Clamp Position::character to line length

LSP says about Position::character

> If the character value is greater than the line length it defaults back to the line length.

but from_proto::offset() doesn't implement this.

A client might for example request code actions for a whole line by sending
Position::character=99999.  I don't think there is ever a reason (besides laziness) why the
client can't specify the line length instead but I guess we should not crash but follow protocol.

Technically it should be a warning, not an error but warning is not shown by default so keep
it at error I guess.

Fixes #18240

Cargo.toml

@@ -96,7 +96,7 @@ test-fixture = { path = "./crates/test-fixture" }
 test-utils = { path = "./crates/test-utils" }
 
 # In-tree crates that are published separately and follow semver. See lib/README.md
-line-index = { version = "0.1.1" }
+line-index = { version = "0.1.2" }
 la-arena = { version = "0.3.1" }
 lsp-server = { version = "0.7.6" }
 

crates/rust-analyzer/src/lsp/from_proto.rs

@@ -35,10 +35,18 @@ pub(crate) fn offset(
                 .ok_or_else(|| format_err!("Invalid wide col offset"))?
         }
     };
-    let text_size = line_index.index.offset(line_col).ok_or_else(|| {
+    let line_range = line_index.index.line(line_col.line).ok_or_else(|| {
         format_err!("Invalid offset {line_col:?} (line index length: {:?})", line_index.index.len())
     })?;
-    Ok(text_size)
+    let col = TextSize::from(line_col.col);
+    let clamped_len = col.min(line_range.len());
+    if clamped_len < col {
+        tracing::error!(
+            "Position {line_col:?} column exceeds line length {}, clamping it",
+            u32::from(line_range.len()),
+        );
+    }
+    Ok(line_range.start() + clamped_len)
 }
 
 pub(crate) fn text_range(