Move non-const ops into their own module

Author: ecstatic-morse

src/librustc_mir/transform/check_consts/mod.rs

@@ -4,8 +4,9 @@ use rustc::ty::{self, TyCtxt};
 
 pub use self::qualifs::Qualif;
 
-mod resolver;
+pub mod ops;
 mod qualifs;
+mod resolver;
 pub mod validation;
 
 /// Information about the item currently being validated, as well as a reference to the global

src/librustc_mir/transform/check_consts/ops.rs

@@ -0,0 +1,337 @@
+use rustc::hir::def_id::DefId;
+use rustc::mir::BorrowKind;
+use rustc::session::config::nightly_options;
+use rustc::ty::TyCtxt;
+use syntax::feature_gate::{emit_feature_err, GateIssue};
+use syntax::symbol::sym;
+use syntax_pos::{Span, Symbol};
+
+use super::Item;
+use super::validation::Mode;
+
+/// An operation that is not *always* allowed in a const context.
+pub trait NonConstOp {
+    /// Whether this operation can be evaluated by miri.
+    const IS_SUPPORTED_IN_MIRI: bool = true;
+
+    /// Returns a boolean indicating whether the feature gate that would allow this operation is
+    /// enabled, or `None` if such a feature gate does not exist.
+    fn feature_gate(_tcx: TyCtxt<'tcx>) -> Option<bool> {
+        None
+    }
+
+    /// Returns `true` if this operation is allowed in the given item.
+    ///
+    /// This check should assume that we are not in a non-const `fn`, where all operations are
+    /// legal.
+    fn is_allowed_in_item(&self, item: &Item<'_, '_>) -> bool {
+        Self::feature_gate(item.tcx).unwrap_or(false)
+    }
+
+    fn emit_error(&self, item: &Item<'_, '_>, span: Span) {
+        let mut err = struct_span_err!(
+            item.tcx.sess,
+            span,
+            E0019,
+            "{} contains unimplemented expression type",
+            item.mode
+        );
+        if item.tcx.sess.teach(&err.get_code().unwrap()) {
+            err.note("A function call isn't allowed in the const's initialization expression \
+                      because the expression's value must be known at compile-time.");
+            err.note("Remember: you can't use a function call inside a const's initialization \
+                      expression! However, you can use it anywhere else.");
+        }
+        err.emit();
+    }
+}
+
+/// A `Downcast` projection.
+#[derive(Debug)]
+pub struct Downcast;
+impl NonConstOp for Downcast {}
+
+/// A function call where the callee is a pointer.
+#[derive(Debug)]
+pub struct FnCallIndirect;
+impl NonConstOp for FnCallIndirect {
+    fn emit_error(&self, item: &Item<'_, '_>, span: Span) {
+        let mut err = item.tcx.sess.struct_span_err(
+            span,
+            &format!("function pointers are not allowed in const fn"));
+        err.emit();
+    }
+}
+
+/// A function call where the callee is not marked as `const`.
+#[derive(Debug)]
+pub struct FnCallNonConst(pub DefId);
+impl NonConstOp for FnCallNonConst {
+    fn emit_error(&self, item: &Item<'_, '_>, span: Span) {
+        let mut err = struct_span_err!(
+            item.tcx.sess,
+            span,
+            E0015,
+            "calls in {}s are limited to constant functions, \
+             tuple structs and tuple variants",
+            item.mode,
+        );
+        err.emit();
+    }
+}
+
+/// A function call where the callee is not a function definition or function pointer, e.g. a
+/// closure.
+///
+/// This can be subdivided in the future to produce a better error message.
+#[derive(Debug)]
+pub struct FnCallOther;
+impl NonConstOp for FnCallOther {
+    const IS_SUPPORTED_IN_MIRI: bool = false;
+}
+
+/// A call to a `#[unstable]` const fn or `#[rustc_const_unstable]` function.
+///
+/// Contains the name of the feature that would allow the use of this function.
+#[derive(Debug)]
+pub struct FnCallUnstable(pub DefId, pub Symbol);
+impl NonConstOp for FnCallUnstable {
+    fn emit_error(&self, item: &Item<'_, '_>, span: Span) {
+        let FnCallUnstable(def_id, feature) = *self;
+
+        let mut err = item.tcx.sess.struct_span_err(span,
