Simplify proto conversion

Trait based infra in conv.rs is significantly more complicated than
what we actually need here.

crates/rust-analyzer/src/from_proto.rs

@@ -0,0 +1,42 @@
+//! Conversion lsp_types types to rust-analyzer specific ones.
+use ra_db::{FileId, FilePosition, FileRange};
+use ra_ide::{LineCol, LineIndex};
+use ra_syntax::{TextRange, TextSize};
+
+use crate::{world::WorldSnapshot, Result};
+
+pub(crate) fn offset(line_index: &LineIndex, position: lsp_types::Position) -> TextSize {
+    let line_col = LineCol { line: position.line as u32, col_utf16: position.character as u32 };
+    line_index.offset(line_col)
+}
+
+pub(crate) fn text_range(line_index: &LineIndex, range: lsp_types::Range) -> TextRange {
+    let start = offset(line_index, range.start);
+    let end = offset(line_index, range.end);
+    TextRange::new(start, end)
+}
+
+pub(crate) fn file_id(world: &WorldSnapshot, url: &lsp_types::Url) -> Result<FileId> {
+    world.uri_to_file_id(url)
+}
+
+pub(crate) fn file_position(
+    world: &WorldSnapshot,
+    tdpp: lsp_types::TextDocumentPositionParams,
+) -> Result<FilePosition> {
+    let file_id = file_id(world, &tdpp.text_document.uri)?;
+    let line_index = world.analysis().file_line_index(file_id)?;
+    let offset = offset(&*line_index, tdpp.position);
+    Ok(FilePosition { file_id, offset })
+}
+
+pub(crate) fn file_range(
+    world: &WorldSnapshot,
+    text_document_identifier: lsp_types::TextDocumentIdentifier,
+    range: lsp_types::Range,
+) -> Result<FileRange> {
+    let file_id = file_id(world, &text_document_identifier.uri)?;
+    let line_index = world.analysis().file_line_index(file_id)?;
+    let range = text_range(&line_index, range);
+    Ok(FileRange { file_id, range })
+}