4 files changed, 353 insertions, 4 deletions

diff --git a/crates/hir/Cargo.toml b/crates/hir/Cargo.toml
index dbb2986b6..d0ddca27f 100644
--- a/crates/hir/Cargo.toml
+++ b/crates/hir/Cargo.toml
@@ -14,6 +14,7 @@ rustc-hash = "1.1.0"
 either = "1.5.3"
 arrayvec = "0.5.1"
 itertools = "0.9.0"
+url = "2.1.1"
 
 stdx = { path = "../stdx" }
 syntax = { path = "../syntax" }
@@ -22,3 +23,4 @@ profile = { path = "../profile" }
 hir_expand = { path = "../hir_expand" }
 hir_def = { path = "../hir_def" }
 hir_ty = { path = "../hir_ty" }
+tt = { path = "../tt" }
diff --git a/crates/hir/src/code_model.rs b/crates/hir/src/code_model.rs
index 3d92d0c0d..9395efe4f 100644
--- a/crates/hir/src/code_model.rs
+++ b/crates/hir/src/code_model.rs
@@ -20,7 +20,7 @@ use hir_def::{
     type_ref::{Mutability, TypeRef},
     AdtId, AssocContainerId, ConstId, DefWithBodyId, EnumId, FunctionId, GenericDefId, HasModule,
     ImplId, LocalEnumVariantId, LocalFieldId, LocalModuleId, Lookup, ModuleId, StaticId, StructId,
-    TraitId, TypeAliasId, TypeParamId, UnionId,
+    TraitId, TypeAliasId, TypeParamId, UnionId, VariantId,
 };
 use hir_expand::{
     diagnostics::DiagnosticSink,
@@ -39,10 +39,12 @@ use syntax::{
     ast::{self, AttrsOwner, NameOwner},
     AstNode, SmolStr,
 };
+use tt::{Ident, Leaf, Literal, TokenTree};
 
 use crate::{
     db::{DefDatabase, HirDatabase},
     has_source::HasSource,
+    link_rewrite::Resolvable,
     HirDisplay, InFile, Name,
 };
 
@@ -122,6 +124,33 @@ impl Crate {
     pub fn all(db: &dyn HirDatabase) -> Vec<Crate> {
         db.crate_graph().iter().map(|id| Crate { id }).collect()
     }
+
+    /// Try to get the root URL of the documentation of a crate.
+    pub fn get_doc_url(self: &Crate, db: &dyn HirDatabase) -> Option<String> {
+        // Look for #![doc(html_root_url = "...")]
+        let attrs = db.attrs(AttrDef::from(self.root_module(db)).into());
+        let doc_attr_q = attrs.by_key("doc");
+
+        let doc_url = if doc_attr_q.exists() {
+            doc_attr_q.tt_values().map(|tt| {
+            let name = tt.token_trees.iter()
+                .skip_while(|tt| !matches!(tt, TokenTree::Leaf(Leaf::Ident(Ident{text: ref ident, ..})) if ident == "html_root_url"))
+                .skip(2)
+                .next();
+
+            match name {
+                Some(TokenTree::Leaf(Leaf::Literal(Literal{ref text, ..}))) => Some(text),
+                _ => None
+            }
+        }).flat_map(|t| t).next().map(|s| s.to_string())
+        } else {
+            None
+        };
+
+        doc_url
+            .map(|s| s.trim_matches('"').trim_end_matches("/").to_owned() + "/")
+            .map(|s| s.to_string())
+    }
 }
 
