Merge pull request #1226 from BatmanAoD/c-unwind-documentation

panic runtime and C-unwind documentation

Author: ehuss

Diff for: src/SUMMARY.md

@@ -114,6 +114,8 @@
     - [Memory allocation and lifetime](memory-allocation-and-lifetime.md)
     - [Variables](variables.md)
 
+- [Panic](panic.md)
+
 - [Linkage](linkage.md)
 
 - [Inline assembly](inline-assembly.md)

Diff for: src/attributes.md

@@ -370,7 +370,7 @@ The following is an index of all built-in attributes.
 [`no_mangle`]: abi.md#the-no_mangle-attribute
 [`no_std`]: names/preludes.md#the-no_std-attribute
 [`non_exhaustive`]: attributes/type_system.md#the-non_exhaustive-attribute
-[`panic_handler`]: runtime.md#the-panic_handler-attribute
+[`panic_handler`]: panic.md#the-panic_handler-attribute
 [`path`]: items/modules.md#the-path-attribute
 [`proc_macro_attribute`]: procedural-macros.md#attribute-macros
 [`proc_macro_derive`]: procedural-macros.md#derive-macros

Diff for: src/attributes/codegen.md

@@ -132,7 +132,7 @@ r[attributes.codegen.target_feature.allowed-positions]
 The `#[target_feature]` attribute is not allowed on the following places:
 
 - [the `main` function][crate.main]
-- a [`panic_handler` function][runtime.panic_handler]
+- a [`panic_handler` function][panic.panic_handler]
 - safe trait methods
 - safe default functions in traits
 

Diff for: src/behavior-considered-undefined.md

@@ -77,7 +77,7 @@ r[undefined.target-feature]
   does not support (see [`target_feature`]), *except* if the platform explicitly documents this to be safe.
 
 r[undefined.call]
-* Calling a function with the wrong call ABI or unwinding from a function with the wrong unwind ABI.
+* Calling a function with the wrong [call ABI][abi], or unwinding past a stack frame that does not allow unwinding (e.g. by calling a `"C-unwind"` function imported or transmuted as a `"C"` function or function pointer).
 
 r[undefined.invalid]
 * Producing an [invalid value][invalid-values]. "Producing" a
@@ -96,6 +96,11 @@ r[undefined.const-transmute-ptr2int]
   'Reinterpreting' refers to loading the pointer value at integer type without a
   cast, e.g. by doing raw pointer casts or using a union.
 
+r[undefined.runtime]
+* Violating assumptions of the Rust runtime. Most assumptions of the Rust runtime are currently not explicitly documented.
+  * For assumptions specifically related to unwinding, see the [panic documentation][unwinding-ffi].
+  * The runtime assumes that a Rust stack frame is not deallocated without executing destructors for local variables owned by the stack frame. This assumption can be violated by C functions like `longjmp`.
+
 > **Note**: Undefined behavior affects the entire program. For example, calling
 > a function in C that exhibits undefined behavior of C means your entire
 > program contains undefined behaviour that can also affect the Rust code. And
@@ -245,6 +250,7 @@ reading uninitialized memory is permitted are inside `union`s and in "padding"
 [`const`]: items/constant-items.md
 [noalias]: http://llvm.org/docs/LangRef.html#noalias
 [pointer aliasing rules]: http://llvm.org/docs/LangRef.html#pointer-aliasing-rules
+[abi]: items/external-blocks.md#abi
 [undef]: http://llvm.org/docs/LangRef.html#undefined-values
 [`target_feature`]: attributes/codegen.md#the-target_feature-attribute
 [`UnsafeCell<U>`]: std::cell::UnsafeCell
@@ -258,5 +264,6 @@ reading uninitialized memory is permitted are inside `union`s and in "padding"
 [project-field]: expressions/field-expr.md
 [project-tuple]: expressions/tuple-expr.md#tuple-indexing-expressions
 [project-slice]: expressions/array-expr.md#array-and-slice-indexing-expressions
+[unwinding-ffi]: panic.md#unwinding-across-ffi-boundaries
 [const-promoted]: destructors.md#constant-promotion
 [lifetime-extended]: destructors.md#temporary-lifetime-extension

