fix(lsp): open library classes behind jar: URIs (#1048)

The official Kotlin LSP answers navigation into a dependency with a
`jar:///…/lib.jar!/pkg/Name.class` URI but does not serve its contents through
`workspace/textDocumentContent`, so opening the target failed with "no
language server provides the contents of jar:…", locally and remotely.

`jar:` URIs are now resolved by the backend, where the jars are:
- An entry that is not a class (a sources jar entry, a resource) is read from
  its archive.
- A class is shown as its source from the artifact's `-sources.jar` in the
  Maven repository or the Gradle cache (`Name.java`/`Name.kt`, an inner
  class's outer file, a `FileKt` facade's `File.kt`, a `@file:JvmName`
  file); Go to Declaration then lands on the declaration of the name it was
  invoked on.
- Otherwise the server's `decompile` command is used when it offers one, and
  the error says that neither a source nor a decompiler is available.
- Library tabs name the archive and package (`spring-boot-3.5.16.jar ›
  org.springframework.boot`).

Fixes #982

Cargo.lock

@@ -4387,6 +4387,7 @@ dependencies = [
  "tokio",
  "toml 0.9.12+spec-1.1.0",
  "tracing",
+ "zip",
 ]
 
 [[package]]

crates/ide-lsp/Cargo.toml

@@ -24,6 +24,7 @@ globset.workspace = true
 ignore.workspace = true
 toml.workspace = true
 percent-encoding = "2"
+zip.workspace = true
 
 [dev-dependencies]
 tempfile.workspace = true

crates/ide-lsp/src/lib.rs

@@ -16,6 +16,7 @@ mod handler;
 mod inspect;
 pub mod jdtls;
 mod launch;
+mod library_source;
 mod scope;
 mod server;
 mod tcp;
@@ -273,6 +274,30 @@ impl<T> Outcome<T> {
 /// Implementation requests of one line-marker pass in flight at once.
 const IMPLEMENTATION_CONCURRENCY: usize = 4;
 
+/// Library classes in `jar:` targets (the official Kotlin LSP) go to their source
+/// in the artifact's sources jar when it is cached, at the declaration of the
+/// name navigated from; [`LspManager::virtual_document`] serves it.
+async fn library_sources(snap: &DocSnap, pos: Pos, locs: Vec<Location>) -> Vec<Location> {
+    if !locs.iter().any(|l| l.uri.as_deref().is_some_and(|u| u.starts_with("jar:") && u.ends_with(".class"))) {
+        return locs;
+    }
+    let name = library_source::identifier_at(snap.text.line(pos.line as usize), pos.col).map(str::to_owned);
+    tokio::task::spawn_blocking(move || {
+        locs.into_iter()
+            .map(|loc| {
+                let Some(uri) = loc.uri.as_deref().filter(|u| u.starts_with("jar:") && u.ends_with(".class")) else {
+                    return loc;
+                };
+                let Some((source, text)) = library_source::class_source(uri) else { return loc };
+                let range = name.as_deref().map(|n| library_source::declaration_range(&text, n)).unwrap_or_default();
+                Location { path: source.clone().into(), range, uri: Some(source) }
+            })
+            .collect()
+    })
+    .await
+    .unwrap_or_default()
+}
+
 fn goto_method(kind: GotoKind) -> (&'static str, &'static str, &'static str) {
     match kind {
         GotoKind::Definition => ("textDocument/definition", "definitionProvider", "go to definition"),
@@ -1187,6 +1212,7 @@ impl LspManager {
             {
                 Ok(Some(r)) => {
                     let locs = convert::goto_response(&self.conv(t.enc()), r);
+                    let locs = library_sources(snap, pos, locs).await;
                     self.note_virtual(&t.server, locs.iter().map(|l| l.uri.as_deref()));
                     if !locs.is_empty() {
                         return Ok(locs);
@@ -2218,12 +2244,35 @@ impl LspManager {
 
