new lint: large_stack_frames

clippy_lints/src/large_stack_frames.rs

@@ -0,0 +1,162 @@
+use std::ops::AddAssign;
+
+use clippy_utils::diagnostics::span_lint_and_then;
+use rustc_hir::def_id::LocalDefId;
+use rustc_hir::intravisit::FnKind;
+use rustc_hir::Body;
+use rustc_hir::FnDecl;
+use rustc_lint::{LateContext, LateLintPass};
+use rustc_session::declare_tool_lint;
+use rustc_session::impl_lint_pass;
+use rustc_span::Span;
+
+declare_clippy_lint! {
+    /// ### What it does
+    /// Checks for functions that use a lot of stack space.
+    ///
+    /// This often happens when constructing a large type, such as an array with a lot of elements,
+    /// or constructing *many* smaller-but-still-large structs, or copying around a lot of large types.
+    ///
+    /// This lint is a more general version of [`large_stack_arrays`](https://rust-lang.github.io/rust-clippy/master/#large_stack_arrays)
+    /// that is intended to look at functions as a whole instead of only individual array expressions inside of a function.
+    ///
+    /// ### Why is this bad?
+    /// The stack region of memory is very limited in size (usually *much* smaller than the heap) and attempting to
+    /// use too much will result in a stack overflow and crash the program.
+    /// To avoid this, you should consider allocating large types on the heap instead (e.g. by boxing them).
+    ///
+    /// Keep in mind that the code path to construction of large types does not even need to be reachable;
+    /// it purely needs to *exist* inside of the function to contribute to the stack size.
+    /// For example, this causes a stack overflow even though the branch is unreachable (with `-Zmir-opt-level=0`):
+    /// ```rust,ignore
+    /// fn main() {
+    ///     if false {
+    ///         let x = [0u8; 10000000]; // 10 MB stack array
+    ///         black_box(&x);
+    ///     }
+    /// }
+    /// ```
+    ///
+    /// ### Drawbacks
+    /// False positives. The stack size that clippy sees is an estimated value and can be vastly different
+    /// from the actual stack usage after optimizations passes have run (especially true in release mode).
+    /// Modern compilers are very smart and are able to optimize away a lot of unnecessary stack allocations.
+    /// In debug mode however, it is usually more accurate.
+    ///
+    /// This lint works by summing up the size of all locals and comparing them against a (configurable, but high-by-default)
+    /// threshold.
+    /// Note that "locals" in this context refers to [MIR locals](https://rustc-dev-guide.rust-lang.org/mir/index.html#key-mir-vocabulary),
+    /// i.e. real local variables that the user typed, storage for temporary values, function arguments
+    /// and the return value.
+    ///
+    /// ### Example
+    /// This function creates four 500 KB arrays on the stack. Quite big but just small enough to not trigger `large_stack_arrays`.
+    /// However, looking at the function as a whole, it's clear that this uses a lot of stack space.
+    /// ```rust
+    /// struct QuiteLargeType([u8; 500_000]);
+    /// fn foo() {
+    ///     // ... some function that uses a lot of stack space ...
+    ///     let _x1 = QuiteLargeType([0; 500_000]);
+    ///     let _x2 = QuiteLargeType([0; 500_000]);
+    ///     let _x3 = QuiteLargeType([0; 500_000]);
+    ///     let _x4 = QuiteLargeType([0; 500_000]);
+    /// }
+    /// ```
+    ///
+    /// Instead of doing this, allocate the arrays on the heap.
+    /// This currently requires going through a `Vec` first and then converting it to a `Box`:
+    /// ```rust
+    /// struct NotSoLargeType(Box<[u8]>);
+    ///
+    /// fn foo() {
+    ///     let _x1 = NotSoLargeType(vec![0; 500_000].into_boxed_slice());
+    /// //                           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^  Now heap allocated.
+    /// //                                                                The size of `NotSoLargeType` is 16 bytes.
+    /// //  ...
+    /// }
+    /// ```
+    #[clippy::version = "1.71.0"]
+    pub LARGE_STACK_FRAMES,
+    nursery,
+    "checks for functions that allocate a lot of stack space"
+}
+
+pub struct LargeStackFrames {
+    maximum_allowed_size: u64,
+}
+
+impl LargeStackFrames {
+    #[must_use]
+    pub fn new(size: u64) -> Self {
+        Self {
+            maximum_allowed_size: size,
+        }
+    }
+}
+
+impl_lint_pass!(LargeStackFrames => [LARGE_STACK_FRAMES]);
+
+#[derive(Copy, Clone)]
+enum Space {
+    Used(u64),
+    Overflow,
+}
+
+impl Space {
+    pub fn exceeds_limit(self, limit: u64) -> bool {
+        match self {
+            Self::Used(used) => used > limit,
+            Self::Overflow => true,
+        }
+    }
+}
+
+impl AddAssign<u64> for Space {
+    fn add_assign(&mut self, rhs: u64) {
+        if let Self::Used(lhs) = self {
+            match lhs.checked_add(rhs) {
+                Some(sum) => *self = Self::Used(sum),
+                None => {
+                    *self = Self::Overflow;
+                },
+            }
+        }
+    }
+}
+
+impl<'tcx> LateLintPass<'tcx> for LargeStackFrames {
+    fn check_fn(
+        &mut self,
+        cx: &LateContext<'tcx>,
+        _: FnKind<'tcx>,
+        _: &'tcx FnDecl<'tcx>,
+        _: &'tcx Body<'tcx>,
+        span: Span,
+        local_def_id: LocalDefId,
+    ) {
+        let def_id = local_def_id.to_def_id();
+
+        let mir = cx.tcx.optimized_mir(def_id);
+        let param_env = cx.tcx.param_env(def_id);
+
+        let mut frame_size = Space::Used(0);
+
+        for local in &mir.local_decls {
+            if let Ok(layout) = cx.tcx.layout_of(param_env.and(local.ty)) {
+                frame_size += layout.size.bytes();
+            }
+        }
+
+        if frame_size.exceeds_limit(self.maximum_allowed_size) {
+            span_lint_and_then(
+                cx,
+                LARGE_STACK_FRAMES,
+                span,
+                "this function allocates a large amount of stack space",
+                |diag| {
+                    diag.note("allocating large amounts of stack space can overflow the stack");
+                },
+            );
+        }
+    }
+}