateLintPass};
+use rustc_session::{declare_lint_pass, declare_tool_lint};
+
+declare_clippy_lint! {
+    /// **What it does:** Checks for implicit saturating subtraction.
+    ///
+    /// **Why is this bad?** Simplicity and readability. Instead we can easily use an builtin function.
+    ///
+    /// **Known problems:** None.
+    ///
+    /// **Example:**
+    ///
+    /// ```rust
+    /// let end: u32 = 10;
+    /// let start: u32 = 5;
+    ///
+    /// let mut i: u32 = end - start;
+    ///
+    /// // Bad
+    /// if i != 0 {
+    ///     i -= 1;
+    /// }
+    /// ```
+    /// Use instead:
+    /// ```rust
+    /// let end: u32 = 10;
+    /// let start: u32 = 5;
+    ///
+    /// let mut i: u32 = end - start;
+    ///
+    /// // Good
+    /// i = i.saturating_sub(1);
+    /// ```
+    pub IMPLICIT_SATURATING_SUB,
+    pedantic,
+    "Perform saturating subtraction instead of implicitly checking lower bound of data type"
+}
+
+declare_lint_pass!(ImplicitSaturatingSub => [IMPLICIT_SATURATING_SUB]);
+
+impl<'a, 'tcx> LateLintPass<'a, 'tcx> for ImplicitSaturatingSub {
+    fn check_expr(&mut self, cx: &LateContext<'a, 'tcx>, expr: &'tcx Expr<'tcx>) {
+        if in_macro(expr.span) {
+            return;
+        }
+        if_chain! {
+            if let Some((ref cond, ref then, None)) = higher::if_block(&expr);
+
+            // Check if the conditional expression is a binary operation
+            if let ExprKind::Binary(ref cond_op, ref cond_left, ref cond_right) = cond.kind;
+
+            // Ensure that the binary operator is >, != and <
+            if BinOpKind::Ne == cond_op.node || BinOpKind::Gt == cond_op.node || BinOpKind::Lt == cond_op.node;
+
+            // Check if the true condition block has only one statement
+            if let ExprKind::Block(ref block, _) = then.kind;
+            if block.stmts.len() == 1 && block.expr.is_none();
+
+            // Check if assign operation is done
+            if let StmtKind::Semi(ref e) = block.stmts[0].kind;
+            if let Some(target) = subtracts_one(cx, e);
+
+            // Extracting out the variable name
+            if let ExprKind::Path(ref assign_path) = target.kind;
+            if let QPath::Resolved(_, ref ares_path) = assign_path;
+
+            then {
+                // Handle symmetric conditions in the if statement
+                let (cond_var, cond_num_val) = if SpanlessEq::new(cx).eq_expr(cond_left, target) {
+                    if BinOpKind::Gt == cond_op.node || BinOpKind::Ne == cond_op.node {
+                        (cond_left, cond_right)
+                    } else {
+                        return;
+                    }
+                } else if SpanlessEq::new(cx).eq_expr(cond_right, target) {
+                    if BinOpKind::Lt == cond_op.node || BinOpKind::Ne == cond_op.node {
+                        (cond_right, cond_left)
+                    } else {
+                        return;
+                    }
+                } else {
+                    return;
+                };
+
+                // Check if the variable in the condition statement is an integer
+                if !cx.tables.expr_ty(cond_var).is_integral() {
+                    return;
+                }
+
+                // Get the variable name
+                let var_name = ares_path.segments[0].ident.name.as_str();
+                const INT_TYPES: [&str; 5] = ["i8", "i16", "i32", "i64", "i128"];
+
+                match cond_num_val.kind {
+                    ExprKind::Lit(ref cond_lit) => {
+                        // Check if the constant is zero
+                        if let LitKind::Int(0, _) = cond_lit.node {
+                            if cx.tables.expr_ty(cond_left).is_signed() {
+                            } else {
+                                print_lint_and_sugg(cx, &var_name, expr);
+                            };
+                        }
+                    },
+                    ExprKind::Path(ref cond_num_path) => {
+                        if INT_TYPES.iter().any(|int_type| match_qpath(cond_num_path, &[int_type, "MIN"])) {
+                            print_lint_and_sugg(cx, &var_name, expr);
+                        };
+                    },
+                    ExprKind::Call(ref func, _) => {
+                        if let ExprKind::Path(ref cond_num_path) = func.kind {
+                            if INT_TYPES.iter().any(|int_type| match_qpath(cond_num_path, &[int_type, "min_value"])) {
+                                print_lint_and_sugg(cx, &var_name, expr);
+                            }
+                        };
+                    },
+                    _ => (),
+                }
+            }
+        }
+    }
+}
+
+fn subtracts_one<'a>(cx: &LateContext<'_, '_>, expr: &Expr<'a>) -> Option<&'a Expr<'a>> {
+    match expr.kind {
+        ExprKind::AssignOp(ref op1, ref target, ref value) => {
+            if_chain! {
+                if BinOpKind::Sub == op1.node;
+                // Check if literal being subtracted is one
+                if let ExprKind::Lit(ref lit1) = value.kind;
+                if let LitKind::Int(1, _) = lit1.node;
+                then {
+                    Some(target)
+                } else {
+                    None
+                }
+            }
+        },
+        ExprKind::Assign(ref target, ref value, _) => {
+            if_chain! {
+                if let ExprKind::Binary(ref op1, ref left1, ref right1) = value.kind;
+                if BinOpKind::Sub == op1.node;
+
+                if SpanlessEq::new(cx).eq_expr(left1, target);
+
+                if let ExprKind::Lit(ref lit1) = right1.kind;
+                if let LitKind::Int(1, _) = lit1.node;
+                then {
+                    Some(target)
+                } else {
+                    None
+                }
+            }
+        },
+        _ => None,
+    }
+}
+
+fn print_lint_and_sugg(cx: &LateContext<'_, '_>, var_name: &str, expr: &Expr<'_>) {
+    span_lint_and_sugg(
+        cx,
+        IMPLICIT_SATURATING_SUB,
+        expr.span,
+        "Implicitly performing saturating subtraction",
+        "try",
+        format!("{} = {}.saturating_sub({});", var_name, var_name, 1.to_string()),
+        Applicability::MachineApplicable,
+    );
+}