Diff for: src/conditional-compilation.md

@@ -304,14 +304,16 @@ r[cfg.panic]
 ### `panic`
 
 r[cfg.panic.general]
-Key-value option set depending on the panic strategy. Note that more values may be added in the future.
+Key-value option set depending on the [panic strategy]. Note that more values may be added in the future.
 
 r[cfg.panic.values]
 Example values:
 
 * `"abort"`
 * `"unwind"`
 
+[panic strategy]: panic.md#panic-strategy
+
 ## Forms of conditional compilation
 
 r[cfg.attr]

Diff for: src/crates-and-source-files.md

@@ -122,13 +122,17 @@ use foo::bar as main;
 <!-- If the previous section needs updating (from "must take no arguments"
   onwards, also update it in the testing.md file -->
 
+r[crate.uncaught-foreign-unwinding]
+### Uncaught foreign unwinding
+
+When a "foreign" unwind (e.g. an exception thrown from C++ code, or a `panic!` in Rust code using a different panic handler) propagates beyond the `main` function, the process will be safely terminated. This may take the form of an abort, in which case it is not guaranteed that any `Drop` calls will be executed, and the error output may be less informative than if the runtime had been terminated by a "native" Rust `panic`.
+
+For more information, see the [panic documentation][panic-docs].
+
 r[crate.no_main]
 ### The `no_main` attribute
 
-
-The *`no_main` [attribute]* may be applied at the crate level to disable
-emitting the `main` symbol for an executable binary. This is useful when some
-other object being linked to defines `main`.
+The *`no_main` [attribute]* may be applied at the crate level to disable emitting the `main` symbol for an executable binary. This is useful when some other object being linked to defines `main`.
 
 r[crate.crate_name]
 ## The `crate_name` attribute
@@ -166,6 +170,7 @@ or `_` (U+005F) characters.
 [function]: items/functions.md
 [module]: items/modules.md
 [module path]: paths.md
+[panic-docs]: panic.md#unwinding-across-ffi-boundaries
 [shebang]: input-format.md#shebang-removal
 [trait or lifetime bounds]: trait-bounds.md
 [where clauses]: items/generics.md#where-clauses

Diff for: src/destructors.md

@@ -274,7 +274,7 @@ Temporaries are also created to hold the result of operands to an expression
 while the other operands are evaluated. The temporaries are associated to the
 scope of the expression with that operand. Since the temporaries are moved from
 once the expression is evaluated, dropping them has no effect unless one of the
-operands to an expression breaks out of the expression, returns, or panics.
+operands to an expression breaks out of the expression, returns, or [panics][panic].
 
 ```rust
 # struct PrintOnDrop(&'static str);
@@ -425,6 +425,8 @@ let x = (&temp()).use_temp();  // ERROR
 r[destructors.forget]
 ## Not running destructors
 
+r[destructors.manually-suppressing]
+### Manually suppressing destructors
 
 [`std::mem::forget`] can be used to prevent the destructor of a variable from being run,
 and [`std::mem::ManuallyDrop`] provides a wrapper to prevent a
@@ -433,6 +435,15 @@ variable or field from being dropped automatically.
 > Note: Preventing a destructor from being run via [`std::mem::forget`] or other means is safe even if it has a type that isn't `'static`.
 > Besides the places where destructors are guaranteed to run as defined by this document, types may *not* safely rely on a destructor being run for soundness.
 
+r[destructors.process-termination]
+### Process termination without unwinding
+
+There are some ways to terminate the process without [unwinding], in which case destructors will not be run.
+
+The standard library provides [`std::process::exit`] and [`std::process::abort`] to do this explicitly. Additionally, if the [panic handler][panic.panic_handler.std] is set to `abort`, panicking will always terminate the process without destructors being run.
+
+There is one additional case to be aware of: when a panic reaches a [non-unwinding ABI boundary], either no destructors will run, or all destructors up until the ABI boundary will run.
+
 [Assignment]: expressions/operator-expr.md#assignment-expressions
 [binding modes]: patterns.md#binding-modes
 [closure]: types/closure.md
@@ -442,11 +453,14 @@ variable or field from being dropped automatically.
 [initialized]: glossary.md#initialized
 [interior mutability]: interior-mutability.md
 [lazy boolean expression]: expressions/operator-expr.md#lazy-boolean-operators
+[non-unwinding ABI boundary]: items/functions.md#unwinding
+[panic]: panic.md
 [place context]: expressions.md#place-expressions-and-value-expressions
 [promoted]: destructors.md#constant-promotion
 [scrutinee]: glossary.md#scrutinee
 [statement]: statements.md
 [temporary]: expressions.md#temporaries
+[unwinding]: panic.md#unwinding
 [variable]: variables.md
 
 [array]: types/array.md

Diff for: src/expressions/array-expr.md

@@ -83,7 +83,7 @@ Indices are zero-based for arrays and slices.
 
 r[expr.array.index.const]
 Array access is a [constant expression], so bounds can be checked at compile-time with a constant index value.
-Otherwise a check will be performed at run-time that will put the thread in a _panicked state_ if it fails.
+Otherwise a check will be performed at run-time that will put the thread in a [_panicked state_][panic] if it fails.
 
