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diff --git a/crates/ide_db/src/symbol_index.rs b/crates/ide_db/src/symbol_index.rs
new file mode 100644
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+++ b/crates/ide_db/src/symbol_index.rs
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+//! This module handles fuzzy-searching of functions, structs and other symbols
+//! by name across the whole workspace and dependencies.
+//!
+//! It works by building an incrementally-updated text-search index of all
+//! symbols. The backbone of the index is the **awesome** `fst` crate by
+//! @BurntSushi.
+//!
+//! In a nutshell, you give a set of strings to `fst`, and it builds a
+//! finite state machine describing this set of strings. The strings which
+//! could fuzzy-match a pattern can also be described by a finite state machine.
+//! What is freaking cool is that you can now traverse both state machines in
+//! lock-step to enumerate the strings which are both in the input set and
+//! fuzz-match the query. Or, more formally, given two languages described by
+//! FSTs, one can build a product FST which describes the intersection of the
+//! languages.
+//!
+//! `fst` does not support cheap updating of the index, but it supports unioning
+//! of state machines. So, to account for changing source code, we build an FST
+//! for each library (which is assumed to never change) and an FST for each Rust
+//! file in the current workspace, and run a query against the union of all
+//! those FSTs.
+
+use std::{
+    cmp::Ordering,
+    fmt,
+    hash::{Hash, Hasher},
+    mem,
+    sync::Arc,
+};
+
+use base_db::{
+    salsa::{self, ParallelDatabase},
+    CrateId, FileId, SourceDatabaseExt, SourceRootId,
+};
+use fst::{self, Streamer};
+use hir::db::DefDatabase;
+use rayon::prelude::*;
+use rustc_hash::{FxHashMap, FxHashSet};
+use syntax::{
+    ast::{self, NameOwner},
+    match_ast, AstNode, Parse, SmolStr, SourceFile,
+    SyntaxKind::{self, *},
+    SyntaxNode, SyntaxNodePtr, TextRange, WalkEvent,
+};
+
+use crate::RootDatabase;
+
+#[derive(Debug)]
+pub struct Query {
+    query: String,
+    lowercased: String,
+    only_types: bool,
+    libs: bool,
+    exact: bool,
+    limit: usize,
+}
+
+impl Query {
+    pub fn new(query: String) -> Query {
+        let lowercased = query.to_lowercase();
+        Query {
+            query,
+            lowercased,
+            only_types: false,
+            libs: false,
+            exact: false,
+            limit: usize::max_value(),
+        }
+    }
+
+    pub fn only_types(&mut self) {
+        self.only_types = true;
+    }
+
+    pub fn libs(&mut self) {
+        self.libs = true;
+    }
+
+    pub fn exact(&mut self) {
+        self.exact = true;
+    }
+
+    pub fn limit(&mut self, limit: usize) {
+        self.limit = limit
+    }
+}
+
+#[salsa::query_group(SymbolsDatabaseStorage)]
+pub trait SymbolsDatabase: hir::db::HirDatabase + SourceDatabaseExt {
+    fn file_symbols(&self, file_id: FileId) -> Arc<SymbolIndex>;
+    fn library_symbols(&self) -> Arc<FxHashMap<SourceRootId, SymbolIndex>>;
+    /// The set of "local" (that is, from the current workspace) roots.
+    /// Files in local roots are assumed to change frequently.
+    #[salsa::input]
+    fn local_roots(&self) -> Arc<FxHashSet<SourceRootId>>;
+    /// The set of roots for crates.io libraries.
+    /// Files in libraries are assumed to never change.
+    #[salsa::input]
+    fn library_roots(&self) -> Arc<FxHashSet<SourceRootId>>;
+}
+
+fn library_symbols(db: &dyn SymbolsDatabase) -> Arc<FxHashMap<SourceRootId, SymbolIndex>> {
+    let _p = profile::span("library_symbols");
+
+    let roots = db.library_roots();
+    let res = roots
+        .iter()
+        .map(|&root_id| {
+            let root = db.source_root(root_id);
+            let files = root
+                .iter()
+                .map(|it| (it, SourceDatabaseExt::file_text(db, it)))
+                .collect::<Vec<_>>();
+            let symbol_index = SymbolIndex::for_files(
+                files.into_par_iter().map(|(file, text)| (file, SourceFile::parse(&text))),
+            );
+            (root_id, symbol_index)
+        })
+        .collect();
+    Arc::new(res)
+}
+
+fn file_symbols(db: &dyn SymbolsDatabase, file_id: FileId) -> Arc<SymbolIndex> {
+    db.check_canceled();
+    let parse = db.parse(file_id);
+
+    let symbols = source_file_to_file_symbols(&parse.tree(), file_id);
+
+    // FIXME: add macros here
+
+    Arc::new(SymbolIndex::new(symbols))
+}
+
+/// Need to wrap Snapshot to provide `Clone` impl for `map_with`
+struct Snap<DB>(DB);
+impl<DB: ParallelDatabase> Clone for Snap<salsa::Snapshot<DB>> {
+    fn clone(&self) -> Snap<salsa::Snapshot<DB>> {
+        Snap(self.0.snapshot())
+    }
+}
+
+// Feature: Workspace Symbol
+//
+// Uses fuzzy-search to find types, modules and functions by name across your
+// project and dependencies. This is **the** most useful feature, which improves code
+// navigation tremendously. It mostly works on top of the built-in LSP
+// functionality, however `#` and `*` symbols can be used to narrow down the
+// search. Specifically,
+//
+// - `Foo` searches for `Foo` type in the current workspace
+// - `foo#` searches for `foo` function in the current workspace
+// - `Foo*` searches for `Foo` type among dependencies, including `stdlib`
+// - `foo#*` searches for `foo` function among dependencies
+//
+// That is, `#` switches from "types" to all symbols, `*` switches from the current
+// workspace to dependencies.
