read/op: add ExpressionKind

DWARF expressions can be used to either compute a value or
a location description, depending on context, and this affects
the result of the evaluation, and which operations are valid
within them.

Use the attribute tag to determine the expression kind for
expressions in DIE attributes.

crates/examples/src/bin/dwarfdump.rs

@@ -1434,8 +1434,8 @@ fn dump_attr_value<R: Reader, W: Write>(
                 }
             };
         }
-        gimli::AttributeValue::Exprloc(ref data) => {
-            if let gimli::AttributeValue::Exprloc(_) = attr.raw_value() {
+        gimli::AttributeValue::Exprloc(ref data, _kind) => {
+            if let gimli::AttributeValue::Exprloc(..) = attr.raw_value() {
                 write!(w, "len 0x{:04x}: ", data.0.len())?;
                 for byte in data.0.to_slice()?.iter() {
                     write!(w, "{:02x}", byte)?;

src/read/mod.rs

@@ -363,6 +363,11 @@ pub enum Error {
     /// An expression-terminating operation was followed by something
     /// other than the end of the expression or a piece operation.
     InvalidExpressionTerminator(u64),
+    /// An expression for a value contained an operation that is only valid for location
+    /// descriptions.
+    InvalidValueExpression,
+    /// Cannot evaluate an expression without knowing its kind.
+    UnknownExpressionKind,
     /// Division or modulus by zero when evaluating an expression.
     DivisionByZero,
     /// An expression operation used mismatching types.
@@ -520,6 +525,13 @@ impl Error {
                 "DWARF expression has piece followed by non-piece expression at end"
             }
             Error::InvalidExpressionTerminator(_) => "Expected DW_OP_piece or DW_OP_bit_piece",
+            Error::InvalidValueExpression => {
+                "An expression for a value contained an operation that is only valid for location \
+                 descriptions"
+            }
+            Error::UnknownExpressionKind => {
+                "Cannot evaluate an expression without knowing its kind"
+            }
             Error::DivisionByZero => "Division or modulus by zero when evaluating expression",
             Error::TypeMismatch => "Type mismatch when evaluating expression",
             Error::IntegralTypeRequired => "Integral type expected when evaluating expression",