     /// Contents of a virtual document, from the server that owns `uri`:
     /// `workspace/textDocumentContent` (LSP 3.18) when the server serves the
-    /// scheme, jdtls' `java/classFileContents` for `jdt:` URIs. Cached.
+    /// scheme, jdtls' `java/classFileContents` for `jdt:` URIs. A `jar:` entry
+    /// is read from its archive, a `jar:` class shown as its library source
+    /// (see [`library_source`]) or else decompiled by the server. Cached.
     pub async fn virtual_document(&self, uri: &str) -> IdeResult<VirtualDocument> {
         if let Some(d) = self.virtual_docs.lock().get(uri) {
             return Ok(d.clone());
         }
         let scheme = virtual_doc::scheme(uri).to_owned();
+        let jar_class = scheme == "jar" && library_source::parse(uri).is_some_and(|e| e.entry.ends_with(".class"));
+        if scheme == "jar" {
+            let found = {
+                let uri = uri.to_owned();
+                tokio::task::spawn_blocking(move || match library_source::parse(&uri) {
+                    Some(e) if !e.entry.ends_with(".class") => {
+                        library_source::read_entry(&e.jar, &e.entry).map(|text| (e.entry, text))
+                    }
+                    Some(_) => library_source::class_source(&uri)
+                        .and_then(|(source, text)| Some((library_source::parse(&source)?.entry, text))),
+                    None => None,
+                })
+                .await
+                .ok()
+                .flatten()
+            };
+            if let Some((entry, text)) = found {
+                let title = entry.rsplit('/').next().unwrap_or(&entry).to_owned();
+                return Ok(self.store_virtual(uri, text, Some(title), &[], self.origin_server(uri).map(|s| s.id)));
+            }
+        }
         let class_file = scheme == "jdt" || (scheme == "file" && uri.ends_with(".class"));
         let origin = self.origin_server(uri);
         let mut servers: Vec<Arc<Server>> = origin.iter().cloned().collect();
@@ -2246,6 +2295,25 @@ impl LspManager {
             } else if class_file && languages.iter().any(|l| l.as_str() == "java") {
                 self.call::<_, Option<String>>(t, "java/classFileContents", json!({ "uri": uri }), None, LONG_TIMEOUT)
                     .await
+            } else if jar_class && t.caps().commands().iter().any(|c| c == "decompile") {
+                // The official Kotlin LSP: `decompile` answers `{ code, language }`.
+                self.call::<_, Option<Value>>(
+                    t,
+                    "workspace/executeCommand",
+                    json!({ "command": "decompile", "arguments": [uri] }),
+                    None,
+                    LONG_TIMEOUT,
+                )
+                .await
+                .map(|v| {
+                    v.and_then(|v| match v {
+                        Value::String(text) => Some(text),
+                        v => ["code", "text", "content"]
+                            .iter()
+                            .find_map(|k| v.get(k).and_then(Value::as_str))
+                            .map(str::to_owned),
+                    })
+                })
             } else {
                 continue;
             };
@@ -2257,33 +2325,53 @@ impl LspManager {
                     continue;
                 }
             };
-            let decompiled = virtual_doc::is_decompiled(&text);
-            let title = virtual_doc::title(uri, decompiled);
-            let detected = self.languages.detect(Path::new(&title), None);
-            let language_id = if detected == LanguageId::plain_text() {
-                languages.first().cloned().unwrap_or(detected)
-            } else {
-                detected
-            };
-            let doc = VirtualDocument {
-                uri: uri.to_owned(),
-                text,
-                language_id,
-                title,
-                description: virtual_doc::description(uri),
-                decompiled,
-            };
-            self.virtual_origins.lock().entry(uri.to_owned()).or_insert(t.server.id);
-            let mut cache = self.virtual_docs.lock();
-            if cache.len() >= VIRTUAL_CACHE_LIMIT {
-                cache.clear();
-            }
-            cache.insert(uri.to_owned(), doc.clone());
-            return Ok(doc);
+            return Ok(self.store_virtual(uri, text, None, &languages, Some(t.server.id)));
+        }
+        if jar_class && error.is_none() {
+            return Err(IdeError::not_found(format!(
+                "no source or decompiler is available for {}: its sources jar is not in the local Maven or Gradle cache, and no language server decompiles it",
+                virtual_doc::title(uri, false)
+            )));
         }
         Err(error.unwrap_or_else(|| IdeError::not_found(format!("no language server provides the contents of {uri}"))))
     }
 
