New excessive precision lint for floats

clippy_lints/src/excessive_precision.rs

@@ -0,0 +1,141 @@
+use rustc::hir;
+use rustc::lint::*;
+use rustc::ty::TypeVariants;
+use std::f32;
+use std::f64;
+use std::fmt;
+use syntax::ast::*;
+use syntax_pos::symbol::Symbol;
+use utils::span_lint_and_sugg;
+
+/// **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
+/// Insert a short example of code that triggers the lint
+///    let v: f32 = 0.123_456_789_9;
+///    println!("{}", v); //  0.123_456_789
+///
+/// // Good
+/// Insert a short example of improved code that doesn't trigger the lint
+///    let v: f64 = 0.123_456_789_9;
+///    println!("{}", v); //  0.123_456_789_9
+/// ```
+declare_clippy_lint! {
+    pub EXCESSIVE_PRECISION,
+    style,
+    "excessive precision for float literal"
+}
+
+pub struct ExcessivePrecision;
+
+impl LintPass for ExcessivePrecision {
+    fn get_lints(&self) -> LintArray {
+        lint_array!(EXCESSIVE_PRECISION)
+    }
+}
+
+impl<'a, 'tcx> LateLintPass<'a, 'tcx> for ExcessivePrecision {
+    fn check_expr(&mut self, cx: &LateContext<'a, 'tcx>, expr: &'tcx hir::Expr) {
+        if_chain! {
+            let ty = cx.tables.expr_ty(expr);
+            if let TypeVariants::TyFloat(ref fty) = ty.sty;
+            if let hir::ExprLit(ref lit) = expr.node;
+            if let LitKind::Float(ref sym, _) | LitKind::FloatUnsuffixed(ref sym) = lit.node;
+            if let Some(sugg) = self.check(sym, fty);
+            then {
+                span_lint_and_sugg(
+                    cx,
+                    EXCESSIVE_PRECISION,
+                    expr.span,
+                    "float has excessive precision",
+                    "consider changing the type or truncating it to",
+                    sugg,
+                );
+            }
+        }
+    }
+}
+
+impl ExcessivePrecision {
+    // None if nothing to lint, Some(suggestion) if lint neccessary
+    fn check(&self, sym: &Symbol, fty: &FloatTy) -> Option<String> {
+        let max = max_digits(fty);
+        let sym_str = sym.as_str();
+        let formatter = FloatFormat::new(&sym_str);
+        let digits = count_digits(&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.
+        if digits > max as usize {
+            let sr = match *fty {
+                FloatTy::F32 => sym_str.parse::<f32>().map(|f| formatter.format(f)),
+                FloatTy::F64 => sym_str.parse::<f64>().map(|f| formatter.format(f)),
+            };
+            // We know this will parse since we are in LatePass
+            let s = sr.unwrap();
+
+            if sym_str == s {
+                None
+            } else {
+                Some(s)
+            }
+        } else {
+            None
+        }
+    }
+}
+
+fn max_digits(fty: &FloatTy) -> u32 {
+    match fty {
+        FloatTy::F32 => f32::DIGITS,
+        FloatTy::F64 => f64::DIGITS,
+    }
+}
+
+fn count_digits(s: &str) -> usize {
+    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 {
+    fn new(s: &str) -> Self {
+        s.chars()
+            .find_map(|x| match x {
+                'e' => Some(FloatFormat::LowerExp),
+                'E' => Some(FloatFormat::UpperExp),
+                _ => None,
+            })
+            .unwrap_or(FloatFormat::Normal)
+    }
+    fn format<T>(&self, f: T) -> String
+    where T: fmt::UpperExp + fmt::LowerExp + fmt::Display {
+        match self {
+            FloatFormat::LowerExp => format!("{:e}", f),
+            FloatFormat::UpperExp => format!("{:E}", f),
+            FloatFormat::Normal => format!("{}", f),
+        }
+    }
+}

clippy_lints/src/lib.rs

@@ -13,6 +13,8 @@
 // FIXME(mark-i-m) remove after i128 stablization merges
 #![allow(stable_features)]
 #![feature(i128, i128_type)]
+#![feature(iterator_find_map)]
+
 
