Auto merge of rust-lang#10942 - Centri3:unnecessary_cast, r=llogiq

Ignore more type aliases in `unnecessary_cast`

This is potentially the worst code I've ever written, and even if not, it's very close to being on par with starb. This will ignore `call() as i32` and `local_obtained_from_call as i32` now.

This should fix every reasonable way to reproduce rust-lang#10555, but likely not entirely.

changelog: Ignore more type aliases in `unnecessary_cast`

Author: bors

clippy_lints/src/casts/unnecessary_cast.rs

@@ -1,15 +1,17 @@
 use clippy_utils::diagnostics::span_lint_and_sugg;
 use clippy_utils::numeric_literal::NumericLiteral;
 use clippy_utils::source::snippet_opt;
-use clippy_utils::{get_parent_expr, is_hir_ty_cfg_dependant, is_ty_alias, path_to_local};
+use clippy_utils::visitors::{for_each_expr, Visitable};
+use clippy_utils::{get_parent_expr, get_parent_node, is_hir_ty_cfg_dependant, is_ty_alias, path_to_local};
 use if_chain::if_chain;
 use rustc_ast::{LitFloatType, LitIntType, LitKind};
 use rustc_errors::Applicability;
-use rustc_hir::def::Res;
+use rustc_hir::def::{DefKind, Res};
 use rustc_hir::{Expr, ExprKind, Lit, Node, Path, QPath, TyKind, UnOp};
 use rustc_lint::{LateContext, LintContext};
 use rustc_middle::lint::in_external_macro;
 use rustc_middle::ty::{self, FloatTy, InferTy, Ty};
+use std::ops::ControlFlow;
 
 use super::UNNECESSARY_CAST;
 
@@ -59,7 +61,7 @@ pub(super) fn check<'tcx>(
         }
     }
 
-    // skip cast of local to type alias
+    // skip cast of local that is a type alias
     if let ExprKind::Cast(inner, ..) = expr.kind
         && let ExprKind::Path(qpath) = inner.kind
         && let QPath::Resolved(None, Path { res, .. }) = qpath
@@ -83,6 +85,11 @@ pub(super) fn check<'tcx>(
         }
     }
 
+    // skip cast of fn call that returns type alias
+    if let ExprKind::Cast(inner, ..) = expr.kind && is_cast_from_ty_alias(cx, inner, cast_from) {
+        return false;
+    }
+
     // skip cast to non-primitive type
     if_chain! {
         if let ExprKind::Cast(_, cast_to) = expr.kind;
@@ -223,3 +230,61 @@ fn fp_ty_mantissa_nbits(typ: Ty<'_>) -> u32 {
         _ => 0,
     }
 }
+
+/// Finds whether an `Expr` returns a type alias.
+///
+/// TODO: Maybe we should move this to `clippy_utils` so others won't need to go down this dark,
+/// dark path reimplementing this (or something similar).
+fn is_cast_from_ty_alias<'tcx>(cx: &LateContext<'tcx>, expr: impl Visitable<'tcx>, cast_from: Ty<'tcx>) -> bool {
+    for_each_expr(expr, |expr| {
+        // Calls are a `Path`, and usage of locals are a `Path`. So, this checks
+        // - call() as i32
+        // - local as i32
+        if let ExprKind::Path(qpath) = expr.kind {
+            let res = cx.qpath_res(&qpath, expr.hir_id);
+            // Function call
+            if let Res::Def(DefKind::Fn, def_id) = res {
+                let Some(snippet) = snippet_opt(cx, cx.tcx.def_span(def_id)) else {
+                    return ControlFlow::Continue(());
+                };
+                // This is the worst part of this entire function. This is the only way I know of to
+                // check whether a function returns a type alias. Sure, you can get the return type
+                // from a function in the current crate as an hir ty, but how do you get it for
+                // external functions?? Simple: It's impossible. So, we check whether a part of the
+                // function's declaration snippet is exactly equal to the `Ty`. That way, we can
+                // see whether it's a type alias.
+                //
+                // Will this work for more complex types? Probably not!
+                if !snippet
+                    .split("->")
+                    .skip(0)
+                    .map(|s| {
+                        s.trim() == cast_from.to_string()
+                            || s.split("where").any(|ty| ty.trim() == cast_from.to_string())
+                    })
+                    .any(|a| a)
+                {
+                    return ControlFlow::Break(());
+                }
+            // Local usage
+            } else if let Res::Local(hir_id) = res
+                && let Some(parent) = get_parent_node(cx.tcx, hir_id)
+                && let Node::Local(l) = parent
+            {
+                if let Some(e) = l.init && is_cast_from_ty_alias(cx, e, cast_from) {
+                    return ControlFlow::Break::<()>(());
+                }
+
+                if let Some(ty) = l.ty
+                    && let TyKind::Path(qpath) = ty.kind
+                    && is_ty_alias(&qpath)
+                {
+                    return ControlFlow::Break::<()>(());
+                }
+            }
+        }
+
+        ControlFlow::Continue(())
+    })
+    .is_some()
+}