+            &format!("`{}` is not yet stable as a const fn",
+                    item.tcx.def_path_str(def_id)));
+        if nightly_options::is_nightly_build() {
+            help!(&mut err,
+                  "add `#![feature({})]` to the \
+                   crate attributes to enable",
+                  feature);
+        }
+        err.emit();
+    }
+}
+
+#[derive(Debug)]
+pub struct HeapAllocation;
+impl NonConstOp for HeapAllocation {
+    const IS_SUPPORTED_IN_MIRI: bool = false;
+
+    fn emit_error(&self, item: &Item<'_, '_>, span: Span) {
+        let mut err = struct_span_err!(item.tcx.sess, span, E0010,
+                                       "allocations are not allowed in {}s", item.mode);
+        err.span_label(span, format!("allocation not allowed in {}s", item.mode));
+        if item.tcx.sess.teach(&err.get_code().unwrap()) {
+            err.note(
+                "The value of statics and constants must be known at compile time, \
+                 and they live for the entire lifetime of a program. Creating a boxed \
+                 value allocates memory on the heap at runtime, and therefore cannot \
+                 be done at compile time."
+            );
+        }
+        err.emit();
+    }
+}
+
+#[derive(Debug)]
+pub struct IfOrMatch;
+impl NonConstOp for IfOrMatch {}
+
+#[derive(Debug)]
+pub struct LiveDrop;
+impl NonConstOp for LiveDrop {
+    fn emit_error(&self, item: &Item<'_, '_>, span: Span) {
+        struct_span_err!(item.tcx.sess, span, E0493,
+                         "destructors cannot be evaluated at compile-time")
+            .span_label(span, format!("{}s cannot evaluate destructors",
+                                      item.mode))
+            .emit();
+    }
+}
+
+#[derive(Debug)]
+pub struct Loop;
+impl NonConstOp for Loop {}
+
+#[derive(Debug)]
+pub struct MutBorrow(pub BorrowKind);
+impl NonConstOp for MutBorrow {
+    fn emit_error(&self, item: &Item<'_, '_>, span: Span) {
+        let kind = self.0;
+        if let BorrowKind::Mut { .. } = kind {
+            let mut err = struct_span_err!(item.tcx.sess, span, E0017,
+                                           "references in {}s may only refer \
+                                            to immutable values", item.mode);
+            err.span_label(span, format!("{}s require immutable values",
+                                                item.mode));
+            if item.tcx.sess.teach(&err.get_code().unwrap()) {
+                err.note("References in statics and constants may only refer \
+                          to immutable values.\n\n\
+                          Statics are shared everywhere, and if they refer to \
+                          mutable data one might violate memory safety since \
+                          holding multiple mutable references to shared data \
+                          is not allowed.\n\n\
+                          If you really want global mutable state, try using \
+                          static mut or a global UnsafeCell.");
+            }
+            err.emit();
+        } else {
+            span_err!(item.tcx.sess, span, E0492,
+                      "cannot borrow a constant which may contain \
+                       interior mutability, create a static instead");
+        }
+    }
+}
+
+#[derive(Debug)]
+pub struct MutDeref;
+impl NonConstOp for MutDeref {}
+
+#[derive(Debug)]
+pub struct Panic;
+impl NonConstOp for Panic {
+    fn feature_gate(tcx: TyCtxt<'_>) -> Option<bool> {
+        Some(tcx.features().const_panic)
+    }
+
+    fn emit_error(&self, item: &Item<'_, '_>, span: Span) {
+        emit_feature_err(
+            &item.tcx.sess.parse_sess,
+            sym::const_panic,
+            span,
+            GateIssue::Language,
+            &format!("panicking in {}s is unstable", item.mode),
+        );
+    }
+}
+
+#[derive(Debug)]
+pub struct RawPtrComparison;
+impl NonConstOp for RawPtrComparison {
+    fn feature_gate(tcx: TyCtxt<'_>) -> Option<bool> {
