rust-clippy/clippy_lints/src/float_literal.rs
2021-01-18 21:10:36 +01:00

182 lines
6.2 KiB
Rust

use crate::utils::{numeric_literal, span_lint_and_sugg};
use if_chain::if_chain;
use rustc_ast::ast::{self, LitFloatType, LitKind};
use rustc_errors::Applicability;
use rustc_hir as hir;
use rustc_lint::{LateContext, LateLintPass};
use rustc_middle::ty::{self, FloatTy};
use rustc_session::{declare_lint_pass, declare_tool_lint};
use std::fmt;
declare_clippy_lint! {
/// **What it does:** Checks for float literals with a precision greater
/// than that supported by the underlying type.
///
/// **Why is this bad?** Rust will truncate the literal silently.
///
/// **Known problems:** None.
///
/// **Example:**
///
/// ```rust
/// // Bad
/// let v: f32 = 0.123_456_789_9;
/// println!("{}", v); // 0.123_456_789
///
/// // Good
/// let v: f64 = 0.123_456_789_9;
/// println!("{}", v); // 0.123_456_789_9
/// ```
pub EXCESSIVE_PRECISION,
style,
"excessive precision for float literal"
}
declare_clippy_lint! {
/// **What it does:** Checks for whole number float literals that
/// cannot be represented as the underlying type without loss.
///
/// **Why is this bad?** Rust will silently lose precision during
/// conversion to a float.
///
/// **Known problems:** None.
///
/// **Example:**
///
/// ```rust
/// // Bad
/// let _: f32 = 16_777_217.0; // 16_777_216.0
///
/// // Good
/// let _: f32 = 16_777_216.0;
/// let _: f64 = 16_777_217.0;
/// ```
pub LOSSY_FLOAT_LITERAL,
restriction,
"lossy whole number float literals"
}
declare_lint_pass!(FloatLiteral => [EXCESSIVE_PRECISION, LOSSY_FLOAT_LITERAL]);
impl<'tcx> LateLintPass<'tcx> for FloatLiteral {
fn check_expr(&mut self, cx: &LateContext<'tcx>, expr: &'tcx hir::Expr<'_>) {
if_chain! {
let ty = cx.typeck_results().expr_ty(expr);
if let ty::Float(fty) = *ty.kind();
if let hir::ExprKind::Lit(ref lit) = expr.kind;
if let LitKind::Float(sym, lit_float_ty) = lit.node;
then {
let sym_str = sym.as_str();
let formatter = FloatFormat::new(&sym_str);
// Try to bail out if the float is for sure fine.
// If its within the 2 decimal digits of being out of precision we
// check if the parsed representation is the same as the string
// since we'll need the truncated string anyway.
let digits = count_digits(&sym_str);
let max = max_digits(fty);
let type_suffix = match lit_float_ty {
LitFloatType::Suffixed(ast::FloatTy::F32) => Some("f32"),
LitFloatType::Suffixed(ast::FloatTy::F64) => Some("f64"),
LitFloatType::Unsuffixed => None
};
let (is_whole, mut float_str) = match fty {
FloatTy::F32 => {
let value = sym_str.parse::<f32>().unwrap();
(value.fract() == 0.0, formatter.format(value))
},
FloatTy::F64 => {
let value = sym_str.parse::<f64>().unwrap();
(value.fract() == 0.0, formatter.format(value))
},
};
if is_whole && !sym_str.contains(|c| c == 'e' || c == 'E') {
// Normalize the literal by stripping the fractional portion
if sym_str.split('.').next().unwrap() != float_str {
// If the type suffix is missing the suggestion would be
// incorrectly interpreted as an integer so adding a `.0`
// suffix to prevent that.
if type_suffix.is_none() {
float_str.push_str(".0");
}
span_lint_and_sugg(
cx,
LOSSY_FLOAT_LITERAL,
expr.span,
"literal cannot be represented as the underlying type without loss of precision",
"consider changing the type or replacing it with",
numeric_literal::format(&float_str, type_suffix, true),
Applicability::MachineApplicable,
);
}
} else if digits > max as usize && sym_str != float_str {
span_lint_and_sugg(
cx,
EXCESSIVE_PRECISION,
expr.span,
"float has excessive precision",
"consider changing the type or truncating it to",
numeric_literal::format(&float_str, type_suffix, true),
Applicability::MachineApplicable,
);
}
}
}
}
}
#[must_use]
fn max_digits(fty: FloatTy) -> u32 {
match fty {
FloatTy::F32 => f32::DIGITS,
FloatTy::F64 => f64::DIGITS,
}
}
/// Counts the digits excluding leading zeros
#[must_use]
fn count_digits(s: &str) -> usize {
// Note that s does not contain the f32/64 suffix, and underscores have been stripped
s.chars()
.filter(|c| *c != '-' && *c != '.')
.take_while(|c| *c != 'e' && *c != 'E')
.fold(0, |count, c| {
// leading zeros
if c == '0' && count == 0 {
count
} else {
count + 1
}
})
}
enum FloatFormat {
LowerExp,
UpperExp,
Normal,
}
impl FloatFormat {
#[must_use]
fn new(s: &str) -> Self {
s.chars()
.find_map(|x| match x {
'e' => Some(Self::LowerExp),
'E' => Some(Self::UpperExp),
_ => None,
})
.unwrap_or(Self::Normal)
}
fn format<T>(&self, f: T) -> String
where
T: fmt::UpperExp + fmt::LowerExp + fmt::Display,
{
match self {
Self::LowerExp => format!("{:e}", f),
Self::UpperExp => format!("{:E}", f),
Self::Normal => format!("{}", f),
}
}
}