 ```rust,should_panic
 // lint is deny by default.
@@ -115,5 +115,6 @@ The array index expression can be implemented for types other than arrays and sl
 [constant item]: ../items/constant-items.md
 [literal]: ../tokens.md#literals
 [memory location]: ../expressions.md#place-expressions-and-value-expressions
+[panic]: ../panic.md
 [path]: path-expr.md
 [slice]: ../types/slice.md

Diff for: src/items/external-blocks.md

@@ -106,8 +106,7 @@ unsafe extern "stdcall" { }
 ```
 
 r[items.extern.abi.standard]
-There are three ABI strings which are cross-platform, and which all compilers
-are guaranteed to support:
+The following ABI strings are supported on all platforms:
 
 r[items.extern.abi.rust]
 * `unsafe extern "Rust"` -- The default ABI when you write a normal `fn foo()` in any
@@ -122,6 +121,9 @@ r[items.extern.abi.system]
   which case it's `"stdcall"`, or what you should use to link to the Windows
   API itself
 
+r[items.extern.abi.unwind]
+* `extern "C-unwind"` and `extern "system-unwind"` -- identical to `"C"` and `"system"`, respectively, but with [different behavior][unwind-behavior] when the callee unwinds (by panicking or throwing a C++ style exception).
+
 r[items.extern.abi.platform]
 There are also some platform-specific ABI strings:
 
@@ -151,6 +153,17 @@ r[items.extern.abi.thiscall]
 r[items.extern.abi.efiapi]
 * `unsafe extern "efiapi"` -- The ABI used for [UEFI] functions.
 
+r[items.extern.abi.platform-unwind-variants]
+Like `"C"` and `"system"`, most platform-specific ABI strings also have a [corresponding `-unwind` variant][unwind-behavior]; specifically, these are:
+
+* `"aapcs-unwind"`
+* `"cdecl-unwind"`
+* `"fastcall-unwind"`
+* `"stdcall-unwind"`
+* `"sysv64-unwind"`
+* `"thiscall-unwind"`
+* `"win64-unwind"`
+
 r[items.extern.variadic]
 ## Variadic functions
 
@@ -428,10 +441,9 @@ Attributes on extern function parameters follow the same rules and
 restrictions as [regular function parameters].
 
 [IDENTIFIER]: ../identifiers.md
+[PE Format]: https://learn.microsoft.com/windows/win32/debug/pe-format#import-name-type
 [UEFI]: https://uefi.org/specifications
 [WebAssembly module]: https://webassembly.github.io/spec/core/syntax/modules.html
-[functions]: functions.md
-[statics]: static-items.md
 [_Abi_]: functions.md
 [_Function_]: functions.md
 [_InnerAttribute_]: ../attributes.md
@@ -441,11 +453,13 @@ restrictions as [regular function parameters].
 [_OuterAttribute_]: ../attributes.md
 [_StaticItem_]: static-items.md
 [_Visibility_]: ../visibility-and-privacy.md
-[attributes]: ../attributes.md
-[regular function parameters]: functions.md#attributes-on-function-parameters
 [`bundle` documentation for rustc]: ../../rustc/command-line-arguments.html#linking-modifiers-bundle
-[`whole-archive` documentation for rustc]: ../../rustc/command-line-arguments.html#linking-modifiers-whole-archive
-[`verbatim` documentation for rustc]: ../../rustc/command-line-arguments.html#linking-modifiers-verbatim
 [`dylib` versus `raw-dylib`]: #dylib-versus-raw-dylib
-[PE Format]: https://learn.microsoft.com/windows/win32/debug/pe-format#import-name-type
+[`verbatim` documentation for rustc]: ../../rustc/command-line-arguments.html#linking-modifiers-verbatim
+[`whole-archive` documentation for rustc]: ../../rustc/command-line-arguments.html#linking-modifiers-whole-archive
+[attributes]: ../attributes.md
+[functions]: functions.md
+[regular function parameters]: functions.md#attributes-on-function-parameters
+[statics]: static-items.md
+[unwind-behavior]: functions.md#unwinding
 [value namespace]: ../names/namespaces.md

Diff for: src/items/functions.md

@@ -254,20 +254,43 @@ let fptr: extern "C" fn() -> i32 = new_i32;
 ```
 