+//
+// |===
+// | Editor  | Shortcut
+//
+// | VS Code | kbd:[Ctrl+T]
+// |===
+pub fn world_symbols(db: &RootDatabase, query: Query) -> Vec<FileSymbol> {
+    let _p = profile::span("world_symbols").detail(|| query.query.clone());
+
+    let tmp1;
+    let tmp2;
+    let buf: Vec<&SymbolIndex> = if query.libs {
+        tmp1 = db.library_symbols();
+        tmp1.values().collect()
+    } else {
+        let mut files = Vec::new();
+        for &root in db.local_roots().iter() {
+            let sr = db.source_root(root);
+            files.extend(sr.iter())
+        }
+
+        let snap = Snap(db.snapshot());
+        tmp2 = files
+            .par_iter()
+            .map_with(snap, |db, &file_id| db.0.file_symbols(file_id))
+            .collect::<Vec<_>>();
+        tmp2.iter().map(|it| &**it).collect()
+    };
+    query.search(&buf)
+}
+
+pub fn crate_symbols(db: &RootDatabase, krate: CrateId, query: Query) -> Vec<FileSymbol> {
+    // FIXME(#4842): This now depends on CrateDefMap, why not build the entire symbol index from
+    // that instead?
+
+    let def_map = db.crate_def_map(krate);
+    let mut files = Vec::new();
+    let mut modules = vec![def_map.root];
+    while let Some(module) = modules.pop() {
+        let data = &def_map[module];
+        files.extend(data.origin.file_id());
+        modules.extend(data.children.values());
+    }
+
+    let snap = Snap(db.snapshot());
+
+    let buf = files
+        .par_iter()
+        .map_with(snap, |db, &file_id| db.0.file_symbols(file_id))
+        .collect::<Vec<_>>();
+    let buf = buf.iter().map(|it| &**it).collect::<Vec<_>>();
+
+    query.search(&buf)
+}
+
+pub fn index_resolve(db: &RootDatabase, name_ref: &ast::NameRef) -> Vec<FileSymbol> {
+    let name = name_ref.text();
+    let mut query = Query::new(name.to_string());
+    query.exact();
+    query.limit(4);
+    world_symbols(db, query)
+}
+
+#[derive(Default)]
+pub struct SymbolIndex {
+    symbols: Vec<FileSymbol>,
+    map: fst::Map<Vec<u8>>,
+}
+
+impl fmt::Debug for SymbolIndex {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        f.debug_struct("SymbolIndex").field("n_symbols", &self.symbols.len()).finish()
+    }
+}
+
+impl PartialEq for SymbolIndex {
+    fn eq(&self, other: &SymbolIndex) -> bool {
+        self.symbols == other.symbols
+    }
+}
+
+impl Eq for SymbolIndex {}
+
+impl Hash for SymbolIndex {
+    fn hash<H: Hasher>(&self, hasher: &mut H) {
+        self.symbols.hash(hasher)
+    }
+}
+
+impl SymbolIndex {
+    fn new(mut symbols: Vec<FileSymbol>) -> SymbolIndex {
+        fn cmp(lhs: &FileSymbol, rhs: &FileSymbol) -> Ordering {
+            let lhs_chars = lhs.name.chars().map(|c| c.to_ascii_lowercase());
+            let rhs_chars = rhs.name.chars().map(|c| c.to_ascii_lowercase());
+            lhs_chars.cmp(rhs_chars)
+        }
+
+        symbols.par_sort_by(cmp);
+
+        let mut builder = fst::MapBuilder::memory();
+
+        let mut last_batch_start = 0;
+
+        for idx in 0..symbols.len() {
+            if let Some(next_symbol) = symbols.get(idx + 1) {
+                if cmp(&symbols[last_batch_start], next_symbol) == Ordering::Equal {
+                    continue;
+                }
+            }
+
+            let start = last_batch_start;
+            let end = idx + 1;
+            last_batch_start = end;
+
+            let key = symbols[start].name.as_str().to_ascii_lowercase();
+            let value = SymbolIndex::range_to_map_value(start, end);
+
+            builder.insert(key, value).unwrap();
+        }
+
+        let map = fst::Map::new(builder.into_inner().unwrap()).unwrap();
+        SymbolIndex { symbols, map }
+    }
+
+    pub fn len(&self) -> usize {
+        self.symbols.len()
+    }
+
+    pub fn memory_size(&self) -> usize {
+        self.map.as_fst().size() + self.symbols.len() * mem::size_of::<FileSymbol>()
+    }
+
+    pub(crate) fn for_files(
+        files: impl ParallelIterator<Item = (FileId, Parse<ast::SourceFile>)>,
+    ) -> SymbolIndex {
+        let symbols = files
+            .flat_map(|(file_id, file)| source_file_to_file_symbols(&file.tree(), file_id))
+            .collect::<Vec<_>>();
+        SymbolIndex::new(symbols)
+    }
+