+    /// Caches `text` as the virtual document `uri`, titled `title` (else after the URI).
+    fn store_virtual(
+        &self,
+        uri: &str,
+        text: String,
+        title: Option<String>,
+        languages: &[LanguageId],
+        server: Option<ide_core::ServerId>,
+    ) -> VirtualDocument {
+        let decompiled = virtual_doc::is_decompiled(&text);
+        let title = title.unwrap_or_else(|| virtual_doc::title(uri, decompiled));
+        let detected = self.languages.detect(Path::new(&title), None);
+        let language_id = if detected == LanguageId::plain_text() {
+            languages.first().cloned().unwrap_or(detected)
+        } else {
+            detected
+        };
+        let doc = VirtualDocument {
+            uri: uri.to_owned(),
+            text,
+            language_id,
+            title,
+            description: virtual_doc::description(uri),
+            decompiled,
+        };
+        if let Some(server) = server {
+            self.virtual_origins.lock().entry(uri.to_owned()).or_insert(server);
+        }
+        let mut cache = self.virtual_docs.lock();
+        if cache.len() >= VIRTUAL_CACHE_LIMIT {
+            cache.clear();
+        }
+        cache.insert(uri.to_owned(), doc.clone());
+        doc
+    }
+
     /// Called when a server sends `workspace/applyEdit`; installed by `src-tauri`
     /// so edits flow through `ide-workspace::Documents`.
     pub fn set_config_hook(&self, hook: Box<ConfigFn>) {

