Method `SourceFile::normalized_byte_pos` is broken

[cpud36]

I have been looking through the source of rustc_span and stumbled upon an inconsistent usage of byte positions.

The problematic code is from SourceFile::normalized_byte_pos:

self
    .normalized_pos
    .binary_search_by(|np| (np.pos.0 + np.diff).cmp(&(self.start_pos.0 + offset)))

Here offset is a byte offset relative to the start of the file. The problem is, that NormalizedPos contains a relative byte pos, but we are comparing it with an absolute one. The current definition of NormalizedPos is the following:

pub struct NormalizedPos {
    /// The relative offset of the character in the `SourceFile`.
    pub pos: RelativeBytePos,
    /// The difference between original and normalized string at position.
    pub diff: u32,
}

When the method normalized_byte_pos was introduced(bba2a1e) the NormalizedPos used an absolute byte pos. But it was later to changed to use relative byte pos in 258ace6

[eggyal]

@rustbot claim