rust-clippy/clippy_lints/src/floating_point_arithmetic.rs

use crate::consts::{
    constant, Constant,
    Constant::{F32, F64},
};
use crate::utils::*;
use if_chain::if_chain;
use rustc::ty;
use rustc_errors::Applicability;
use rustc_hir::*;
use rustc_lint::{LateContext, LateLintPass};
use rustc_session::{declare_lint_pass, declare_tool_lint};
use std::f32::consts as f32_consts;
use std::f64::consts as f64_consts;
use sugg::{format_numeric_literal, Sugg};
use syntax::ast;

declare_clippy_lint! {
    /// **What it does:** Looks for floating-point expressions that
    /// can be expressed using built-in methods to improve both
    /// accuracy and performance.
    ///
    /// **Why is this bad?** Negatively impacts accuracy and performance.
    ///
    /// **Known problems:** None
    ///
    /// **Example:**
    ///
    /// ```rust
    /// use std::f32::consts::E;
    ///
    /// let a = 3f32;
    /// let _ = (2f32).powf(a);
    /// let _ = E.powf(a);
    /// let _ = a.powf(1.0 / 2.0);
    /// let _ = a.powf(1.0 / 3.0);
    /// let _ = a.log(2.0);
    /// let _ = a.log(10.0);
    /// let _ = a.log(E);
    /// let _ = (1.0 + a).ln();
    /// let _ = a.exp() - 1.0;
    /// let _ = a.powf(2.0);
    /// ```
    ///
    /// is better expressed as
    ///
    /// ```rust
    /// use std::f32::consts::E;
    ///
    /// let a = 3f32;
    /// let _ = a.exp2();
    /// let _ = a.exp();
    /// let _ = a.sqrt();
    /// let _ = a.cbrt();
    /// let _ = a.log2();
    /// let _ = a.log10();
    /// let _ = a.ln();
    /// let _ = a.ln_1p();
    /// let _ = a.exp_m1();
    /// let _ = a.powi(2);
    /// ```
    pub SUBOPTIMAL_FLOPS,
    nursery,
    "usage of sub-optimal floating point operations"
}

declare_lint_pass!(FloatingPointArithmetic => [SUBOPTIMAL_FLOPS]);

// Returns the specialized log method for a given base if base is constant
// and is one of 2, 10 and e
fn get_specialized_log_method(cx: &LateContext<'_, '_>, base: &Expr<'_>) -> Option<&'static str> {
    if let Some((value, _)) = constant(cx, cx.tables, base) {
        if F32(2.0) == value || F64(2.0) == value {
            return Some("log2");
        } else if F32(10.0) == value || F64(10.0) == value {
            return Some("log10");
        } else if F32(f32_consts::E) == value || F64(f64_consts::E) == value {
            return Some("ln");
        }
    }

    None
}

// Adds type suffixes and parenthesis to method receivers if necessary
fn prepare_receiver_sugg<'a>(cx: &LateContext<'_, '_>, mut expr: &'a Expr<'a>) -> Sugg<'a> {
    let mut suggestion = Sugg::hir(cx, expr, "..");

    if let ExprKind::Unary(UnOp::UnNeg, inner_expr) = &expr.kind {
        expr = &inner_expr;
    }

    if_chain! {
        // if the expression is a float literal and it is unsuffixed then
        // add a suffix so the suggestion is valid and unambiguous
        if let ty::Float(float_ty) = cx.tables.expr_ty(expr).kind;
        if let ExprKind::Lit(lit) = &expr.kind;
        if let ast::LitKind::Float(sym, ast::LitFloatType::Unsuffixed) = lit.node;
        then {
            let op = format!(
                "{}{}{}",
                suggestion,
                // Check for float literals without numbers following the decimal
                // separator such as `2.` and adds a trailing zero
                if sym.as_str().ends_with('.') {
                    "0"
                } else {
                    ""
                },
                float_ty.name_str()
            ).into();

            suggestion = match suggestion {
                Sugg::MaybeParen(_) => Sugg::MaybeParen(op),
                _ => Sugg::NonParen(op)
            };
        }
    }

    suggestion.maybe_par()
}

fn check_log_base(cx: &LateContext<'_, '_>, expr: &Expr<'_>, args: &[Expr<'_>]) {
    if let Some(method) = get_specialized_log_method(cx, &args[1]) {
        span_lint_and_sugg(
            cx,
            SUBOPTIMAL_FLOPS,
            expr.span,
            "logarithm for bases 2, 10 and e can be computed more accurately",
            "consider using",
            format!("{}.{}()", Sugg::hir(cx, &args[0], ".."), method),
            Applicability::MachineApplicable,
        );
    }
}

