Merge #8955

8955: feature: Support standalone Rust files r=matklad a=SomeoneToIgnore

![standalone](https://user-images.githubusercontent.com/2690773/119277037-0b579380-bc26-11eb-8d77-20d46ab4916a.gif)

Closes https://github.com/rust-analyzer/rust-analyzer/issues/6388

Caveats: 

* I've decided to support multiple detached files in the code (anticipating the scratch files), but I found no way to open multiple files in VSCode at once: running `code *.rs` makes the plugin to register in the `vscode.workspace.textDocuments` only the first file, while code actually displays all files later.
Apparently what happens is the same as when you have VSCode open at some workplace already and then run `code some_other_file.rs`: it gets opened in the same workspace of the same VSCode with no server to support it.
If there's a way to override it, I'd appreciate the pointer.

* No way to toggle inlay hints, since the setting is updated for the workspace (which does not exist for a single file opened)
> [2021-05-24 00:22:49.100] [exthost] [error] Error: Unable to write to Workspace Settings because no workspace is opened. Please open a workspace first and try again.

* No runners/lens to run or check the code are implemented for this mode. 
In theory, we can detect `rustc`, run it on a file and run the resulting binary, but not sure if worth doing it at this stage.

Otherwise imports, hints, completion and other features work.

Co-authored-by: Kirill Bulatov <[email protected]>

2 files changed, 91 insertions, 2 deletions

diff --git a/crates/project_model/src/sysroot.rs b/crates/project_model/src/sysroot.rs
index 3b0ff506d..4e39d6dd3 100644
--- a/crates/project_model/src/sysroot.rs
+++ b/crates/project_model/src/sysroot.rs
@@ -50,7 +50,9 @@ impl Sysroot {
 
     pub fn discover(cargo_toml: &AbsPath) -> Result<Sysroot> {
         log::debug!("Discovering sysroot for {}", cargo_toml.display());
-        let current_dir = cargo_toml.parent().unwrap();
+        let current_dir = cargo_toml.parent().ok_or_else(|| {
+            format_err!("Failed to find the parent directory for {}", cargo_toml.display())
+        })?;
         let sysroot_dir = discover_sysroot_dir(current_dir)?;
         let sysroot_src_dir = discover_sysroot_src_dir(&sysroot_dir, current_dir)?;
         let res = Sysroot::load(&sysroot_src_dir)?;
diff --git a/crates/project_model/src/workspace.rs b/crates/project_model/src/workspace.rs
index 607e62ea5..84990075f 100644
--- a/crates/project_model/src/workspace.rs
+++ b/crates/project_model/src/workspace.rs
@@ -4,7 +4,7 @@
 
 use std::{collections::VecDeque, fmt, fs, path::Path, process::Command};
 
-use anyhow::{Context, Result};
+use anyhow::{format_err, Context, Result};
 use base_db::{CrateDisplayName, CrateGraph, CrateId, CrateName, Edition, Env, FileId, ProcMacro};
 use cargo_workspace::DepKind;
 use cfg::CfgOptions;
@@ -49,6 +49,18 @@ pub enum ProjectWorkspace {
     },
     /// Project workspace was manually specified using a `rust-project.json` file.
     Json { project: ProjectJson, sysroot: Option<Sysroot>, rustc_cfg: Vec<CfgFlag> },
+
+    // FIXME: The primary limitation of this approach is that the set of detached files needs to be fixed at the beginning.
+    // That's not the end user experience we should strive for.
+    // Ideally, you should be able to just open a random detached file in existing cargo projects, and get the basic features working.
+    // That needs some changes on the salsa-level though.
+    // In particular, we should split the unified CrateGraph (which currently has maximal durability) into proper crate graph, and a set of ad hoc roots (with minimal durability).
+    // Then, we need to hide the graph behind the queries such that most queries look only at the proper crate graph, and fall back to ad hoc roots only if there's no results.
+    // After this, we should be able to tweak the logic in reload.rs to add newly opened files, which don't belong to any existing crates, to the set of the detached files.
+    // //
+    /// Project with a set of disjoint files, not belonging to any particular workspace.
+    /// Backed by basic sysroot crates for basic completion and highlighting.
+    DetachedFiles { files: Vec<AbsPathBuf>, sysroot: Sysroot, rustc_cfg: Vec<CfgFlag> },
 }
 
 impl fmt::Debug for ProjectWorkspace {
@@ -75,6 +87,12 @@ impl fmt::Debug for ProjectWorkspace {
                 debug_struct.field("n_rustc_cfg", &rustc_cfg.len());
                 debug_struct.finish()
             }
+            ProjectWorkspace::DetachedFiles { files, sysroot, rustc_cfg } => f
+                .debug_struct("DetachedFiles")
+                .field("n_files", &files.len())
+                .field("n_sysroot_crates", &sysroot.crates().len())
+                .field("n_rustc_cfg", &rustc_cfg.len())
+                .finish(),
         }
     }
 }
@@ -165,6 +183,14 @@ impl ProjectWorkspace {
         Ok(ProjectWorkspace::Json { project: project_json, sysroot, rustc_cfg })
     }
 
