fn not_const(&mut self) {
self.add(Qualif::NOT_CONST);
if self.mode != Mode::Fn {
- span_err!(self.tcx.sess, self.span, E0019,
- "{} contains unimplemented expression type", self.mode);
+ let mut err = struct_span_err!(
+ self.tcx.sess,
+ self.span,
+ E0019,
+ "{} contains unimplemented expression type",
+ self.mode
+ );
+ if self.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();
}
}
fn statement_like(&mut self) {
self.add(Qualif::NOT_CONST);
if self.mode != Mode::Fn {
- span_err!(self.tcx.sess, self.span, E0016,
- "blocks in {}s are limited to items and tail expressions",
- self.mode);
+ let mut err = struct_span_err!(
+ self.tcx.sess,
+ self.span,
+ E0016,
+ "blocks in {}s are limited to items and tail expressions",
+ self.mode
+ );
+ if self.tcx.sess.teach(&err.get_code().unwrap()) {
+ err.note("Blocks in constants may only contain items (such as constant, function \
+ definition, etc...) and a tail expression.");
+ err.help("To avoid it, you have to replace the non-item object.");
+ }
+ err.emit();
}
}
TerminatorKind::GeneratorDrop |
TerminatorKind::Yield { .. } |
TerminatorKind::Unreachable |
- TerminatorKind::FalseEdges { .. } => None,
+ TerminatorKind::FalseEdges { .. } |
+ TerminatorKind::FalseUnwind { .. } => None,
TerminatorKind::Return => {
// Check for unused values. This usually means
}
if self.mode == Mode::Const || self.mode == Mode::ConstFn {
- span_err!(self.tcx.sess, self.span, E0013,
- "{}s cannot refer to statics, use \
- a constant instead", self.mode);
+ let mut err = struct_span_err!(self.tcx.sess, self.span, E0013,
+ "{}s cannot refer to statics, use \
+ a constant instead", self.mode);
+ if self.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()
}
}
Place::Projection(ref proj) => {
if let ty::TyRawPtr(_) = base_ty.sty {
this.add(Qualif::NOT_CONST);
if this.mode != Mode::Fn {
- struct_span_err!(this.tcx.sess,
- this.span, E0396,
+ let mut err = struct_span_err!(
+ this.tcx.sess,
+ this.span,
+ E0396,
"raw pointers cannot be dereferenced in {}s",
- this.mode)
- .span_label(this.span,
- "dereference of raw pointer in constant")
- .emit();
+ this.mode
+ );
+ err.span_label(this.span,
+ "dereference of raw pointer in constant");
+ if this.tcx.sess.teach(&err.get_code().unwrap()) {
+ err.note(
+ "The value behind a raw pointer can't be determined \
+ at compile-time (or even link-time), which means it \
+ can't be used in a constant expression."
+ );
+ err.help("A possible fix is to dereference your pointer \
+ at some point in run-time.");
+ }
+ err.emit();
}
}
}
}
let ty = place.ty(self.mir, self.tcx).to_ty(self.tcx);
- if kind == BorrowKind::Mut {
+ if let BorrowKind::Mut { .. } = kind {
// In theory, any zero-sized value could be borrowed
// mutably without consequences. However, only &mut []
// is allowed right now, and only in functions.
if !allow {
self.add(Qualif::NOT_CONST);
if self.mode != Mode::Fn {
- struct_span_err!(self.tcx.sess, self.span, E0017,
- "references in {}s may only refer \
- to immutable values", self.mode)
- .span_label(self.span, format!("{}s require immutable values",
- self.mode))
- .emit();
+ let mut err = struct_span_err!(self.tcx.sess, self.span, E0017,
+ "references in {}s may only refer \
+ to immutable values", self.mode);
+ err.span_label(self.span, format!("{}s require immutable values",
+ self.mode));
+ if self.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 {
(CastTy::FnPtr, CastTy::Int(_)) => {
self.add(Qualif::NOT_CONST);
if self.mode != Mode::Fn {
- span_err!(self.tcx.sess, self.span, E0018,
- "raw pointers cannot be cast to integers in {}s",
- self.mode);
+ let mut err = struct_span_err!(
+ self.tcx.sess,
+ self.span,
+ E0018,
+ "raw pointers cannot be cast to integers in {}s",
+ self.mode
+ );
+ if self.tcx.sess.teach(&err.get_code().unwrap()) {
+ err.note("\
+ The value of static and constant integers must be known at compile time. You can't cast a pointer \
+ to an integer because the address of a pointer can vary.
+
+ For example, if you write:
+
+ ```
+ static MY_STATIC: u32 = 42;
+ static MY_STATIC_ADDR: usize = &MY_STATIC as *const _ as usize;
+ static WHAT: usize = (MY_STATIC_ADDR^17) + MY_STATIC_ADDR;
+ ```
+
+ Then `MY_STATIC_ADDR` would contain the address of `MY_STATIC`. However, the address can change \
+ when the program is linked, as well as change between different executions due to ASLR, and many \
+ linkers would not be able to calculate the value of `WHAT`.
+
+ On the other hand, static and constant pointers can point either to a known numeric address or to \
+ the address of a symbol.
+
+ ```
+ static MY_STATIC: u32 = 42;
+ static MY_STATIC_ADDR: &'static u32 = &MY_STATIC;
+ const CONST_ADDR: *const u8 = 0x5f3759df as *const u8;
+ ```
+
+ This does not pose a problem by itself because they can't be accessed directly.");
+ }
+ err.emit();
}
}
_ => {}
Rvalue::NullaryOp(NullOp::Box, _) => {
self.add(Qualif::NOT_CONST);
if self.mode != Mode::Fn {
- struct_span_err!(self.tcx.sess, self.span, E0010,
- "allocations are not allowed in {}s", self.mode)
- .span_label(self.span, format!("allocation not allowed in {}s", self.mode))
- .emit();
+ let mut err = struct_span_err!(self.tcx.sess, self.span, E0010,
+ "allocations are not allowed in {}s", self.mode);
+ err.span_label(self.span, format!("allocation not allowed in {}s", self.mode));
+ if self.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();
}
}
// Avoid a generic error for other uses of arguments.
if self.qualif.intersects(Qualif::FN_ARGUMENT) {
let decl = &self.mir.local_decls[index];
- span_err!(self.tcx.sess, decl.source_info.span, E0022,
- "arguments of constant functions can only \
- be immutable by-value bindings");
+ let mut err = struct_span_err!(
+ self.tcx.sess,
+ decl.source_info.span,
+ E0022,
+ "arguments of constant functions can only be immutable by-value bindings"
+ );
+ if self.tcx.sess.teach(&err.get_code().unwrap()) {
+ err.note("Constant functions are not allowed to mutate anything. Thus, \
+ binding to an argument with a mutable pattern is not allowed.");
+ err.note("Remove any mutable bindings from the argument list to fix this \
+ error. In case you need to mutate the argument, try lazily \
+ initializing a global variable instead of using a const fn, or \
+ refactoring the code to a functional style to avoid mutation if \
+ possible.");
+ }
+ err.emit();
return;
}
}
projects / rust.git / commitdiff