 #[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
@@ -198,7 +227,6 @@ impl ModuleDef {
             ModuleDef::Function(it) => Some(it.name(db)),
             ModuleDef::EnumVariant(it) => Some(it.name(db)),
             ModuleDef::TypeAlias(it) => Some(it.name(db)),
-
             ModuleDef::Module(it) => it.name(db),
             ModuleDef::Const(it) => it.name(db),
             ModuleDef::Static(it) => it.name(db),
@@ -206,6 +234,23 @@ impl ModuleDef {
             ModuleDef::BuiltinType(it) => Some(it.as_name()),
         }
     }
+
+    pub fn resolver<D: DefDatabase + HirDatabase>(&self, db: &D) -> Option<Resolver> {
+        Some(match self {
+            ModuleDef::Module(m) => ModuleId::from(m.clone()).resolver(db),
+            ModuleDef::Function(f) => FunctionId::from(f.clone()).resolver(db),
+            ModuleDef::Adt(adt) => AdtId::from(adt.clone()).resolver(db),
+            ModuleDef::EnumVariant(ev) => {
+                GenericDefId::from(GenericDef::from(ev.clone())).resolver(db)
+            }
+            ModuleDef::Const(c) => GenericDefId::from(GenericDef::from(c.clone())).resolver(db),
+            ModuleDef::Static(s) => StaticId::from(s.clone()).resolver(db),
+            ModuleDef::Trait(t) => TraitId::from(t.clone()).resolver(db),
+            ModuleDef::TypeAlias(t) => ModuleId::from(t.module(db)).resolver(db),
+            // FIXME: This should be a resolver relative to `std/core`
+            ModuleDef::BuiltinType(_t) => None?,
+        })
+    }
 }
 
 pub use hir_def::{
@@ -1771,3 +1816,76 @@ pub trait HasVisibility {
         vis.is_visible_from(db.upcast(), module.id)
     }
 }
+
+impl Resolvable for ModuleDef {
+    fn resolver<D: DefDatabase + HirDatabase>(&self, db: &D) -> Option<Resolver> {
+        Some(match self {
+            ModuleDef::Module(m) => ModuleId::from(m.clone()).resolver(db),
+            ModuleDef::Function(f) => FunctionId::from(f.clone()).resolver(db),
+            ModuleDef::Adt(adt) => AdtId::from(adt.clone()).resolver(db),
+            ModuleDef::EnumVariant(ev) => {
+                GenericDefId::from(GenericDef::from(ev.clone())).resolver(db)
+            }
+            ModuleDef::Const(c) => GenericDefId::from(GenericDef::from(c.clone())).resolver(db),
+            ModuleDef::Static(s) => StaticId::from(s.clone()).resolver(db),
+            ModuleDef::Trait(t) => TraitId::from(t.clone()).resolver(db),
+            ModuleDef::TypeAlias(t) => ModuleId::from(t.module(db)).resolver(db),
+            // FIXME: This should be a resolver relative to `std/core`
+            ModuleDef::BuiltinType(_t) => None?,
+        })
+    }
+
+    fn try_into_module_def(self) -> Option<ModuleDef> {
+        Some(self)
+    }
+}
+
+impl Resolvable for TypeParam {
+    fn resolver<D: DefDatabase + HirDatabase>(&self, db: &D) -> Option<Resolver> {
+        Some(Into::<ModuleId>::into(self.module(db)).resolver(db))
+    }
+
+    fn try_into_module_def(self) -> Option<ModuleDef> {
+        None
+    }
+}
+
+impl Resolvable for MacroDef {
+    fn resolver<D: DefDatabase + HirDatabase>(&self, db: &D) -> Option<Resolver> {
+        Some(Into::<ModuleId>::into(self.module(db)?).resolver(db))
+    }
+
+    fn try_into_module_def(self) -> Option<ModuleDef> {
+        None
+    }
+}
+
+impl Resolvable for Field {
+    fn resolver<D: DefDatabase + HirDatabase>(&self, db: &D) -> Option<Resolver> {
+        Some(Into::<VariantId>::into(Into::<VariantDef>::into(self.parent_def(db))).resolver(db))
+    }
+
+    fn try_into_module_def(self) -> Option<ModuleDef> {
+        None
+    }
+}
+
+impl Resolvable for ImplDef {
+    fn resolver<D: DefDatabase + HirDatabase>(&self, db: &D) -> Option<Resolver> {
+        Some(Into::<ModuleId>::into(self.module(db)).resolver(db))
+    }
+
+    fn try_into_module_def(self) -> Option<ModuleDef> {
+        None
+    }
+}
+
+impl Resolvable for Local {
+    fn resolver<D: DefDatabase + HirDatabase>(&self, db: &D) -> Option<Resolver> {
+        Some(Into::<ModuleId>::into(self.module(db)).resolver(db))
+    }
+
+    fn try_into_module_def(self) -> Option<ModuleDef> {
+        None
+    }
+}
diff --git a/crates/hir/src/lib.rs b/crates/hir/src/lib.rs
index 78d8651cb..ae2f1fd4d 100644
--- a/crates/hir/src/lib.rs
+++ b/crates/hir/src/lib.rs
@@ -27,6 +27,7 @@ pub mod diagnostics;
 