+    pub fn load_detached_files(detached_files: Vec<AbsPathBuf>) -> Result<ProjectWorkspace> {
+        let sysroot = Sysroot::discover(
+            &detached_files.first().ok_or_else(|| format_err!("No detached files to load"))?,
+        )?;
+        let rustc_cfg = rustc_cfg::get(None, None);
+        Ok(ProjectWorkspace::DetachedFiles { files: detached_files, sysroot, rustc_cfg })
+    }
+
     /// Returns the roots for the current `ProjectWorkspace`
     /// The return type contains the path and whether or not
     /// the root is a member of the current workspace
@@ -224,6 +250,19 @@ impl ProjectWorkspace {
                     })
                 }))
                 .collect(),
+            ProjectWorkspace::DetachedFiles { files, sysroot, .. } => files
+                .into_iter()
+                .map(|detached_file| PackageRoot {
+                    is_member: true,
+                    include: vec![detached_file.clone()],
+                    exclude: Vec::new(),
+                })
+                .chain(sysroot.crates().map(|krate| PackageRoot {
+                    is_member: false,
+                    include: vec![sysroot[krate].root_dir().to_path_buf()],
+                    exclude: Vec::new(),
+                }))
+                .collect(),
         }
     }
 
@@ -234,6 +273,9 @@ impl ProjectWorkspace {
                 let rustc_package_len = rustc.as_ref().map_or(0, |rc| rc.packages().len());
                 cargo.packages().len() + sysroot.crates().len() + rustc_package_len
             }
+            ProjectWorkspace::DetachedFiles { sysroot, files, .. } => {
+                sysroot.crates().len() + files.len()
+            }
         }
     }
 
@@ -267,6 +309,9 @@ impl ProjectWorkspace {
                 rustc,
                 rustc.as_ref().zip(build_data).and_then(|(it, map)| map.get(it.workspace_root())),
             ),
+            ProjectWorkspace::DetachedFiles { files, sysroot, rustc_cfg } => {
+                detached_files_to_crate_graph(rustc_cfg.clone(), load, files, sysroot)
+            }
         };
         if crate_graph.patch_cfg_if() {
             log::debug!("Patched std to depend on cfg-if")
@@ -474,6 +519,48 @@ fn cargo_to_crate_graph(
     crate_graph
 }
 
+fn detached_files_to_crate_graph(
+    rustc_cfg: Vec<CfgFlag>,
+    load: &mut dyn FnMut(&AbsPath) -> Option<FileId>,
+    detached_files: &[AbsPathBuf],
+    sysroot: &Sysroot,
+) -> CrateGraph {
+    let _p = profile::span("detached_files_to_crate_graph");
+    let mut crate_graph = CrateGraph::default();
+    let (public_deps, _libproc_macro) =
+        sysroot_to_crate_graph(&mut crate_graph, sysroot, rustc_cfg.clone(), load);
+
+    let mut cfg_options = CfgOptions::default();
+    cfg_options.extend(rustc_cfg);
+
+    for detached_file in detached_files {
+        let file_id = match load(&detached_file) {
+            Some(file_id) => file_id,
+            None => {
+                log::error!("Failed to load detached file {:?}", detached_file);
+                continue;
+            }
+        };
+        let display_name = detached_file
+            .file_stem()
+            .and_then(|os_str| os_str.to_str())
+            .map(|file_stem| CrateDisplayName::from_canonical_name(file_stem.to_string()));
+        let detached_file_crate = crate_graph.add_crate_root(
+            file_id,
+            Edition::Edition2018,
+            display_name,
+            cfg_options.clone(),
+            Env::default(),
+            Vec::new(),
+        );
+
+        for (name, krate) in public_deps.iter() {
+            add_dep(&mut crate_graph, detached_file_crate, name.clone(), *krate);
+        }
+    }
+    crate_graph
+}
+
 fn handle_rustc_crates(
     rustc_workspace: &CargoWorkspace,
     load: &mut dyn FnMut(&AbsPath) -> Option<FileId>,