Defects vs subsets

src/coding-guidelines/expressions.rst

@@ -17,14 +17,14 @@ Expressions
    :scope: module
    :tags: readability, reduce-human-error
 
-   Code must not rely on Rust's type inference when doing explicit pointer casts via ``var as Type`` or ``core::mem::transmute``.
-   Instead, explicitly specify the complete target type in the ``as`` expression or ``core::mem::transmute`` call expression.
+   Code must not rely on Rust's type inference when doing explicit pointer casts via ``var as Type`` or :std:`core::mem::transmute`.
+   Instead, explicitly specify the complete target type in the ``as`` expression or :std:`core::mem::transmute` call expression.
 
    .. rationale::
       :id: rat_h8LdJQ1MNKu9
       :status: draft
 
-      ``var as Type`` casts and ``core::mem::transmute``\s between raw pointer types are generally valid and unchecked by the compiler as long the target pointer type is a thin pointer.
+      ``var as Type`` casts and :std:`core::mem::transmute`\s between raw pointer types are generally valid and unchecked by the compiler as long the target pointer type is a thin pointer.
       Not specifying the concrete target pointer type allows the compiler to infer it from the surroundings context which may result in the cast accidentally changing due to surrounding type changes resulting in semantically invalid pointer casts.
 
       Raw pointers have a variety of invariants to manually keep track of.
@@ -81,3 +81,252 @@ Expressions
          }
 