 mod from_id;
 mod code_model;
+mod link_rewrite;
 
 mod has_source;
 
@@ -38,6 +39,7 @@ pub use crate::{
         ScopeDef, Static, Struct, Trait, Type, TypeAlias, TypeParam, Union, VariantDef, Visibility,
     },
     has_source::HasSource,
+    link_rewrite::resolve_doc_link,
     semantics::{original_range, PathResolution, Semantics, SemanticsScope},
 };
 
@@ -47,13 +49,14 @@ pub use hir_def::{
     body::scope::ExprScopes,
     builtin_type::BuiltinType,
     docs::Documentation,
+    item_scope::ItemInNs,
     nameres::ModuleSource,
     path::ModPath,
     type_ref::{Mutability, TypeRef},
 };
 pub use hir_expand::{
-    name::Name, HirFileId, InFile, MacroCallId, MacroCallLoc, /* FIXME */ MacroDefId,
-    MacroFile, Origin,
+    name::AsName, name::Name, HirFileId, InFile, MacroCallId, MacroCallLoc,
+    /* FIXME */ MacroDefId, MacroFile, Origin,
 };
 pub use hir_ty::display::HirDisplay;
 
diff --git a/crates/hir/src/link_rewrite.rs b/crates/hir/src/link_rewrite.rs
new file mode 100644
index 000000000..dad3a39cf
--- /dev/null
+++ b/crates/hir/src/link_rewrite.rs
@@ -0,0 +1,226 @@
+//! Resolves and rewrites links in markdown documentation for hovers/completion windows.
+
+use std::iter::once;
+
+use itertools::Itertools;
+use url::Url;
+
+use crate::{db::HirDatabase, Adt, AsName, Crate, Hygiene, ItemInNs, ModPath, ModuleDef};
+use hir_def::{db::DefDatabase, resolver::Resolver};
+use syntax::ast::Path;
+
+pub fn resolve_doc_link<T: Resolvable + Clone, D: DefDatabase + HirDatabase>(
+    db: &D,
+    definition: &T,
+    link_text: &str,
+    link_target: &str,
+) -> Option<(String, String)> {
+    try_resolve_intra(db, definition, link_text, &link_target).or_else(|| {
+        if let Some(definition) = definition.clone().try_into_module_def() {
+            try_resolve_path(db, &definition, &link_target)
+                .map(|target| (target, link_text.to_string()))
+        } else {
+            None
+        }
+    })
+}
+
+/// Try to resolve path to local documentation via intra-doc-links (i.e. `super::gateway::Shard`).
+///
+/// See [RFC1946](https://github.com/rust-lang/rfcs/blob/master/text/1946-intra-rustdoc-links.md).
+fn try_resolve_intra<T: Resolvable, D: DefDatabase + HirDatabase>(
+    db: &D,
+    definition: &T,
+    link_text: &str,
+    link_target: &str,
+) -> Option<(String, String)> {
+    // Set link_target for implied shortlinks
+    let link_target =
+        if link_target.is_empty() { link_text.trim_matches('`') } else { link_target };
+
+    // Namespace disambiguation
+    let namespace = Namespace::from_intra_spec(link_target);
+
+    // Strip prefixes/suffixes
+    let link_target = strip_prefixes_suffixes(link_target);
+
+    // Parse link as a module path
+    let path = Path::parse(link_target).ok()?;
+    let modpath = ModPath::from_src(path, &Hygiene::new_unhygienic()).unwrap();
+
+    // Resolve it relative to symbol's location (according to the RFC this should consider small scopes)
+    let resolver = definition.resolver(db)?;
+
+    let resolved = resolver.resolve_module_path_in_items(db, &modpath);
+    let (defid, namespace) = match namespace {
+        // FIXME: .or(resolved.macros)
+        None => resolved