 r[items.fn.extern.unwind]
-Functions with an ABI that differs from `"Rust"` do not support unwinding in the
-exact same way that Rust does. Therefore, unwinding past the end of functions
-with such ABIs causes the process to abort.
+### Unwinding
 
-> **Note**: The LLVM backend of the `rustc` implementation
-aborts the process by executing an illegal instruction.
+r[items.fn.extern.unwind.intro]
+Most ABI strings come in two variants, one with an `-unwind` suffix and one without. The `Rust` ABI always permits unwinding, so there is no `Rust-unwind` ABI. The choice of ABI, together with the runtime [panic handler], determines the behavior when unwinding out of a function.
+
+r[items.fn.extern.unwind.behavior]
+The table below indicates the behavior of an unwinding operation reaching each type of ABI boundary (function declaration or definition using the corresponding ABI string). Note that the Rust runtime is not affected by, and cannot have an effect on, any unwinding that occurs entirely within another language's runtime, that is, unwinds that are thrown and caught without reaching a Rust ABI boundary.
+
+The `panic`-unwind column refers to [panicking] via the `panic!` macro and similar standard library mechanisms, as well as to any other Rust operations that cause a panic, such as out-of-bounds array indexing or integer overflow.
+
+The "unwinding" ABI category refers to `"Rust"` (the implicit ABI of Rust functions not marked `extern`), `"C-unwind"`, and any other ABI with `-unwind` in its name. The "non-unwinding" ABI category refers to all other ABI strings, including `"C"` and `"stdcall"`.
+
+Native unwinding is defined per-target. On targets that support throwing and catching C++ exceptions, it refers to the mechanism used to implement this feature. Some platforms implement a form of unwinding referred to as ["forced unwinding"][forced-unwinding]; `longjmp` on Windows and `pthread_exit` in `glibc` are implemented this way. Forced unwinding is explicitly excluded from the "Native unwind" column in the table.
+
+| panic runtime  | ABI           | `panic`-unwind                        | Native unwind (unforced) |
+| -------------- | ------------  | ------------------------------------- | -----------------------  |
+| `panic=unwind` | unwinding     | unwind                                | unwind                   |
+| `panic=unwind` | non-unwinding | abort (see notes below)               | [undefined behavior]     |
+| `panic=abort`  | unwinding     | `panic` aborts without unwinding      | abort                    |
+| `panic=abort`  | non-unwinding | `panic` aborts without unwinding      | [undefined behavior]     |
+
+r[items.fn.extern.abort]
+With `panic=unwind`, when a `panic` is turned into an abort by a non-unwinding ABI boundary, either no destructors (`Drop` calls) will run, or all destructors up until the ABI boundary will run. It is unspecified which of those two behaviors will happen.
+
+For other considerations and limitations regarding unwinding across FFI boundaries, see the [relevant section in the Panic documentation][panic-ffi].
+
+[forced-unwinding]: https://rust-lang.github.io/rfcs/2945-c-unwind-abi.html#forced-unwinding
+[panic handler]: ../panic.md#the-panic_handler-attribute
+[panic-ffi]: ../panic.md#unwinding-across-ffi-boundaries
+[panicking]: ../panic.md
+[undefined behavior]: ../behavior-considered-undefined.md
 
