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| <h1 id="panicking-in-rust"><a class="header" href="#panicking-in-rust">Panicking in Rust</a></h1> |
| <ul> |
| <li><a href="#step-1-invocation-of-the-panic-macro">Step 1: Invocation of the <code>panic!</code> macro.</a> |
| <ul> |
| <li><a href="#core-definition-of-panic">core definition of panic!</a></li> |
| <li><a href="#std-implementation-of-panic">std implementation of panic!</a></li> |
| </ul> |
| </li> |
| <li><a href="#step-2-the-panic-runtime">Step 2: The panic runtime</a></li> |
| </ul> |
| <h2 id="step-1-invocation-of-the-panic-macro"><a class="header" href="#step-1-invocation-of-the-panic-macro">Step 1: Invocation of the <code>panic!</code> macro.</a></h2> |
| <p>There are actually two panic macros - one defined in <code>core</code>, and one defined in <code>std</code>. |
| This is due to the fact that code in <code>core</code> can panic. <code>core</code> is built before <code>std</code>, |
| but we want panics to use the same machinery at runtime, whether they originate in <code>core</code> |
| or <code>std</code>.</p> |
| <h3 id="core-definition-of-panic"><a class="header" href="#core-definition-of-panic">core definition of panic!</a></h3> |
| <p>The <code>core</code> <code>panic!</code> macro eventually makes the following call (in <code>library/core/src/panicking.rs</code>):</p> |
| <pre><pre class="playground"><code class="language-rust"><span class="boring">#![allow(unused)] |
| </span><span class="boring">fn main() { |
| </span>// NOTE This function never crosses the FFI boundary; it's a Rust-to-Rust call |
| extern "Rust" { |
| #[lang = "panic_impl"] |
| fn panic_impl(pi: &PanicInfo<'_>) -> !; |
| } |
| |
| let pi = PanicInfo::internal_constructor(Some(&fmt), location); |
| unsafe { panic_impl(&pi) } |
| <span class="boring">}</span></code></pre></pre> |
| <p>Actually resolving this goes through several layers of indirection:</p> |
| <ol> |
| <li> |
| <p>In <code>compiler/rustc_middle/src/middle/weak_lang_items.rs</code>, <code>panic_impl</code> is |
| declared as 'weak lang item', with the symbol <code>rust_begin_unwind</code>. This is |
| used in <code>rustc_hir_analysis/src/collect.rs</code> to set the actual symbol name to |
| <code>rust_begin_unwind</code>.</p> |
| <p>Note that <code>panic_impl</code> is declared in an <code>extern "Rust"</code> block, |
| which means that core will attempt to call a foreign symbol called <code>rust_begin_unwind</code> |
| (to be resolved at link time)</p> |
| </li> |
| <li> |
| <p>In <code>library/std/src/panicking.rs</code>, we have this definition:</p> |
| </li> |
| </ol> |
| <pre><pre class="playground"><code class="language-rust"><span class="boring">#![allow(unused)] |
| </span><span class="boring">fn main() { |
| </span>/// Entry point of panic from the core crate. |
| #[cfg(not(test))] |
| #[panic_handler] |
| #[unwind(allowed)] |
| pub fn begin_panic_handler(info: &PanicInfo<'_>) -> ! { |
| ... |
| } |
| <span class="boring">}</span></code></pre></pre> |
| <p>The special <code>panic_handler</code> attribute is resolved via <code>compiler/rustc_middle/src/middle/lang_items</code>. |
| The <code>extract</code> function converts the <code>panic_handler</code> attribute to a <code>panic_impl</code> lang item.</p> |
| <p>Now, we have a matching <code>panic_handler</code> lang item in the <code>std</code>. This function goes |
| through the same process as the <code>extern { fn panic_impl }</code> definition in <code>core</code>, ending |
| up with a symbol name of <code>rust_begin_unwind</code>. At link time, the symbol reference in <code>core</code> |
| will be resolved to the definition of <code>std</code> (the function called <code>begin_panic_handler</code> in the |
| Rust source).</p> |