 #[macro_use]
 extern crate rustc;
@@ -124,6 +126,7 @@ pub mod erasing_op;
 pub mod escape;
 pub mod eta_reduction;
 pub mod eval_order_dependence;
+pub mod excessive_precision;
 pub mod explicit_write;
 pub mod fallible_impl_from;
 pub mod format;
@@ -294,6 +297,7 @@ pub fn register_plugins(reg: &mut rustc_plugin::Registry) {
     reg.register_early_lint_pass(box enum_variants::EnumVariantNames::new(conf.enum_variant_name_threshold));
     reg.register_late_lint_pass(box enum_glob_use::EnumGlobUse);
     reg.register_late_lint_pass(box enum_clike::UnportableVariant);
+    reg.register_late_lint_pass(box excessive_precision::ExcessivePrecision);
     reg.register_late_lint_pass(box bit_mask::BitMask::new(conf.verbose_bit_mask_threshold));
     reg.register_late_lint_pass(box ptr::PointerPass);
     reg.register_late_lint_pass(box needless_bool::NeedlessBool);

tests/ui/approx_const.rs

@@ -42,7 +42,7 @@ fn main() {
     let my_ln_2 = 0.6931471805599453;
     let no_ln_2 = 0.693;
 
-    let my_log10_e = 0.43429448190325182;
+    let my_log10_e = 0.4342944819032518;
     let no_log10_e = 0.434;
 
     let my_log2_e = 1.4426950408889634;

tests/ui/approx_const.stderr

@@ -87,8 +87,8 @@ error: approximate value of `f{32, 64}::consts::LN_2` found. Consider using it d
 error: approximate value of `f{32, 64}::consts::LOG10_E` found. Consider using it directly
   --> $DIR/approx_const.rs:45:22
    |
-45 |     let my_log10_e = 0.43429448190325182;
-   |                      ^^^^^^^^^^^^^^^^^^^
+45 |     let my_log10_e = 0.4342944819032518;
+   |                      ^^^^^^^^^^^^^^^^^^
 
 error: approximate value of `f{32, 64}::consts::LOG2_E` found. Consider using it directly
   --> $DIR/approx_const.rs:48:21

tests/ui/excessive_precision.rs

@@ -0,0 +1,52 @@
+#![feature(plugin, custom_attribute)]
+#![warn(excessive_precision)]
+#![allow(print_literal)]
+
+fn main() {
+    // TODO add prefix tests
+    // Consts
+    const GOOD32_SUF: f32 = 0.123_456_f32;
+    const GOOD32: f32 = 0.123_456;
+    const GOOD32_SM: f32 = 0.000_000_000_1;
+    const GOOD64: f64 = 0.123_456_789_012;
+    const GOOD64_SM: f32 = 0.000_000_000_000_000_1;
+
+    const BAD32_1: f32 = 0.123_456_789_f32;
+    const BAD32_2: f32 = 0.123_456_789;
+    const BAD32_3: f32 = 0.100_000_000_000_1;
+
+    const BAD64_1: f64 = 0.123_456_789_012_345_67f64;
+    const BAD64_2: f64 = 0.123_456_789_012_345_67;
+    const BAD64_3: f64 = 0.100_000_000_000_000_000_1;
+
+    // Literal
+    println!("{}", 8.888_888_888_888_888_888_888);
+
+    // TODO add inferred type tests for f32
+    // TODO add tests cases exactly on the edge
+    // Locals
+    let good32: f32 = 0.123_456_f32;
+    let good32_2: f32 = 0.123_456;
+
+    let good64: f64 = 0.123_456_789_012f64;
+    let good64: f64 = 0.123_456_789_012;
+    let good64_2 = 0.123_456_789_012;
+
+    let bad32_1: f32 = 1.123_456_789_f32;
+    let bad32_2: f32 = 1.123_456_789;
+
+    let bad64_1: f64 = 0.123_456_789_012_345_67f64;
+    let bad64_2: f64 = 0.123_456_789_012_345_67;
+    let bad64_3 = 0.123_456_789_012_345_67;
+
+    // TODO Vectors / nested vectors
+    let vec32: Vec<f32> = vec![0.123_456_789];
+    let vec64: Vec<f64> = vec![0.123_456_789_123_456_789];
+
+    // Exponential float notation
+    let good_e32: f32 = 1e-10;
+    let bad_e32: f32 = 1.123_456_788_888e-10;
+
+    let good_bige32: f32 = 1E-10;
+    let bad_bige32: f32 = 1.123_456_788_888E-10;
+}