crates/ide-lsp/src/library_source.rs

@@ -0,0 +1,299 @@
+//! `jar:` URIs (`jar:///…/lib.jar!/pkg/Name.class`, also `jar:file:///…`) that
+//! language servers return for library code: the official Kotlin LSP for the
+//! classes of a dependency, Metals and others for entries of a sources jar.
+//! Not every server can serve their contents (`workspace/textDocumentContent`),
+//! so they are read here, where the jars are (also on a remote backend): an
+//! entry as is, a class as its source from the artifact's `-sources.jar` next to
+//! it in the Maven or Gradle cache.
+
+use ide_core::{Pos, Range};
+use percent_encoding::{AsciiSet, CONTROLS, percent_decode_str, utf8_percent_encode};
+use std::io::Read;
+use std::path::{Path, PathBuf};
+
+/// An entry of an archive named by a `jar:` URI.
+#[derive(Debug, Clone, PartialEq, Eq)]
+pub(crate) struct JarEntry {
+    /// The URI up to the archive path (`jar:///` or `jar:file:///`), kept for new URIs.
+    prefix: String,
+    pub jar: PathBuf,
+    /// `/`-separated, without a leading `/`.
+    pub entry: String,
+}
+
+pub(crate) fn parse(uri: &str) -> Option<JarEntry> {
+    let rest = uri.strip_prefix("jar:")?;
+    let (archive, entry) = rest.split_once("!/")?;
+    let (prefix, path) = if let Some(path) = archive.strip_prefix("file://") {
+        ("jar:file://", path)
+    } else if let Some(path) = archive.strip_prefix("//") {
+        ("jar://", path)
+    } else {
+        ("jar:", archive)
+    };
+    let path = percent_decode_str(path).decode_utf8().ok()?;
+    let path = if cfg!(windows) {
+        // `/C:/x` → `C:\x`
+        let trimmed = path.strip_prefix('/').filter(|s| s.as_bytes().get(1) == Some(&b':')).unwrap_or(&path);
+        PathBuf::from(trimmed.replace('/', "\\"))
+    } else {
+        PathBuf::from(path.into_owned())
+    };
+    let entry = percent_decode_str(entry.split(['?', '#']).next().unwrap_or(entry)).decode_utf8().ok()?;
+    Some(JarEntry { prefix: prefix.to_owned(), jar: path, entry: entry.trim_start_matches('/').to_owned() })
+}
+
+const PATH: &AsciiSet = &CONTROLS.add(b' ').add(b'#').add(b'%').add(b'?').add(b'!');
+
+impl JarEntry {
+    /// The URI of `entry` in `jar`, written like the URI this one came from.
+    fn uri_of(&self, jar: &Path, entry: &str) -> String {
+        let mut path = jar.to_string_lossy().replace('\\', "/");
+        if !path.starts_with('/') {
+            path.insert(0, '/');
+        }
+        let encode = |s: &str| utf8_percent_encode(s, PATH).to_string();
+        format!("{}{}!/{}", self.prefix, encode(&path), encode(entry))
+    }
+}
+
+/// The text of `entry` in the archive `jar`, when it is a UTF-8 file.
+pub(crate) fn read_entry(jar: &Path, entry: &str) -> Option<String> {
+    let mut zip = zip::ZipArchive::new(std::fs::File::open(jar).ok()?).ok()?;
+    let mut file = zip.by_name(entry).ok()?;
+    let mut text = String::new();
+    file.read_to_string(&mut text).ok()?;
+    Some(text)
+}
+
+/// The library source for a class entry: its `.kt` / `.java` file in the
+/// artifact's sources jar, as `(uri, text)`.
+pub(crate) fn class_source(uri: &str) -> Option<(String, String)> {
+    let jar = parse(uri)?;
+    let class = jar.entry.strip_suffix(".class")?;
+    for sources in sources_jars(&jar.jar) {
+        if let Some((entry, text)) = source_entry(&sources, class) {
+            return Some((jar.uri_of(&sources, &entry), text));
+        }
+    }
+    None
+}
+
+/// Sources jars of the artifact `jar`: `<name>-sources.jar` next to it (Maven
+/// repository), or in a sibling hash folder (Gradle `files-2.1/<g>/<a>/<v>/<hash>/`).
+fn sources_jars(jar: &Path) -> Vec<PathBuf> {
+    let (Some(dir), Some(stem)) = (jar.parent(), jar.file_stem().and_then(|s| s.to_str())) else { return vec![] };
+    let name = format!("{stem}-sources.jar");
+    let mut out: Vec<PathBuf> = vec![dir.join(&name)];
+    if let Some(version_dir) = dir.parent()
+        && version_dir.ancestors().any(|a| a.file_name().is_some_and(|n| n == "files-2.1"))
+        && let Ok(entries) = std::fs::read_dir(version_dir)
+    {
+        out.extend(entries.flatten().map(|e| e.path().join(&name)));
+    }
+    out.retain(|p| p.is_file());
+    out
+}
+
+/// The source file of `class` (`pkg/Name`, `pkg/Outer$Inner`, `pkg/FileKt`) in `sources`.
+fn source_entry(sources: &Path, class: &str) -> Option<(String, String)> {
+    let mut zip = zip::ZipArchive::new(std::fs::File::open(sources).ok()?).ok()?;
+    let (package, name) = class.rsplit_once('/').unwrap_or(("", class));
+    let outer = name.split('$').next().unwrap_or(name);
+    let dir = if package.is_empty() { String::new() } else { format!("{package}/") };
+    let mut candidates = vec![format!("{dir}{outer}.java"), format!("{dir}{outer}.kt")];
+    // Top-level Kotlin declarations compile to a `<File>Kt` facade class.
+    if let Some(file) = outer.strip_suffix("Kt").filter(|f| !f.is_empty()) {