+            .types
+            .map(|t| (t.0, Namespace::Types))
+            .or(resolved.values.map(|t| (t.0, Namespace::Values)))?,
+        Some(ns @ Namespace::Types) => (resolved.types?.0, ns),
+        Some(ns @ Namespace::Values) => (resolved.values?.0, ns),
+        // FIXME:
+        Some(Namespace::Macros) => None?,
+    };
+
+    // Get the filepath of the final symbol
+    let def: ModuleDef = defid.into();
+    let module = def.module(db)?;
+    let krate = module.krate();
+    let ns = match namespace {
+        Namespace::Types => ItemInNs::Types(defid),
+        Namespace::Values => ItemInNs::Values(defid),
+        // FIXME:
+        Namespace::Macros => None?,
+    };
+    let import_map = db.import_map(krate.into());
+    let path = import_map.path_of(ns)?;
+
+    Some((
+        get_doc_url(db, &krate)?
+            .join(&format!("{}/", krate.display_name(db)?))
+            .ok()?
+            .join(&path.segments.iter().map(|name| name.to_string()).join("/"))
+            .ok()?
+            .join(&get_symbol_filename(db, &def)?)
+            .ok()?
+            .into_string(),
+        strip_prefixes_suffixes(link_text).to_string(),
+    ))
+}
+
+/// Try to resolve path to local documentation via path-based links (i.e. `../gateway/struct.Shard.html`).
+fn try_resolve_path(db: &dyn HirDatabase, moddef: &ModuleDef, link_target: &str) -> Option<String> {
+    if !link_target.contains("#") && !link_target.contains(".html") {
+        return None;
+    }
+    let ns = ItemInNs::Types(moddef.clone().into());
+
+    let module = moddef.module(db)?;
+    let krate = module.krate();
+    let import_map = db.import_map(krate.into());
+    let base = once(format!("{}", krate.display_name(db)?))
+        .chain(import_map.path_of(ns)?.segments.iter().map(|name| format!("{}", name)))
+        .join("/");
+
+    get_doc_url(db, &krate)
+        .and_then(|url| url.join(&base).ok())
+        .and_then(|url| {
+            get_symbol_filename(db, moddef).as_deref().map(|f| url.join(f).ok()).flatten()
+        })
+        .and_then(|url| url.join(link_target).ok())
+        .map(|url| url.into_string())
+}
+
+// Strip prefixes, suffixes, and inline code marks from the given string.
+fn strip_prefixes_suffixes(mut s: &str) -> &str {
+    s = s.trim_matches('`');
+
+    [
+        (TYPES.0.iter(), TYPES.1.iter()),
+        (VALUES.0.iter(), VALUES.1.iter()),
+        (MACROS.0.iter(), MACROS.1.iter()),
+    ]
+    .iter()
+    .for_each(|(prefixes, suffixes)| {
+        prefixes.clone().for_each(|prefix| s = s.trim_start_matches(*prefix));
+        suffixes.clone().for_each(|suffix| s = s.trim_end_matches(*suffix));
+    });
+    let s = s.trim_start_matches("@").trim();
+    s
+}
+
+fn get_doc_url(db: &dyn HirDatabase, krate: &Crate) -> Option<Url> {
+    krate
+        .get_doc_url(db)
+        .or_else(||
+        // Fallback to docs.rs
+        // FIXME: Specify an exact version here. This may be difficult, as multiple versions of the same crate could exist.
+        Some(format!("https://docs.rs/{}/*/", krate.display_name(db)?)))
+        .and_then(|s| Url::parse(&s).ok())
+}
+
+/// Get the filename and extension generated for a symbol by rustdoc.
+///