tests/ui/excessive_precision.stderr

@@ -0,0 +1,100 @@
+error: float has excessive precision
+  --> $DIR/excessive_precision.rs:14:26
+   |
+14 |     const BAD32_1: f32 = 0.123_456_789_f32;
+   |                          ^^^^^^^^^^^^^^^^^ help: consider changing the type or truncating it to: `0.12345679`
+   |
+   = note: `-D excessive-precision` implied by `-D warnings`
+
+error: float has excessive precision
+  --> $DIR/excessive_precision.rs:15:26
+   |
+15 |     const BAD32_2: f32 = 0.123_456_789;
+   |                          ^^^^^^^^^^^^^ help: consider changing the type or truncating it to: `0.12345679`
+
+error: float has excessive precision
+  --> $DIR/excessive_precision.rs:16:26
+   |
+16 |     const BAD32_3: f32 = 0.100_000_000_000_1;
+   |                          ^^^^^^^^^^^^^^^^^^^ help: consider changing the type or truncating it to: `0.1`
+
+error: float has excessive precision
+  --> $DIR/excessive_precision.rs:18:26
+   |
+18 |     const BAD64_1: f64 = 0.123_456_789_012_345_67f64;
+   |                          ^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: consider changing the type or truncating it to: `0.12345678901234566`
+
+error: float has excessive precision
+  --> $DIR/excessive_precision.rs:19:26
+   |
+19 |     const BAD64_2: f64 = 0.123_456_789_012_345_67;
+   |                          ^^^^^^^^^^^^^^^^^^^^^^^^ help: consider changing the type or truncating it to: `0.12345678901234566`
+
+error: float has excessive precision
+  --> $DIR/excessive_precision.rs:20:26
+   |
+20 |     const BAD64_3: f64 = 0.100_000_000_000_000_000_1;
+   |                          ^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: consider changing the type or truncating it to: `0.1`
+
+error: float has excessive precision
+  --> $DIR/excessive_precision.rs:23:20
+   |
+23 |     println!("{}", 8.888_888_888_888_888_888_888);
+   |                    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: consider changing the type or truncating it to: `8.88888888888889`
+
+error: float has excessive precision
+  --> $DIR/excessive_precision.rs:35:24
+   |
+35 |     let bad32_1: f32 = 1.123_456_789_f32;
+   |                        ^^^^^^^^^^^^^^^^^ help: consider changing the type or truncating it to: `1.1234568`
+
+error: float has excessive precision
+  --> $DIR/excessive_precision.rs:36:24
+   |
+36 |     let bad32_2: f32 = 1.123_456_789;
+   |                        ^^^^^^^^^^^^^ help: consider changing the type or truncating it to: `1.1234568`
+
+error: float has excessive precision
+  --> $DIR/excessive_precision.rs:38:24
+   |
+38 |     let bad64_1: f64 = 0.123_456_789_012_345_67f64;
+   |                        ^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: consider changing the type or truncating it to: `0.12345678901234566`
+
+error: float has excessive precision
+  --> $DIR/excessive_precision.rs:39:24
+   |
+39 |     let bad64_2: f64 = 0.123_456_789_012_345_67;
+   |                        ^^^^^^^^^^^^^^^^^^^^^^^^ help: consider changing the type or truncating it to: `0.12345678901234566`
+
+error: float has excessive precision
+  --> $DIR/excessive_precision.rs:40:19
+   |
+40 |     let bad64_3 = 0.123_456_789_012_345_67;
+   |                   ^^^^^^^^^^^^^^^^^^^^^^^^ help: consider changing the type or truncating it to: `0.12345678901234566`
+
+error: float has excessive precision
+  --> $DIR/excessive_precision.rs:43:32
+   |
+43 |     let vec32: Vec<f32> = vec![0.123_456_789];
+   |                                ^^^^^^^^^^^^^ help: consider changing the type or truncating it to: `0.12345679`
+
+error: float has excessive precision
+  --> $DIR/excessive_precision.rs:44:32
+   |
+44 |     let vec64: Vec<f64> = vec![0.123_456_789_123_456_789];
+   |                                ^^^^^^^^^^^^^^^^^^^^^^^^^ help: consider changing the type or truncating it to: `0.12345678912345678`
+
+error: float has excessive precision
+  --> $DIR/excessive_precision.rs:48:24
+   |
+48 |     let bad_e32: f32 = 1.123_456_788_888e-10;
+   |                        ^^^^^^^^^^^^^^^^^^^^^ help: consider changing the type or truncating it to: `1.1234568e-10`
+
+error: float has excessive precision
+  --> $DIR/excessive_precision.rs:51:27
+   |
+51 |     let bad_bige32: f32 = 1.123_456_788_888E-10;
+   |                           ^^^^^^^^^^^^^^^^^^^^^ help: consider changing the type or truncating it to: `1.1234568E-10`
+
+error: aborting due to 16 previous errors
+