 r[items.fn.const]
 ## Const functions
 
 r[items.fn.const.intro]
-Functions qualified with the `const` keyword are [const functions], as are
-[tuple struct] and [tuple variant] constructors. _Const functions_  can be
-called from within [const contexts].
+Functions qualified with the `const` keyword are [const functions], as are [tuple struct] and [tuple variant] constructors. _Const functions_  can be called from within [const contexts].
 
 r[items.fn.const.extern]
 Const functions may use the [`extern`] function qualifier.

Diff for: src/linkage.md

@@ -253,6 +253,9 @@ RUSTFLAGS='-C target-feature=+crt-static' cargo build --target x86_64-pc-windows
 
 ## Mixed Rust and foreign codebases
 
+r[link.foreign-code]
+
+r[link.foreign-code.foreign-linkers]
 If you are mixing Rust with foreign code (e.g. C, C++) and wish to make a single
 binary containing both types of code, you have two approaches for the final
 binary link:
@@ -268,6 +271,36 @@ binary link:
 
 Passing `rlib`s directly into your foreign linker is currently unsupported.
 
+> [!NOTE]
+>  Rust code compiled or linked with a different instance of the Rust runtime counts as "foreign code" for the purpose of this section.
+
+r[link.unwinding]
+### Prohibited linkage and unwinding
+
+r[link.unwinding.intro]
+Panic unwinding can only be used if the binary is built consistently according to the following rules.
+
+r[link.unwinding.potential]
+A Rust artifact is called *potentially unwinding* if any of the following conditions is met:
+- The artifact uses the [`unwind` panic handler][panic.panic_handler].
+- The artifact contains a crate built with the `unwind` [panic strategy] that makes a call to a function using a `-unwind` ABI.
+- The artifact makes a `"Rust"` ABI call to code running in another Rust artifact that has a separate copy of the Rust runtime, and that other artifact is potentially unwinding.
+
+> [!NOTE]
+> This definition captures whether a `"Rust"` ABI call inside a Rust artifact can ever unwind.
+
+r[link.unwinding.prohibited]
+If a Rust artifact is potentially unwinding, then all its crates must be built with the `unwind` [panic strategy]. Otherwise, unwinding can cause undefined behavior.
+
+> [!NOTE]
+> If you are using `rustc` to link, these rules are enforced automatically. If you are *not* using `rustc` to link, you must take care to ensure that unwinding is handled consistently across the entire binary. Linking without `rustc` includes using `dlopen` or similar facilities where linking is done by the system runtime without `rustc` being involved. This can only happen when mixing code with different [`-C panic`] flags, so most users do not have to be concerned about this.
+
+> [!NOTE]
+> To guarantee that a library will be sound (and linkable with `rustc`) regardless of the panic runtime used at link-time, the [`ffi_unwind_calls` lint] may be used. The lint flags any calls to `-unwind` foreign functions or function pointers.
+
 [`cfg` attribute `target_feature` option]: conditional-compilation.md#target_feature
+[`ffi_unwind_calls` lint]: ../rustc/lints/listing/allowed-by-default.html#ffi-unwind-calls
 [configuration option]: conditional-compilation.md
 [procedural macros]: procedural-macros.md
+[panic strategy]: panic.md#panic-strategy
+[`-C panic`]: ../rustc/codegen-options/index.html#panic