          fn with_base(_: &Base) { ... }
+
+
+.. guideline:: The 'as' operator should not be used with numeric operands
+   :id: gui_ADHABsmK9FXz
+   :category: advisory
+   :status: draft
+   :release: <TODO>
+   :fls: fls_otaxe9okhdr1
+   :decidability: decidable
+   :scope: module
+   :tags: subset, reduce-human-error
+
+   The binary operator ``as`` should not be used with:
+
+   * a numeric type, including all supported integer, floating, and machine-dependent arithmetic types; or
+   * ``bool``; or
+   * ``char``
+
+   as either the right operand or the type of the left operand.
+
+   **Exception:** ``as`` may be used with ``usize`` as the right operand and an expression of raw pointer
+   type as the left operand.
+
+   .. rationale::
+      :id: rat_v56bjjcveLxQ
+      :status: draft
+
+      Although the conversions performed by ``as`` between numeric types are all well-defined, ``as`` coerces
+      the value to fit in the destination type, which may result in unexpected data loss if the value needs to
+      be truncated, rounded, or produce a nearest possible non-equal value.
+
+      Although some conversions are lossless, others are not symmetrical. Instead of relying on either a defined
+      lossy behaviour or risking loss of precision, the code can communicate intent by using ``Into`` or ``From``
+      and ``TryInto`` or ``TryFrom`` to signal which conversions are intended to perfectly preserve the original
+      value, and which are intended to be fallible. The latter cannot be used from const functions, indicating
+      that these should avoid using fallible conversions.
+
+      A pointer-to-address cast does not lose value, but will be truncated unless the destination type is large
+      enough to hold the address value. The ``usize`` type is guaranteed to be wide enough for this purpose.
+
+      A pointer-to-address cast is not symmetrical because the resulting pointer may not point to a valid object,
+      may not point to an object of the right type, or may not be properly aligned.
+      If a conversion in this direction is needed, :std:`std::mem::transmute` will communicate the intent to perform
+      an unsafe operation.
+
+   .. non_compliant_example::
+      :id: non_compl_ex_hzGUYoMnK59w
+      :status: draft
+
+      ``as`` used here can change the value range or lose precision.
+      Even when it doesn't, nothing enforces the correct behaviour or communicates whether
+      we intend to allow lossy conversions, or only expect valid conversions.
+
+      .. code-block:: rust
+
+         fn f1(x: u16, y: i32, z: u64, w: u8) {
+           let a = w as char;           // non-compliant
+           let b = y as u32;            // non-compliant - changes value range, converting negative values
+           let c = x as i64;            // non-compliant - could use .into()
+
+           let d = y as f32;            // non-compliant - lossy
+           let e = d as f64;            // non-compliant - could use .into()
+           let f = e as f32;            // non-compliant - lossy
+
+           let g = e as i64;            // non-compliant - lossy despite object size
+
+           let p1: * const u32 = &b;
+           let a1 = p1 as usize;        // compliant by exception
+           let a2 = p1 as u16;          // non-compliant - may lose address range
+           let a3 = p1 as u64;          // non-compliant - use usize to indicate intent
+
+           let p2 = a1 as * const u32;  // non-compliant - prefer transmute
+           let p3 = a2 as * const u32;  // non-compliant (and most likely not in a valid address range)
+         }
+
+   .. compliant_example::
+      :id: compl_ex_uilHTIOgxD37
+      :status: draft
+
+      Valid conversions that are guaranteed to preserve exact values can be communicated
+      better with ``into()`` or ``from()``.
+      Valid conversions that risk losing value, where doing so would be an error, can
+      communicate this and include an error check, with ``try_into`` or ``try_from``.
+      Other forms of conversion may find ``transmute`` better communicates their intent.
+
+      .. code-block:: rust
+
+         fn f2(x: u16, y: i32, z: u64, w: u8) {
+           let a: char            = w.into();
+           let b: Result <u32, _> = y.try_into(); // produce an error on range clip
+           let c: i64             = x.into();
+
+           let d = f32::from(x);  // u16 is within range, u32 is not
+           let e = f64::from(d);
+           // let f = f32::from(e); // no From exists
+
+           // let g = ...            // no From exists
+
+           let h: u32 = 0;
+           let p1: * const u32 = &h;
+           let a1 = p1 as usize;     // (compliant)
+
+           unsafe {
+             let a2: usize = std::mem::transmute(p1);  // OK
+             let a3: u64   = std::mem::transmute(p1);  // OK, size is checked
+             // let a3: u16   = std::mem::transmute(p1);  // invalid, different sizes
+
+             let p2: * const u32 = std::mem::transmute(a1); // OK
+             let p3: * const u32 = std::mem::transmute(a1); // OK
+           }
+
+           unsafe {