+        Some(tcx.features().const_compare_raw_pointers)
+    }
+
+    fn emit_error(&self, item: &Item<'_, '_>, span: Span) {
+        emit_feature_err(
+            &item.tcx.sess.parse_sess,
+            sym::const_compare_raw_pointers,
+            span,
+            GateIssue::Language,
+            &format!("comparing raw pointers inside {}", item.mode),
+        );
+    }
+}
+
+#[derive(Debug)]
+pub struct RawPtrDeref;
+impl NonConstOp for RawPtrDeref {
+    fn feature_gate(tcx: TyCtxt<'_>) -> Option<bool> {
+        Some(tcx.features().const_raw_ptr_deref)
+    }
+
+    fn emit_error(&self, item: &Item<'_, '_>, span: Span) {
+        emit_feature_err(
+            &item.tcx.sess.parse_sess, sym::const_raw_ptr_deref,
+            span, GateIssue::Language,
+            &format!(
+                "dereferencing raw pointers in {}s is unstable",
+                item.mode,
+            ),
+        );
+    }
+}
+
+#[derive(Debug)]
+pub struct RawPtrToIntCast;
+impl NonConstOp for RawPtrToIntCast {
+    fn feature_gate(tcx: TyCtxt<'_>) -> Option<bool> {
+        Some(tcx.features().const_raw_ptr_to_usize_cast)
+    }
+
+    fn emit_error(&self, item: &Item<'_, '_>, span: Span) {
+        emit_feature_err(
+            &item.tcx.sess.parse_sess, sym::const_raw_ptr_to_usize_cast,
+            span, GateIssue::Language,
+            &format!(
+                "casting pointers to integers in {}s is unstable",
+                item.mode,
+            ),
+        );
+    }
+}
+
+/// An access to a (non-thread-local) `static`.
+#[derive(Debug)]
+pub struct StaticAccess;
+impl NonConstOp for StaticAccess {
+    fn is_allowed_in_item(&self, item: &Item<'_, '_>) -> bool {
+        item.mode.is_static()
+    }
+
+    fn emit_error(&self, item: &Item<'_, '_>, span: Span) {
+        let mut err = struct_span_err!(item.tcx.sess, span, E0013,
+                                        "{}s cannot refer to statics, use \
+                                        a constant instead", item.mode);
+        if item.tcx.sess.teach(&err.get_code().unwrap()) {
+            err.note(
+                "Static and const variables can refer to other const variables. \
+                    But a const variable cannot refer to a static variable."
+            );
+            err.help(
+                "To fix this, the value can be extracted as a const and then used."
+            );
+        }
+        err.emit();
+    }
+}
+
+/// An access to a thread-local `static`.
+#[derive(Debug)]
+pub struct ThreadLocalAccess;
+impl NonConstOp for ThreadLocalAccess {
+    const IS_SUPPORTED_IN_MIRI: bool = false;
+
+    fn emit_error(&self, item: &Item<'_, '_>, span: Span) {
+        span_err!(item.tcx.sess, span, E0625,
+            "thread-local statics cannot be \
+            accessed at compile-time");
+    }
+}
+
+#[derive(Debug)]
+pub struct Transmute;
+impl NonConstOp for Transmute {
+    fn feature_gate(tcx: TyCtxt<'_>) -> Option<bool> {
+        Some(tcx.features().const_transmute)
+    }
+
+    fn emit_error(&self, item: &Item<'_, '_>, span: Span) {
+        emit_feature_err(
+            &item.tcx.sess.parse_sess, sym::const_transmute,
+            span, GateIssue::Language,
+            &format!("The use of std::mem::transmute() \
+            is gated in {}s", item.mode));
+    }
+}
+
+#[derive(Debug)]
+pub struct UnionAccess;
+impl NonConstOp for UnionAccess {
+    fn is_allowed_in_item(&self, item: &Item<'_, '_>) -> bool {
+        // Union accesses are stable in all contexts except `const fn`.
+        item.mode != Mode::ConstFn || Self::feature_gate(item.tcx).unwrap()
+    }
+
+    fn feature_gate(tcx: TyCtxt<'_>) -> Option<bool> {
+        Some(tcx.features().const_fn_union)
+    }
+
+    fn emit_error(&self, item: &Item<'_, '_>, span: Span) {
+        emit_feature_err(
+            &item.tcx.sess.parse_sess, sym::const_fn_union,
+            span, GateIssue::Language,
+            "unions in const fn are unstable",
+        );
+    }
+}