clippy_lints/src/lib.rs

@@ -225,6 +225,7 @@ mod identity_op;
 mod if_let_some_result;
 mod if_not_else;
 mod implicit_return;
+mod implicit_saturating_sub;
 mod indexing_slicing;
 mod infinite_iter;
 mod inherent_impl;
@@ -574,6 +575,7 @@ pub fn register_plugins(store: &mut rustc_lint::LintStore, sess: &Session, conf:
         &if_let_some_result::IF_LET_SOME_RESULT,
         &if_not_else::IF_NOT_ELSE,
         &implicit_return::IMPLICIT_RETURN,
+        &implicit_saturating_sub::IMPLICIT_SATURATING_SUB,
         &indexing_slicing::INDEXING_SLICING,
         &indexing_slicing::OUT_OF_BOUNDS_INDEXING,
         &infinite_iter::INFINITE_ITER,
@@ -888,6 +890,7 @@ pub fn register_plugins(store: &mut rustc_lint::LintStore, sess: &Session, conf:
     store.register_late_pass(|| box unicode::Unicode);
     store.register_late_pass(|| box strings::StringAdd);
     store.register_late_pass(|| box implicit_return::ImplicitReturn);
+    store.register_late_pass(|| box implicit_saturating_sub::ImplicitSaturatingSub);
     store.register_late_pass(|| box methods::Methods);
     store.register_late_pass(|| box map_clone::MapClone);
     store.register_late_pass(|| box shadow::Shadow);
@@ -1111,6 +1114,7 @@ pub fn register_plugins(store: &mut rustc_lint::LintStore, sess: &Session, conf:
         LintId::of(&functions::MUST_USE_CANDIDATE),
         LintId::of(&functions::TOO_MANY_LINES),
         LintId::of(&if_not_else::IF_NOT_ELSE),
+        LintId::of(&implicit_saturating_sub::IMPLICIT_SATURATING_SUB),
         LintId::of(&infinite_iter::MAYBE_INFINITE_ITER),
         LintId::of(&items_after_statements::ITEMS_AFTER_STATEMENTS),
         LintId::of(&large_stack_arrays::LARGE_STACK_ARRAYS),

src/lintlist/mod.rs

@@ -773,6 +773,13 @@ pub static ref ALL_LINTS: Vec<Lint> = vec![
         deprecation: None,
         module: "implicit_return",
     },
+    Lint {
+        name: "implicit_saturating_sub",
+        group: "pedantic",
+        desc: "Perform saturating subtraction instead of implicitly checking lower bound of data type",
+        deprecation: None,
+        module: "implicit_saturating_sub",
+    },
     Lint {
         name: "imprecise_flops",
         group: "nursery",