+        candidates.push(format!("{dir}{file}.kt"));
+    }
+    let read = |zip: &mut zip::ZipArchive<std::fs::File>, entry: &str| {
+        let mut text = String::new();
+        zip.by_name(entry).ok()?.read_to_string(&mut text).ok()?;
+        Some(text)
+    };
+    for entry in &candidates {
+        if let Some(text) = read(&mut zip, entry) {
+            return Some((entry.clone(), text));
+        }
+    }
+    // A facade renamed with `@file:JvmName("…")`.
+    let kotlin_files: Vec<String> = zip
+        .file_names()
+        .filter_map(|n| n.ok().map(|n| n.into_owned()))
+        .filter(|n| n.ends_with(".kt") && n.strip_prefix(&dir).is_some_and(|rest| !rest.contains('/')))
+        .collect();
+    let jvm_name = format!("@file:JvmName(\"{outer}\")");
+    for entry in kotlin_files {
+        if let Some(text) = read(&mut zip, &entry)
+            && text.contains(&jvm_name)
+        {
+            return Some((entry, text));
+        }
+    }
+    None
+}
+
+/// Where `name` is declared in a Kotlin or Java source `text` (the start of the
+/// name), else the start of the file.
+pub(crate) fn declaration_range(text: &str, name: &str) -> Range {
+    const KEYWORDS: [&str; 9] = ["fun", "class", "interface", "object", "val", "var", "typealias", "enum", "record"];
+    let is_ident = |c: char| c.is_alphanumeric() || c == '_' || c == '$';
+    let mut fallback = None;
+    for (line_no, line) in text.lines().enumerate() {
+        let mut from = 0;
+        while let Some(i) = line[from..].find(name) {
+            let start = from + i;
+            let end = start + name.len();
+            from = end;
+            let before = line[..start].chars().next_back();
+            let after = line[end..].chars().next();
+            if before.is_some_and(is_ident) || after.is_some_and(is_ident) {
+                continue;
+            }
+            let head = &line[..start];
+            let words: Vec<&str> = head.split(|c: char| !is_ident(c)).filter(|w| !w.is_empty()).collect();
+            let col = head.encode_utf16().count() as u32;
+            let range = Range {
+                start: Pos::new(line_no as u32, col),
+                end: Pos::new(line_no as u32, col + name.encode_utf16().count() as u32),
+            };
+            // Kotlin: `fun <T> Foo.name(`, `class Name`; Java: `… Type name(`.
+            if words.iter().any(|w| KEYWORDS.contains(w)) {
+                return range;
+            }
+            let java_member =
+                after == Some('(') && !head.trim_end().ends_with('.') && !head.trim().is_empty() && !head.contains('=');
+            if java_member && fallback.is_none() {
+                fallback = Some(range);
+            }
+        }
+    }
+    fallback.unwrap_or_default()
+}
+
+/// The identifier around `col` (UTF-16) of `line`.
+pub(crate) fn identifier_at(line: &str, col: u32) -> Option<&str> {
+    let is_ident = |c: char| c.is_alphanumeric() || c == '_' || c == '$';
+    let mut utf16 = 0u32;
+    let mut at = line.len();
+    for (i, c) in line.char_indices() {
+        if utf16 >= col {
+            at = i;
+            break;
+        }
+        utf16 += c.len_utf16() as u32;
+    }
+    let start =
+        line[..at].rfind(|c: char| !is_ident(c)).map_or(0, |i| i + line[i..].chars().next().map_or(1, char::len_utf8));
+    let end = line[at..].find(|c: char| !is_ident(c)).map_or(line.len(), |i| at + i);
+    let word = &line[start..end];
+    (!word.is_empty() && !word.starts_with(|c: char| c.is_ascii_digit())).then_some(word)
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use std::io::Write;
+
+    fn jar(path: &Path, entries: &[(&str, &str)]) {
+        std::fs::create_dir_all(path.parent().unwrap()).unwrap();
+        let mut zip = zip::ZipWriter::new(std::fs::File::create(path).unwrap());
+        for (name, text) in entries {
+            zip.start_file(*name, zip::write::SimpleFileOptions::default()).unwrap();
+            zip.write_all(text.as_bytes()).unwrap();
+        }
+        zip.finish().unwrap();
+    }
+
+    const EXTENSIONS: &str = "package org.springframework.boot\n\nimport kotlin.reflect.KClass\n\n/**\n * Top-level function acting as a Kotlin shortcut allowing to write\n * `runApplication<Foo>(arg1, arg2)` instead of `SpringApplication.run(Foo::class.java, arg1, arg2)`.\n */\ninline fun <reified T : Any> runApplication(vararg args: String): ConfigurableApplicationContext =\n\t\tSpringApplication.run(T::class.java, *args)\n";
+
+    #[test]
+    fn parses_both_jar_uri_forms() {
+        for uri in [
+            "jar:///home/me/.m2/repository/b/spring-boot-3.5.16.jar!/org/springframework/boot/SpringApplicationExtensionsKt.class",
+            "jar:file:///home/me/.m2/repository/b/spring-boot-3.5.16.jar!/org/springframework/boot/SpringApplicationExtensionsKt.class",
+        ] {
+            let e = parse(uri).unwrap();
+            if !cfg!(windows) {