+             // does something entirely different,
+             // reinterpreting the bits of z as the IEEE bit pattern of a double
+             // precision object, rather than converting the integer value
+             let f1: f64 = std::mem::transmute(z);
+           }
+         }
+
+
+.. guideline:: An integer shall not be converted to a pointer
+   :id: gui_PM8Vpf7lZ51U
+   :category: <TODO>
+   :status: draft
+   :release: <TODO>
+   :fls: fls_59mpteeczzo
+   :decidability: decidable
+   :scope: module
+   :tags: subset, undefined-behavior
+
+   The ``as`` operator shall not be used with an expression of numeric type as the left operand,
+   and any pointer type as the right operand.
+
+   :std:`std::mem::transmute` shall not be used with any numeric type (including floating point types)
+   as the argument to the ``Src`` parameter, and any pointer type as the argument to the ``Dst`` parameter.
+
+   .. rationale::
+      :id: rat_YqhEiWTj9z6L
+      :status: draft
+
+      A pointer created from an arbitrary arithmetic expression may designate an invalid address,
+      including an address that does not point to a valid object, an address that points to an
+      object of the wrong type, or an address that is not properly aligned. Use of such a pointer
+      to access memory will result in undefined behavior.
+
+      The ``as`` operator also does not check that the size of the source operand is the same as
+      the size of a pointer, which may lead to unexpected results if the address computation was
+      originally performed in a differently-sized address space.
+
+      While ``as`` can notionally be used to create a null pointer, the functions
+      :std:`core::ptr::null` and :std:`core::ptr::null_mut` are the more idiomatic way to do this.
+
+   .. non_compliant_example::
+      :id: non_compl_ex_0ydPk7VENSrA
+      :status: draft
+
+      Any use of ``as`` or ``transmute`` to create a pointer from an arithmetic address value
+      is non-compliant:
+
+      .. code-block:: rust
+
+        fn f1(x: u16, y: i32, z: u64, w: usize) {
+          let p1 = x as * const u32;  // not compliant
+          let p2 = y as * const u32;  // not compliant
+          let p3 = z as * const u32;  // not compliant
+          let p4 = w as * const u32;  // not compliant despite being the right size
+
+          let f: f64 = 10.0;
+          // let p5 = f as * const u32;  // not valid
+
+          unsafe {
+            // let p5: * const u32 = std::mem::transmute(x);  // not valid
+            // let p6: * const u32 = std::mem::transmute(y);  // not valid
+
+            let p7: * const u32 = std::mem::transmute(z); // not compliant
+            let p8: * const u32 = std::mem::transmute(w); // not compliant
+
+            let p9: * const u32 = std::mem::transmute(f); // not compliant, and very strange
+          }
+        }
+
+   .. compliant_example::
+      :id: compl_ex_oneKuF52yzrx
+      :status: draft
+
+      There is no compliant example of this operation.
+
+
+.. guideline:: An integer shall not be converted to an invalid pointer
+   :id: gui_iv9yCMHRgpE0
+   :category: <TODO>
+   :status: draft
+   :release: <TODO>
+   :fls: fls_9wgldua1u8yt
+   :decidability: undecidable
+   :scope: system
+   :tags: defect, undefined-behavior
+
+   An expression of numeric type shall not be converted to a pointer if the resulting pointer
+   is incorrectly aligned, does not point to an entity of the referenced type, or is an invalid representation.
+
+   .. rationale::
+      :id: rat_OhxKm751axKw
+      :status: draft
+
+      The mapping between pointers and integers must be consistent with the addressing structure of the
+      execution environment. Issues may arise, for example, on architectures that have a segmented memory model.
+
+   .. non_compliant_example::
+      :id: non_compl_ex_CkytKjRQezfQ
+      :status: draft
+
+      This example makes assumptions about the layout of the address space that do not hold on all platforms.
+      The manipulated address may have discarded part of the original address space, and the flag may
+      silently interfere with the address value. On platforms where pointers are 64-bits this may have
+      particularly unexpected results.
+
+      .. code-block:: rust
+
+        fn f1(flag: u32, ptr: * const u32) {
+          /* ... */
+          let mut rep = ptr as usize;
+          rep = (rep & 0x7fffff) | ((flag as usize) << 23);
+          let p2 = rep as * const u32;
+        }
+
+   .. compliant_example::
+      :id: compl_ex_oBoluiKSvREu
+      :status: draft
+
+      This compliant solution uses a struct to provide storage for both the pointer and the flag value.
+      This solution is portable to machines of different word sizes, both smaller and larger than 32 bits,
+      working even when pointers cannot be represented in any integer type.
+
+      .. code-block:: rust
+
+        struct PtrFlag {
+          pointer: * const u32,
+          flag: u32
+        }
+
+        fn f2(flag: u32, ptr: * const u32) {
+          let ptrflag = PtrFlag {
+            pointer: ptr,
+            flag: flag
+          };
+          /* ... */
+        }
+