// TODO: Lint expressions of the form `(x + y).ln()` where y > 1 and
// suggest usage of `(x + (y - 1)).ln_1p()` instead
fn check_ln1p(cx: &LateContext<'_, '_>, expr: &Expr<'_>, args: &[Expr<'_>]) {
    if_chain! {
        if let ExprKind::Binary(op, ref lhs, ref rhs) = &args[0].kind;
        if op.node == BinOpKind::Add;
        then {
            let recv = match (constant(cx, cx.tables, lhs), constant(cx, cx.tables, rhs)) {
                (Some((value, _)), _) if F32(1.0) == value || F64(1.0) == value => rhs,
                (_, Some((value, _))) if F32(1.0) == value || F64(1.0) == value => lhs,
                _ => return,
            };

            span_lint_and_sugg(
                cx,
                SUBOPTIMAL_FLOPS,
                expr.span,
                "ln(1 + x) can be computed more accurately",
                "consider using",
                format!("{}.ln_1p()", prepare_receiver_sugg(cx, recv)),
                Applicability::MachineApplicable,
            );
        }
    }
}

// Returns an integer if the float constant is a whole number and it can be
// converted to an integer without loss of precision. For now we only check
// ranges [-16777215, 16777216) for type f32 as whole number floats outside
// this range are lossy and ambiguous.
#[allow(clippy::cast_possible_truncation)]
fn get_integer_from_float_constant(value: &Constant) -> Option<i32> {
    match value {
        F32(num) if num.fract() == 0.0 => {
            if (-16_777_215.0..16_777_216.0).contains(num) {
                Some(num.round() as i32)
            } else {
                None
            }
        },
        F64(num) if num.fract() == 0.0 => {
            if (-2_147_483_648.0..2_147_483_648.0).contains(num) {
                Some(num.round() as i32)
            } else {
                None
            }
        },
        _ => None,
    }
}

fn check_powf(cx: &LateContext<'_, '_>, expr: &Expr<'_>, args: &[Expr<'_>]) {
    // Check receiver
    if let Some((value, _)) = constant(cx, cx.tables, &args[0]) {
        let method = if F32(f32_consts::E) == value || F64(f64_consts::E) == value {
            "exp"
        } else if F32(2.0) == value || F64(2.0) == value {
            "exp2"
        } else {
            return;
        };

        span_lint_and_sugg(
            cx,
            SUBOPTIMAL_FLOPS,
            expr.span,
            "exponent for bases 2 and e can be computed more accurately",
            "consider using",
            format!("{}.{}()", prepare_receiver_sugg(cx, &args[1]), method),
            Applicability::MachineApplicable,
        );
    }

    // Check argument
    if let Some((value, _)) = constant(cx, cx.tables, &args[1]) {
        let (help, suggestion) = if F32(1.0 / 2.0) == value || F64(1.0 / 2.0) == value {
            (
                "square-root of a number can be computed more efficiently and accurately",
                format!("{}.sqrt()", Sugg::hir(cx, &args[0], ".."))
            )
        } else if F32(1.0 / 3.0) == value || F64(1.0 / 3.0) == value {
            (
                "cube-root of a number can be computed more accurately",
                format!("{}.cbrt()", Sugg::hir(cx, &args[0], ".."))
            )
        } else if let Some(exponent) = get_integer_from_float_constant(&value) {
            (
                "exponentiation with integer powers can be computed more efficiently",
                format!(
                    "{}.powi({})",
                    Sugg::hir(cx, &args[0], ".."),
                    format_numeric_literal(&exponent.to_string(), None, false)
                )
            )
        } else {
            return;
        };

        span_lint_and_sugg(
            cx,
            SUBOPTIMAL_FLOPS,
            expr.span,
            help,
            "consider using",
            suggestion,
            Applicability::MachineApplicable,
        );
    }
}

// TODO: Lint expressions of the form `x.exp() - y` where y > 1
// and suggest usage of `x.exp_m1() - (y - 1)` instead
fn check_expm1(cx: &LateContext<'_, '_>, expr: &Expr<'_>) {
    if_chain! {
        if let ExprKind::Binary(op, ref lhs, ref rhs) = expr.kind;
        if op.node == BinOpKind::Sub;
        if cx.tables.expr_ty(lhs).is_floating_point();
        if let Some((value, _)) = constant(cx, cx.tables, rhs);
        if F32(1.0) == value || F64(1.0) == value;
        if let ExprKind::MethodCall(ref path, _, ref method_args) = lhs.kind;
        if cx.tables.expr_ty(&method_args[0]).is_floating_point();
        if path.ident.name.as_str() == "exp";
        then {
            span_lint_and_sugg(
                cx,
                SUBOPTIMAL_FLOPS,
                expr.span,
                "(e.pow(x) - 1) can be computed more accurately",
                "consider using",
                format!(
                    "{}.exp_m1()",
                    Sugg::hir(cx, &method_args[0], "..")
                ),
                Applicability::MachineApplicable,
            );
        }
    }
}

impl<'a, 'tcx> LateLintPass<'a, 'tcx> for FloatingPointArithmetic {
    fn check_expr(&mut self, cx: &LateContext<'a, 'tcx>, expr: &'tcx Expr<'_>) {
        if let ExprKind::MethodCall(ref path, _, args) = &expr.kind {
            let recv_ty = cx.tables.expr_ty(&args[0]);

            if recv_ty.is_floating_point() {
                match &*path.ident.name.as_str() {
                    "ln" => check_ln1p(cx, expr, args),
                    "log" => check_log_base(cx, expr, args),
                    "powf" => check_powf(cx, expr, args),
                    _ => {},
                }
            }
        } else {
            check_expm1(cx, expr);
        }
    }
}