+                assert_eq!(e.jar, PathBuf::from("/home/me/.m2/repository/b/spring-boot-3.5.16.jar"));
+            }
+            assert_eq!(e.entry, "org/springframework/boot/SpringApplicationExtensionsKt.class");
+            assert_eq!(e.uri_of(&e.jar, &e.entry), uri, "written back the same way");
+        }
+        let spaced = parse("jar:///tmp/my%20libs/a.jar!/A.class").unwrap();
+        assert!(spaced.jar.to_string_lossy().contains("my libs"));
+        assert_eq!(spaced.uri_of(&spaced.jar, "A.kt"), "jar:///tmp/my%20libs/a.jar!/A.kt");
+        assert_eq!(parse("jdt://contents/a.jar/A.class"), None);
+        assert_eq!(parse("jar:///no/entry.jar"), None);
+    }
+
+    #[test]
+    fn finds_the_source_of_each_kind_of_class_in_maven_and_gradle_caches() {
+        let tmp = tempfile::tempdir().unwrap();
+        let m2 = tmp.path().join("m2/org/springframework/boot/spring-boot/3.5.16");
+        let gradle = tmp.path().join("gradle/caches/modules-2/files-2.1/org.springframework.boot/spring-boot/3.5.16");
+        let sources: &[(&str, &str)] = &[
+            ("org/springframework/boot/SpringApplicationExtensions.kt", EXTENSIONS),
+            ("org/springframework/boot/SpringApplication.java", "public class SpringApplication {}"),
+            (
+                "org/springframework/boot/Renamed.kt",
+                "@file:JvmName(\"BootUtils\")\npackage org.springframework.boot\nfun util() {}",
+            ),
+            ("org/springframework/boot/Builder.kt", "class Builder"),
+            ("Root.java", "class Root {}"),
+        ];
+        jar(&m2.join("spring-boot-3.5.16.jar"), &[]);
+        jar(&m2.join("spring-boot-3.5.16-sources.jar"), sources);
+        // Gradle keeps each file under its own hash folder.
+        jar(&gradle.join("aaa/spring-boot-3.5.16.jar"), &[]);
+        jar(&gradle.join("bbb/spring-boot-3.5.16-sources.jar"), sources);
+        for binary in [m2.join("spring-boot-3.5.16.jar"), gradle.join("aaa/spring-boot-3.5.16.jar")] {
+            let uri = |class: &str| format!("jar://{}!/{class}", binary.to_string_lossy().replace('\\', "/"));
+            for (class, file) in [
+                ("org/springframework/boot/SpringApplicationExtensionsKt.class", "SpringApplicationExtensions.kt"),
+                ("org/springframework/boot/SpringApplication.class", "SpringApplication.java"),
+                ("org/springframework/boot/SpringApplication$Running.class", "SpringApplication.java"),
+                ("org/springframework/boot/BootUtils.class", "Renamed.kt"),
+                ("org/springframework/boot/Builder.class", "Builder.kt"),
+                ("Root.class", "Root.java"),
+            ] {
+                let (source_uri, text) = class_source(&uri(class)).unwrap_or_else(|| panic!("{class} in {binary:?}"));
+                let dir = class.rsplit_once('/').map_or(String::new(), |(p, _)| format!("{p}/"));
+                assert!(source_uri.ends_with(&format!("-sources.jar!/{dir}{file}")), "{source_uri}");
+                assert_eq!(
+                    parse(&source_uri).and_then(|e| read_entry(&e.jar, &e.entry)).as_deref(),
+                    Some(text.as_str())
+                );
+            }
+            assert_eq!(class_source(&uri("org/springframework/boot/Missing.class")), None);
+        }
+        // No sources jar: nothing to show from here.
+        let lone = tmp.path().join("lone/x-1.jar");
+        jar(&lone, &[]);
+        assert_eq!(class_source(&format!("jar://{}!/X.class", lone.to_string_lossy().replace('\\', "/"))), None);
+    }
+
+    #[test]
+    fn locates_declarations_in_kotlin_and_java_sources() {
+        let at = |text: &str, name: &str| {
+            let r = declaration_range(text, name);
+            (r.start.line, r.start.col, r.end.col)
+        };
+        assert_eq!(at(EXTENSIONS, "runApplication"), (8, 29, 43), "the declaration, not the KDoc mention");
+        let java = "package a;\n\npublic class SpringApplication {\n    public static ConfigurableApplicationContext run(Class<?> source, String... args) {\n        return run(new Class<?>[] { source }, args);\n    }\n}\n";
+        assert_eq!(at(java, "SpringApplication"), (2, 13, 30));
+        assert_eq!(at(java, "run"), (3, 49, 52), "the member, not the call in its body");
+        assert_eq!(at("val x = 1", "missing"), (0, 0, 0));
+        assert_eq!(at("// é\nfun ü() {}", "ü"), (1, 4, 5), "UTF-16 columns");
+    }
+
+    #[test]
+    fn identifier_under_the_caret() {
+        let line = "    runApplication<Application>(*args)";
+        assert_eq!(identifier_at(line, 4), Some("runApplication"));
+        assert_eq!(identifier_at(line, 10), Some("runApplication"));
+        assert_eq!(identifier_at(line, 18), Some("runApplication"), "right after it");
+        assert_eq!(identifier_at(line, 20), Some("Application"));
+        assert_eq!(identifier_at("a + b", 2), None);
+    }
+}