tests/ui/auxiliary/extern_fake_libc.rs

@@ -0,0 +1,10 @@
+#![allow(nonstandard_style)]
+#![allow(clippy::missing_safety_doc, unused)]
+
+type pid_t = i32;
+pub unsafe fn getpid() -> pid_t {
+    pid_t::from(0)
+}
+pub fn getpid_SAFE_TRUTH() -> pid_t {
+    unsafe { getpid() }
+}

tests/ui/unnecessary_cast.fixed

@@ -1,13 +1,17 @@
 //@run-rustfix
+//@aux-build:extern_fake_libc.rs
 #![warn(clippy::unnecessary_cast)]
 #![allow(
-    unused,
     clippy::borrow_as_ptr,
     clippy::no_effect,
     clippy::nonstandard_macro_braces,
-    clippy::unnecessary_operation
+    clippy::unnecessary_operation,
+    nonstandard_style,
+    unused
 )]
 
+extern crate extern_fake_libc;
+
 type PtrConstU8 = *const u8;
 type PtrMutU8 = *mut u8;
 
@@ -19,6 +23,21 @@ fn uwu<T, U>(ptr: *const T) -> *const U {
     ptr as *const U
 }
 
+mod fake_libc {
+    type pid_t = i32;
+    pub unsafe fn getpid() -> pid_t {
+        pid_t::from(0)
+    }
+    // Make sure a where clause does not break it
+    pub fn getpid_SAFE_TRUTH<T: Clone>(t: &T) -> pid_t
+    where
+        T: Clone,
+    {
+        t;
+        unsafe { getpid() }
+    }
+}
+
 #[rustfmt::skip]
 fn main() {
     // Test cast_unnecessary
@@ -82,6 +101,10 @@ fn main() {
     // or from
     let x: I32Alias = 1;
     let y = x as u64;
+    fake_libc::getpid_SAFE_TRUTH(&0u32) as i32;
+    extern_fake_libc::getpid_SAFE_TRUTH() as i32;
+    let pid = unsafe { fake_libc::getpid() };
+    pid as i32;
 
     let i8_ptr: *const i8 = &1;
     let u8_ptr: *const u8 = &1;

tests/ui/unnecessary_cast.rs

@@ -1,13 +1,17 @@
 //@run-rustfix
+//@aux-build:extern_fake_libc.rs
 #![warn(clippy::unnecessary_cast)]
 #![allow(
-    unused,
     clippy::borrow_as_ptr,
     clippy::no_effect,
     clippy::nonstandard_macro_braces,
-    clippy::unnecessary_operation
+    clippy::unnecessary_operation,
+    nonstandard_style,
+    unused
 )]
 
+extern crate extern_fake_libc;
+
 type PtrConstU8 = *const u8;
 type PtrMutU8 = *mut u8;
 
@@ -19,6 +23,21 @@ fn uwu<T, U>(ptr: *const T) -> *const U {
     ptr as *const U
 }
 
+mod fake_libc {
+    type pid_t = i32;
+    pub unsafe fn getpid() -> pid_t {
+        pid_t::from(0)
+    }
+    // Make sure a where clause does not break it
+    pub fn getpid_SAFE_TRUTH<T: Clone>(t: &T) -> pid_t
+    where
+        T: Clone,
+    {
+        t;
+        unsafe { getpid() }
+    }
+}
+
 #[rustfmt::skip]
 fn main() {
     // Test cast_unnecessary
@@ -82,6 +101,10 @@ fn main() {
     // or from
     let x: I32Alias = 1;
     let y = x as u64;
+    fake_libc::getpid_SAFE_TRUTH(&0u32) as i32;
+    extern_fake_libc::getpid_SAFE_TRUTH() as i32;
+    let pid = unsafe { fake_libc::getpid() };
+    pid as i32;
 
     let i8_ptr: *const i8 = &1;
     let u8_ptr: *const u8 = &1;