write: add LineConvert

This allow reuse of the implementation of `LineProgram::from` as part
of a more complex transformation.

Operation of `LineProgram::from` is mostly unchanged. The one difference
is that it now uses the string form chosen by `LineString::new`
instead of copying the form of the input.

src/read/dwarf.rs

@@ -420,6 +420,30 @@ impl<R: Reader> Dwarf<R> {
         }
     }
 
+    /// Return an attribute value as a string slice.
+    /// This only handles forms that are usable without an associated unit.
+    ///
+    /// If the attribute value is one of:
+    ///
+    /// - an inline `DW_FORM_string` string
+    /// - a `DW_FORM_strp` reference to an offset into the `.debug_str` section
+    /// - a `DW_FORM_strp_sup` reference to an offset into a supplementary
+    ///   object file
+    /// - a `DW_FORM_line_strp` reference to an offset into the `.debug_line_str`
+    ///   section
+    ///
+    /// then return the attribute's string value. Returns an error if the attribute
+    /// value does not have a string form, or if a string form has an invalid value.
+    pub fn attr_line_string(&self, attr: AttributeValue<R>) -> Result<R> {
+        match attr {
+            AttributeValue::String(string) => Ok(string),
+            AttributeValue::DebugStrRef(offset) => self.string(offset),
+            AttributeValue::DebugStrRefSup(offset) => self.sup_string(offset),
+            AttributeValue::DebugLineStrRef(offset) => self.line_string(offset),
+            _ => Err(Error::ExpectedStringAttributeValue),
+        }
+    }
+
     /// Return the address at the given index.
     pub fn address(&self, unit: &Unit<R>, index: DebugAddrIndex<R::Offset>) -> Result<u64> {
         self.debug_addr

src/write/dwarf.rs

@@ -2,8 +2,8 @@ use alloc::vec::Vec;
 
 use crate::common::Encoding;
 use crate::write::{
-    AbbreviationTable, LineProgram, LineStringTable, Result, Sections, StringTable, Unit,
-    UnitTable, Writer,
+    AbbreviationTable, LineProgram, LineString, LineStringTable, Result, Sections, StringTable,
+    Unit, UnitTable, Writer,
 };
 
 /// Writable DWARF information for more than one unit.
@@ -48,6 +48,11 @@ impl Dwarf {
         }
         Ok(())
     }
+
+    /// Get a reference to the data for a line string.
+    pub fn get_line_string<'a>(&'a self, string: &'a LineString) -> &'a [u8] {
+        string.get(&self.strings, &self.line_strings)
+    }
 }
 
 /// Writable DWARF information for a single unit.
@@ -102,6 +107,11 @@ impl DwarfUnit {
         abbrevs.write(&mut sections.debug_abbrev)?;
         Ok(())
     }
+
+    /// Get a reference to the data for a line string.
+    pub fn get_line_string<'a>(&'a self, string: &'a LineString) -> &'a [u8] {
+        string.get(&self.strings, &self.line_strings)
+    }
 }
 
 #[cfg(feature = "read")]