src/conf.py

@@ -99,6 +99,10 @@
     dict(name="readability", description="Readability-related guideline"),
     dict(name="reduce-human-error", description="Reducing human error guideline"),
     dict(name="numerics", description="Numerics-related guideline"),
+    dict(name="undefined-behavior", description="Numerics-related guideline"),
+
+    dict(name="defect", description="Guideline associated with the language-subset profile"),
+    dict(name="subset", description="Guideline associated with the defect-prevention profile"),
 ]
 
 needs_categories = [

src/process/style-guideline.rst

@@ -92,10 +92,6 @@ We will examine each part:
       :scope: module
       :tags: numerics
 
-      Code must not rely on Rust's implicit integer wrapping behavior that occurs in release builds.
-      Instead, explicitly handle potential overflows using the standard library's checked,
-      saturating, or wrapping operations.
-
 ``guideline`` Title
 -------------------
 
@@ -308,15 +304,29 @@ what it covers.
 Content **SHOULD** aim to be as short and self-contained as possible, while still explaining
 the scope of the guideline.
 
-Content **SHOULD NOT** cover the rationale for the guideline, which is done in the ``rationale`` section.
+Guideline content consists of an Amplification and any Exceptions, which are normative,
+supported by a Rationale and examples, which are not normative.
+The justification extended explanation for the guideline **SHOULD** appear in the non-normative
+Rationale rather than in the normative content.
 
 Amplification
 ^^^^^^^^^^^^^
 
-Guideline Content **MAY** contain a section titled *Amplification* followed by text that provides a more
-precise description of the guideline title. An amplification is normative; if it conflicts with the
-``guideline`` Title, the amplification **MUST** take precedence. This mechanism is convenient as it allows
-a complicated concept to be conveyed using a short Title.
+The *Amplification* is the block of text that **MAY** appear immediately below the guideline
+attribute block, before any other subheadings.
+If it is provided, the Amplification is normative; if it conflicts with the ``guideline`` Title,
+the Amplification **MUST** take precedence. This mechanism is convenient as it allows a complicated
+concept to be conveyed using a short Title and refined by the text below.
+
+Content in the Amplification **SHOULD NOT** cover the rationale for the guideline or any
+non-normative explanations, which **SHOULD** be provided in the ``rationale`` and examples sections
+where helpful.
+
+::
+
+      Code must not rely on Rust's implicit integer wrapping behavior that occurs in release builds.
+      Instead, explicitly handle potential overflows using the standard library's checked,
+      saturating, or wrapping operations.
 
 Exception
 ^^^^^^^^^
@@ -327,9 +337,17 @@ some guidelines to be simplified. It is important to note that an exception is a
 a guideline does not apply. Code that complies with a guideline by virtue of an exception does not
 require a deviation.
 
+If it is provided, the Exception is normative; if it conflicts with the ``guideline`` Title or the
+Amplification, the Exception takes precedence over both. Depending on the individual guideline, it
+may be clearer to have an Amplification or Title with an explicit Exception overriding parts of
+their description, or it may be clearer to express excepted cases as integrated sentences in the
+Amplification. This decision is editorial.
+
 ``rationale``
 =============
 
+Each Guideline **MUST** provide a *Rationale* for its inclusion and enforcement.
+
 ::
 
       .. rationale::
@@ -363,7 +381,13 @@ The ``status`` option of a ``rationale`` **MUST** match the ``status`` of its pa
 Rationale Content
 -----------------
 
-TODO(pete.levasseur)
+The content of the rationale **SHOULD** provide the relevant context for why this guideline is useful.
+The Rationale **SHOULD** make reference to any undefined behaviors or known errors associated
+with the subject of the guideline.
+The Rationale **MAY** make reference to other guidelines or to external documents cited in the
+References.
+
+The Rationale **SHOULD** be supported by code examples wherever concise examples are possible.
 
 ``non_compliant_example``
 =========================
@@ -408,10 +432,10 @@ The ``non_compliant_example`` is neither normative, nor exhaustive. ``guideline`
 ``non_compliant_example`` Code Explanation
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
-The Code Explanation of a `non_compliant_example` **MUST** explain in prose the reason the guideline
+The Code Explanation of a ``non_compliant_example`` **MUST** explain in prose the reason the guideline
 when not applied results in code which is undesirable.
 
-The Code Explanation of a `non_compliant_example` **MAY** be a simple explanation no longer than
+The Code Explanation of a ``non_compliant_example`` **MAY** be a simple explanation no longer than
 a sentence.
 
 The Code Explanation of a ``non_compliant_example`` **SHOULD** be no longer than necessary to explain
@@ -435,6 +459,12 @@ than the current guideline.
 ``compliant_example``
 =====================
 
+A compliant example **SHOULD** be omitted when the guideline forbids an action entirely, i.e. there
+is no compliant way to achieve the goal of the non-compliant code, rather than giving an irrelevant
+example (or encouraging strange workarounds).
+When there is a clear and idiomatic compliant way to achieve the goal, a compliant example **SHOULD**
+be provided after the corresponding non-compliant example.
+
 ::
 
       .. compliant_example::