crates/ide-lsp/src/virtual_doc.rs

@@ -60,6 +60,9 @@ pub fn title(uri: &str, decompiled: bool) -> String {
 
 /// Where the document comes from, e.g. the jar path of a `jdt://` class.
 pub fn description(uri: &str) -> Option<String> {
+    if scheme(uri) == "jar" {
+        return crate::library_source::parse(uri).map(|e| e.jar.to_string_lossy().into_owned());
+    }
     if scheme(uri) != "jdt" {
         return None;
     }

crates/ide-lsp/tests/mock_server.rs

@@ -1234,6 +1234,71 @@ async fn open_java_files_are_checked_again_after_a_project_update() {
     eventually(|| refreshed() == expected).await;
 }
 
+#[tokio::test(flavor = "multi_thread")]
+async fn library_classes_in_jars_open_as_their_source_or_decompiled() {
+    use std::io::Write;
+    let jar = |path: &Path, entries: &[(&str, &str)]| {
+        std::fs::create_dir_all(path.parent().unwrap()).unwrap();
+        let mut zip = zip::ZipWriter::new(std::fs::File::create(path).unwrap());
+        for (name, text) in entries {
+            zip.start_file(*name, zip::write::SimpleFileOptions::default()).unwrap();
+            zip.write_all(text.as_bytes()).unwrap();
+        }
+        zip.finish().unwrap();
+    };
+    let repo = tempfile::tempdir().unwrap();
+    let dir = repo.path().join("org/springframework/boot/spring-boot/3.5.16");
+    let source = "package org.springframework.boot\n\ninline fun <reified T : Any> runApplication(vararg args: String) =\n    SpringApplication.run(T::class.java, *args)\n";
+    jar(
+        &dir.join("spring-boot-3.5.16.jar"),
+        &[("org/springframework/boot/SpringApplicationExtensionsKt.class", "\u{0}")],
+    );
+    jar(
+        &dir.join("spring-boot-3.5.16-sources.jar"),
+        &[("org/springframework/boot/SpringApplicationExtensions.kt", source)],
+    );
+    let other = repo.path().join("acme/lib/1.0/lib-1.0.jar");
+    jar(&other, &[("acme/Thing.class", "\u{0}")]);
+    let jar_uri = |jar: &Path, entry: &str| format!("jar://{}!/{entry}", uri_of(jar).trim_start_matches("file://"));
+    let r = |l: u32, c0: u32, c1: u32| json!({"start":{"line":l,"character":c0},"end":{"line":l,"character":c1}});
+
+    let mut caps = incremental_caps();
+    caps["executeCommandProvider"] = json!({ "commands": ["decompile"] });
+    let h = harness_for("kotlin", "kt", &[("kotlin-lsp", caps)], &[]);
+    let (doc, _) = h.open("Main.kt", "fun main(args: Array<String>) {\n    runApplication<App>(*args)\n}\n");
+    h.wait_status("kotlin-lsp", ServerStatus::Ready).await;
+    let s = h.launcher.state("kotlin-lsp");
+
+    // A class with a cached sources jar: navigation lands on the declaration in its source.
+    let class =
+        jar_uri(&dir.join("spring-boot-3.5.16.jar"), "org/springframework/boot/SpringApplicationExtensionsKt.class");
+    s.respond("textDocument/definition", json!({ "uri": class, "range": r(7, 43, 57) }));
+    let defs = h.mgr.goto(doc, Pos::new(1, 8), GotoKind::Definition).await.unwrap();
+    let sources =
+        jar_uri(&dir.join("spring-boot-3.5.16-sources.jar"), "org/springframework/boot/SpringApplicationExtensions.kt");
+    assert_eq!(
+        defs,
+        vec![ide_core::Location { path: sources.clone().into(), range: rng(2, 29, 2, 43), uri: Some(sources.clone()) }]
+    );
+    let vd = h.mgr.virtual_document(&sources).await.unwrap();
+    assert_eq!((vd.text.as_str(), vd.title.as_str(), vd.decompiled), (source, "SpringApplicationExtensions.kt", false));
+    assert_eq!(vd.language_id.as_str(), "kotlin");
+    // The class URI itself (from usages, say) shows the same source.
+    assert_eq!(h.mgr.virtual_document(&class).await.unwrap().text, source);
+    assert!(s.requested("workspace/executeCommand").is_empty(), "no server round trip for cached sources");
+
+    // No sources jar: the server's `decompile` command.
+    let thing = jar_uri(&other, "acme/Thing.class");
+    s.respond("textDocument/definition", json!({ "uri": thing, "range": r(0, 6, 11) }));
+    let defs = h.mgr.goto(doc, Pos::new(1, 8), GotoKind::Definition).await.unwrap();
+    assert_eq!(defs[0].uri.as_deref(), Some(thing.as_str()), "kept as the server returned it");
+    s.respond("workspace/executeCommand", json!({ "code": "package acme\n\nclass Thing", "language": "kotlin" }));
+    let vd = h.mgr.virtual_document(&thing).await.unwrap();
+    assert_eq!(vd.text, "package acme\n\nclass Thing");
+    let asked = s.requested("workspace/executeCommand");
+    assert_eq!(asked[0], json!({ "command": "decompile", "arguments": [thing] }));
+}
+
 #[tokio::test(flavor = "multi_thread")]
 async fn fallback_servers_only_run_when_the_preferred_one_is_missing() {
     // Preferred installed: the alternative is never started.

