New lint: default_numeric_fallback

CHANGELOG.md

@@ -1909,6 +1909,7 @@ Released 2018-09-13
 [`debug_assert_with_mut_call`]: https://rust-lang.github.io/rust-clippy/master/index.html#debug_assert_with_mut_call
 [`decimal_literal_representation`]: https://rust-lang.github.io/rust-clippy/master/index.html#decimal_literal_representation
 [`declare_interior_mutable_const`]: https://rust-lang.github.io/rust-clippy/master/index.html#declare_interior_mutable_const
+[`default_numeric_fallback`]: https://rust-lang.github.io/rust-clippy/master/index.html#default_numeric_fallback
 [`default_trait_access`]: https://rust-lang.github.io/rust-clippy/master/index.html#default_trait_access
 [`deprecated_cfg_attr`]: https://rust-lang.github.io/rust-clippy/master/index.html#deprecated_cfg_attr
 [`deprecated_semver`]: https://rust-lang.github.io/rust-clippy/master/index.html#deprecated_semver

clippy_lints/src/default_numeric_fallback.rs

@@ -0,0 +1,62 @@
+use rustc_ast::ast::{LitFloatType, LitIntType, LitKind};
+use rustc_hir::{Expr, ExprKind};
+use rustc_lint::{LateContext, LateLintPass};
+use rustc_middle::ty;
+use rustc_session::{declare_lint_pass, declare_tool_lint};
+
+use crate::utils::span_lint_and_help;
+
+declare_clippy_lint! {
+    /// **What it does:** Checks for usage of unconstrained numeric literals which may cause default numeric fallback in type
+    /// inference.
+    ///
+    /// Default numeric fallback means that if numeric types have not yet been bound to concrete
+    /// types at the end of type inference, then integer type is bound to `i32`, and similarly
+    /// floating type is bound to `f64`.
+    ///
+    /// See [RFC0212](https://github.com/rust-lang/rfcs/blob/master/text/0212-restore-int-fallback.md) for more information about the fallback.
+    ///
+    /// **Why is this bad?** For those who are very careful about types, default numeric fallback
+    /// may be a pitfall that cause unexpected runtime behavior.
+    ///
+    /// **Known problems:** None.
+    ///
+    /// **Example:**
+    /// ```rust
+    /// let i = 10;
+    /// let f = 1.23;
+    /// ```
+    ///
+    /// Use instead:
+    /// ```rust
+    /// let i = 10i32;
+    /// let f = 1.23f64;
+    /// ```
+    pub DEFAULT_NUMERIC_FALLBACK,
+    restriction,
+    "usage of unconstrained numeric literals which may cause default numeric fallback."
+}
+
+declare_lint_pass!(DefaultNumericFallback => [DEFAULT_NUMERIC_FALLBACK]);
+
+impl LateLintPass<'_> for DefaultNumericFallback {
+    fn check_expr(&mut self, cx: &LateContext<'tcx>, expr: &'tcx Expr<'_>) {
+        let lit = match &expr.kind {
+            ExprKind::Lit(lit) => lit,
+            _ => return,
+        };
+
+        match (&lit.node, cx.typeck_results().expr_ty(expr).kind()) {
+            (LitKind::Int(_, LitIntType::Unsuffixed), ty::Int(ty::IntTy::I32))
+            | (LitKind::Float(_, LitFloatType::Unsuffixed), ty::Float(ty::FloatTy::F64)) => span_lint_and_help(
+                cx,
+                DEFAULT_NUMERIC_FALLBACK,
+                lit.span,
+                "default numeric fallback may occured",
+                None,
+                "Consider adding suffix to avoid default numeric fallback",
+            ),
+            _ => {},
+        }
+    }
+}

clippy_lints/src/lib.rs

@@ -181,6 +181,7 @@ mod copy_iterator;
 mod create_dir;
 mod dbg_macro;
 mod default;
+mod default_numeric_fallback;
 mod dereference;
 mod derive;
 mod disallowed_method;
@@ -584,6 +585,7 @@ pub fn register_plugins(store: &mut rustc_lint::LintStore, sess: &Session, conf:
         &dbg_macro::DBG_MACRO,
         &default::DEFAULT_TRAIT_ACCESS,
         &default::FIELD_REASSIGN_WITH_DEFAULT,
+        &default_numeric_fallback::DEFAULT_NUMERIC_FALLBACK,
         &dereference::EXPLICIT_DEREF_METHODS,
         &derive::DERIVE_HASH_XOR_EQ,
         &derive::DERIVE_ORD_XOR_PARTIAL_ORD,
@@ -1025,6 +1027,7 @@ pub fn register_plugins(store: &mut rustc_lint::LintStore, sess: &Session, conf:
     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 default_numeric_fallback::DefaultNumericFallback);
 
     let msrv = conf.msrv.as_ref().and_then(|s| {
         parse_msrv(s, None, None).or_else(|| {
@@ -1257,6 +1260,7 @@ pub fn register_plugins(store: &mut rustc_lint::LintStore, sess: &Session, conf:
         LintId::of(&asm_syntax::INLINE_ASM_X86_INTEL_SYNTAX),
         LintId::of(&create_dir::CREATE_DIR),
         LintId::of(&dbg_macro::DBG_MACRO),
+        LintId::of(&default_numeric_fallback::DEFAULT_NUMERIC_FALLBACK),
         LintId::of(&else_if_without_else::ELSE_IF_WITHOUT_ELSE),
         LintId::of(&exhaustive_items::EXHAUSTIVE_ENUMS),
         LintId::of(&exhaustive_items::EXHAUSTIVE_STRUCTS),

tests/ui/default_numeric_fallback.rs

@@ -0,0 +1,24 @@
+#![warn(clippy::default_numeric_fallback)]
+#![allow(unused)]
+
+fn foo_mono(x: u32) -> u32 {
+    x
+}
+
+fn foo_poly<T>(t: T) -> T {
+    t
+}
+
+fn main() {
+    let i = 23;
+    let f = 1.23;
+    foo_poly(10);
+    for _ in 0..10 {}
+
+    let x = 23i32;
+    let f = 1.23f64;
+    for i in 0..10 {
+        let _ = 10usize + i;
+    }
+    foo_mono(10);
+}

tests/ui/default_numeric_fallback.stderr

@@ -0,0 +1,43 @@
+error: default numeric fallback may occured
+  --> $DIR/default_numeric_fallback.rs:13:13
+   |
+LL |     let i = 23;
+   |             ^^
+   |
+   = note: `-D clippy::default-numeric-fallback` implied by `-D warnings`
+   = help: Consider adding suffix to avoid default numeric fallback
+
+error: default numeric fallback may occured
+  --> $DIR/default_numeric_fallback.rs:14:13
+   |
+LL |     let f = 1.23;
+   |             ^^^^
+   |
+   = help: Consider adding suffix to avoid default numeric fallback
+
+error: default numeric fallback may occured
+  --> $DIR/default_numeric_fallback.rs:15:14
+   |
+LL |     foo_poly(10);
+   |              ^^
+   |
+   = help: Consider adding suffix to avoid default numeric fallback
+
+error: default numeric fallback may occured
+  --> $DIR/default_numeric_fallback.rs:16:14
+   |
+LL |     for _ in 0..10 {}
+   |              ^
+   |
+   = help: Consider adding suffix to avoid default numeric fallback
+
+error: default numeric fallback may occured
+  --> $DIR/default_numeric_fallback.rs:16:17
+   |
+LL |     for _ in 0..10 {}
+   |                 ^^
+   |
+   = help: Consider adding suffix to avoid default numeric fallback
+
+error: aborting due to 5 previous errors
+