+/// Example: `struct.Shard.html`
+fn get_symbol_filename(db: &dyn HirDatabase, definition: &ModuleDef) -> Option<String> {
+    Some(match definition {
+        ModuleDef::Adt(adt) => match adt {
+            Adt::Struct(s) => format!("struct.{}.html", s.name(db)),
+            Adt::Enum(e) => format!("enum.{}.html", e.name(db)),
+            Adt::Union(u) => format!("union.{}.html", u.name(db)),
+        },
+        ModuleDef::Module(_) => "index.html".to_string(),
+        ModuleDef::Trait(t) => format!("trait.{}.html", t.name(db)),
+        ModuleDef::TypeAlias(t) => format!("type.{}.html", t.name(db)),
+        ModuleDef::BuiltinType(t) => format!("primitive.{}.html", t.as_name()),
+        ModuleDef::Function(f) => format!("fn.{}.html", f.name(db)),
+        ModuleDef::EnumVariant(ev) => {
+            format!("enum.{}.html#variant.{}", ev.parent_enum(db).name(db), ev.name(db))
+        }
+        ModuleDef::Const(c) => format!("const.{}.html", c.name(db)?),
+        ModuleDef::Static(s) => format!("static.{}.html", s.name(db)?),
+    })
+}
+
+#[derive(PartialEq, Eq, Hash, Copy, Clone, Debug)]
+enum Namespace {
+    Types,
+    Values,
+    Macros,
+}
+
+static TYPES: ([&str; 7], [&str; 0]) =
+    (["type", "struct", "enum", "mod", "trait", "union", "module"], []);
+static VALUES: ([&str; 8], [&str; 1]) =
+    (["value", "function", "fn", "method", "const", "static", "mod", "module"], ["()"]);
+static MACROS: ([&str; 1], [&str; 1]) = (["macro"], ["!"]);
+
+impl Namespace {
+    /// Extract the specified namespace from an intra-doc-link if one exists.
+    ///
+    /// # Examples
+    ///
+    /// * `struct MyStruct` -> `Namespace::Types`
+    /// * `panic!` -> `Namespace::Macros`
+    /// * `fn@from_intra_spec` -> `Namespace::Values`
+    fn from_intra_spec(s: &str) -> Option<Self> {
+        [
+            (Namespace::Types, (TYPES.0.iter(), TYPES.1.iter())),
+            (Namespace::Values, (VALUES.0.iter(), VALUES.1.iter())),
+            (Namespace::Macros, (MACROS.0.iter(), MACROS.1.iter())),
+        ]
+        .iter()
+        .filter(|(_ns, (prefixes, suffixes))| {
+            prefixes
+                .clone()
+                .map(|prefix| {
+                    s.starts_with(*prefix)
+                        && s.chars()
+                            .nth(prefix.len() + 1)
+                            .map(|c| c == '@' || c == ' ')
+                            .unwrap_or(false)
+                })
+                .any(|cond| cond)
+                || suffixes
+                    .clone()
+                    .map(|suffix| {
+                        s.starts_with(*suffix)
+                            && s.chars()
+                                .nth(suffix.len() + 1)
+                                .map(|c| c == '@' || c == ' ')
+                                .unwrap_or(false)
+                    })
+                    .any(|cond| cond)
+        })
+        .map(|(ns, (_, _))| *ns)
+        .next()
+    }
+}
+
+/// Sealed trait used solely for the generic bound on [`resolve_doc_link`].
+pub trait Resolvable {
+    fn resolver<D: DefDatabase + HirDatabase>(&self, db: &D) -> Option<Resolver>;
+    fn try_into_module_def(self) -> Option<ModuleDef>;
+}