| <p>Thus, control flow will pass from core to std at runtime. This allows panics from <code>core</code> |
| to go through the same infrastructure that other panics use (panic hooks, unwinding, etc)</p> |
| <h3 id="std-implementation-of-panic"><a class="header" href="#std-implementation-of-panic">std implementation of panic!</a></h3> |
| <p>This is where the actual panic-related logic begins. In <code>library/std/src/panicking.rs</code>, |
| control passes to <code>rust_panic_with_hook</code>. This method is responsible |
| for invoking the global panic hook, and checking for double panics. Finally, |
| we call <code>__rust_start_panic</code>, which is provided by the panic runtime.</p> |
| <p>The call to <code>__rust_start_panic</code> is very weird - it is passed a <code>*mut &mut dyn PanicPayload</code>, |
| converted to an <code>usize</code>. Let's break this type down:</p> |
| <ol> |
| <li> |
| <p><code>PanicPayload</code> is an internal trait. It is implemented for <code>PanicPayload</code> |
| (a wrapper around the user-supplied payload type), and has a method |
| <code>fn take_box(&mut self) -> *mut (dyn Any + Send)</code>. |
| This method takes the user-provided payload (<code>T: Any + Send</code>), |
| boxes it, and converts the box to a raw pointer.</p> |
| </li> |
| <li> |
| <p>When we call <code>__rust_start_panic</code>, we have an <code>&mut dyn PanicPayload</code>. |
| However, this is a fat pointer (twice the size of a <code>usize</code>). |
| To pass this to the panic runtime across an FFI boundary, we take a mutable |
| reference <em>to this mutable reference</em> (<code>&mut &mut dyn PanicPayload</code>), and convert it to a raw |
| pointer (<code>*mut &mut dyn PanicPayload</code>). The outer raw pointer is a thin pointer, since it points to |
| a <code>Sized</code> type (a mutable reference). Therefore, we can convert this thin pointer into a <code>usize</code>, |
| which is suitable for passing across an FFI boundary.</p> |
| </li> |
| </ol> |
| <p>Finally, we call <code>__rust_start_panic</code> with this <code>usize</code>. We have now entered the panic runtime.</p> |
| <h2 id="step-2-the-panic-runtime"><a class="header" href="#step-2-the-panic-runtime">Step 2: The panic runtime</a></h2> |
| <p>Rust provides two panic runtimes: <code>panic_abort</code> and <code>panic_unwind</code>. The user chooses |
| between them at build time via their <code>Cargo.toml</code></p> |
| <p><code>panic_abort</code> is extremely simple: its implementation of <code>__rust_start_panic</code> just aborts, |
| as you would expect.</p> |
| <p><code>panic_unwind</code> is the more interesting case.</p> |
| <p>In its implementation of <code>__rust_start_panic</code>, we take the <code>usize</code>, convert |
| it back to a <code>*mut &mut dyn PanicPayload</code>, dereference it, and call <code>take_box</code> |
| on the <code>&mut dyn PanicPayload</code>. At this point, we have a raw pointer to the payload |
| itself (a <code>*mut (dyn Send + Any)</code>): that is, a raw pointer to the actual value |
| provided by the user who called <code>panic!</code>.</p> |
| <p>At this point, the platform-independent code ends. We now call into |
| platform-specific unwinding logic (e.g <code>unwind</code>). This code is |
| responsible for unwinding the stack, running any 'landing pads' associated |
| with each frame (currently, running destructors), and transferring control |
| to the <code>catch_unwind</code> frame.</p> |
| <p>Note that all panics either abort the process or get caught by some call to <code>catch_unwind</code>. |
| In particular, in std's <a href="https://github.com/rust-lang/rust/blob/master/library/std/src/rt.rs">runtime service</a>, |
| the call to the user-provided <code>main</code> function is wrapped in <code>catch_unwind</code>.</p> |
| |
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