src/platform/virtualDocs.test.ts

@@ -19,6 +19,15 @@ describe("virtual document URIs", () => {
     expect(virtualName(JDT)).toBe("SpringApplication.class");
     expect(virtualParent(JDT)).toBe("jdt://contents/spring-boot-4.1.1.jar/org.springframework.boot");
     expect(virtualContainer(JDT)).toBe("spring-boot-4.1.1.jar › org.springframework.boot");
+    // `jar:` targets (official Kotlin LSP, sources jars), in both URI forms.
+    for (const prefix of ["jar:///", "jar:file:///"])
+      expect(
+        virtualContainer(
+          `${prefix}home/me/.m2/repository/spring-boot-3.5.16-sources.jar!/org/springframework/boot/SpringApplicationExtensions.kt`,
+        ),
+      ).toBe("spring-boot-3.5.16-sources.jar › org.springframework.boot");
+    expect(virtualContainer("jar:///tmp/my%20libs/a.jar!/Root.class")).toBe("a.jar");
+    expect(virtualName("jar:///tmp/a.jar!/org/x/Y.class")).toBe("Y.class");
   });
 
   it("makes path helpers URI-aware", () => {

src/platform/virtualDocs.ts

@@ -37,6 +37,20 @@ export function virtualParent(uri: string): string {
  */
 export function virtualContainer(uri: string): string {
   const scheme = uri.slice(0, uri.indexOf(":"));
+  // `jar:///…/spring-boot-3.5.16.jar!/org/springframework/boot/X.class` → `spring-boot-3.5.16.jar › org.springframework.boot`
+  const jar = scheme === "jar" ? /^jar:(?:file:)?(?:\/\/)?(.*?)!\/(.*)$/.exec(uri.split(/[?#]/)[0]!) : null;
+  if (jar) {
+    const decode = (s: string) => {
+      try {
+        return decodeURIComponent(s);
+      } catch {
+        return s;
+      }
+    };
+    const archive = decode(jar[1]!.split("/").pop() ?? jar[1]!);
+    const pkg = jar[2]!.split("/").slice(0, -1).map(decode).join(".");
+    return pkg ? `${archive} › ${pkg}` : archive;
+  }
   const rest = uri.replace(/^[^:]+:(\/\/)?/, "").split(/[?#]/)[0]!;
   const segs = rest
     .split("/")