tests/ui/implicit_saturating_sub.fixed

@@ -0,0 +1,160 @@
+// run-rustfix
+#![allow(unused_assignments, unused_mut, clippy::assign_op_pattern)]
+#![warn(clippy::implicit_saturating_sub)]
+
+fn main() {
+    // Tests for unsigned integers
+
+    let end_8: u8 = 10;
+    let start_8: u8 = 5;
+    let mut u_8: u8 = end_8 - start_8;
+
+    // Lint
+    u_8 = u_8.saturating_sub(1);
+
+    match end_8 {
+        10 => {
+            // Lint
+            u_8 = u_8.saturating_sub(1);
+        },
+        11 => u_8 += 1,
+        _ => u_8 = 0,
+    }
+
+    let end_16: u16 = 35;
+    let start_16: u16 = 40;
+
+    let mut u_16: u16 = end_16 - start_16;
+
+    // Lint
+    u_16 = u_16.saturating_sub(1);
+
+    let mut end_32: u32 = 7000;
+    let mut start_32: u32 = 7010;
+
+    let mut u_32: u32 = end_32 - start_32;
+
+    // Lint
+    u_32 = u_32.saturating_sub(1);
+
+    // No Lint
+    if u_32 > 0 {
+        u_16 += 1;
+    }
+
+    // No Lint
+    if u_32 != 0 {
+        end_32 -= 1;
+        start_32 += 1;
+    }
+
+    let mut end_64: u64 = 75001;
+    let mut start_64: u64 = 75000;
+
+    let mut u_64: u64 = end_64 - start_64;
+
+    // Lint
+    u_64 = u_64.saturating_sub(1);
+
+    // Lint
+    u_64 = u_64.saturating_sub(1);
+
+    // Lint
+    u_64 = u_64.saturating_sub(1);
+
+    // No Lint
+    if u_64 >= 1 {
+        u_64 -= 1;
+    }
+
+    // No Lint
+    if u_64 > 0 {
+        end_64 -= 1;
+    }
+
+    // Tests for usize
+    let end_usize: usize = 8054;
+    let start_usize: usize = 8050;
+
+    let mut u_usize: usize = end_usize - start_usize;
+
+    // Lint
+    u_usize = u_usize.saturating_sub(1);
+
+    // Tests for signed integers
+
+    let endi_8: i8 = 10;
+    let starti_8: i8 = 50;
+
+    let mut i_8: i8 = endi_8 - starti_8;
+
+    // Lint
+    i_8 = i_8.saturating_sub(1);
+
+    // Lint
+    i_8 = i_8.saturating_sub(1);
+
+    // Lint
+    i_8 = i_8.saturating_sub(1);
+
+    // Lint
+    i_8 = i_8.saturating_sub(1);
+
+    let endi_16: i16 = 45;
+    let starti_16: i16 = 44;
+
+    let mut i_16: i16 = endi_16 - starti_16;
+
+    // Lint
+    i_16 = i_16.saturating_sub(1);
+
+    // Lint
+    i_16 = i_16.saturating_sub(1);
+
+    // Lint
+    i_16 = i_16.saturating_sub(1);
+
+    // Lint
+    i_16 = i_16.saturating_sub(1);
+
+    let endi_32: i32 = 45;
+    let starti_32: i32 = 44;
+
+    let mut i_32: i32 = endi_32 - starti_32;
+
+    // Lint
+    i_32 = i_32.saturating_sub(1);
+
+    // Lint
+    i_32 = i_32.saturating_sub(1);
+
+    // Lint
+    i_32 = i_32.saturating_sub(1);
+
+    // Lint
+    i_32 = i_32.saturating_sub(1);
+
+    let endi_64: i64 = 45;
+    let starti_64: i64 = 44;
+
+    let mut i_64: i64 = endi_64 - starti_64;
+
+    // Lint
+    i_64 = i_64.saturating_sub(1);
+
+    // Lint
+    i_64 = i_64.saturating_sub(1);
+
+    // Lint
+    i_64 = i_64.saturating_sub(1);
+
+    // No Lint
+    if i_64 > 0 {
+        i_64 -= 1;
+    }
+
+    // No Lint
+    if i_64 != 0 {
+        i_64 -= 1;
+    }
+}