+    fn range_to_map_value(start: usize, end: usize) -> u64 {
+        debug_assert![start <= (std::u32::MAX as usize)];
+        debug_assert![end <= (std::u32::MAX as usize)];
+
+        ((start as u64) << 32) | end as u64
+    }
+
+    fn map_value_to_range(value: u64) -> (usize, usize) {
+        let end = value as u32 as usize;
+        let start = (value >> 32) as usize;
+        (start, end)
+    }
+}
+
+impl Query {
+    pub(crate) fn search(self, indices: &[&SymbolIndex]) -> Vec<FileSymbol> {
+        let mut op = fst::map::OpBuilder::new();
+        for file_symbols in indices.iter() {
+            let automaton = fst::automaton::Subsequence::new(&self.lowercased);
+            op = op.add(file_symbols.map.search(automaton))
+        }
+        let mut stream = op.union();
+        let mut res = Vec::new();
+        while let Some((_, indexed_values)) = stream.next() {
+            for indexed_value in indexed_values {
+                let symbol_index = &indices[indexed_value.index];
+                let (start, end) = SymbolIndex::map_value_to_range(indexed_value.value);
+
+                for symbol in &symbol_index.symbols[start..end] {
+                    if self.only_types && !is_type(symbol.kind) {
+                        continue;
+                    }
+                    if self.exact && symbol.name != self.query {
+                        continue;
+                    }
+
+                    res.push(symbol.clone());
+                    if res.len() >= self.limit {
+                        return res;
+                    }
+                }
+            }
+        }
+        res
+    }
+}
+
+fn is_type(kind: SyntaxKind) -> bool {
+    matches!(kind, STRUCT | ENUM | TRAIT | TYPE_ALIAS)
+}
+
+/// The actual data that is stored in the index. It should be as compact as
+/// possible.
+#[derive(Debug, Clone, PartialEq, Eq, Hash)]
+pub struct FileSymbol {
+    pub file_id: FileId,
+    pub name: SmolStr,
+    pub kind: SyntaxKind,
+    pub range: TextRange,
+    pub ptr: SyntaxNodePtr,
+    pub name_range: Option<TextRange>,
+    pub container_name: Option<SmolStr>,
+}
+
+fn source_file_to_file_symbols(source_file: &SourceFile, file_id: FileId) -> Vec<FileSymbol> {
+    let mut symbols = Vec::new();
+    let mut stack = Vec::new();
+
+    for event in source_file.syntax().preorder() {
+        match event {
+            WalkEvent::Enter(node) => {
+                if let Some(mut symbol) = to_file_symbol(&node, file_id) {
+                    symbol.container_name = stack.last().cloned();
+
+                    stack.push(symbol.name.clone());
+                    symbols.push(symbol);
+                }
+            }
+
+            WalkEvent::Leave(node) => {
+                if to_symbol(&node).is_some() {
+                    stack.pop();
+                }
+            }
+        }
+    }
+
+    symbols
+}
+
+fn to_symbol(node: &SyntaxNode) -> Option<(SmolStr, SyntaxNodePtr, TextRange)> {
+    fn decl<N: NameOwner>(node: N) -> Option<(SmolStr, SyntaxNodePtr, TextRange)> {
+        let name = node.name()?;
+        let name_range = name.syntax().text_range();
+        let name = name.text().clone();
+        let ptr = SyntaxNodePtr::new(node.syntax());
+
+        Some((name, ptr, name_range))
+    }
+    match_ast! {
+        match node {
+            ast::Fn(it) => decl(it),
+            ast::Struct(it) => decl(it),
+            ast::Enum(it) => decl(it),
+            ast::Trait(it) => decl(it),
+            ast::Module(it) => decl(it),
+            ast::TypeAlias(it) => decl(it),
+            ast::Const(it) => decl(it),
+            ast::Static(it) => decl(it),
+            ast::MacroCall(it) => {
+                if it.is_macro_rules().is_some() {
+                    decl(it)
+                } else {
+                    None
+                }
+            },
+            _ => None,
+        }
+    }
+}
+
+fn to_file_symbol(node: &SyntaxNode, file_id: FileId) -> Option<FileSymbol> {
+    to_symbol(node).map(move |(name, ptr, name_range)| FileSymbol {
+        name,
+        kind: node.kind(),
+        range: node.text_range(),
+        ptr,
+        file_id,
+        name_range: Some(name_range),
+        container_name: None,
+    })
+}