Use rustc --print sysroot for default sources

Previously, we were used /usr/share/doc/rust{,-doc}/html as a default
source, assuming that it would contain the standard library
documentation.  This is true if the user installed the Rust
documentation manually or using a package manager.  If they use rustup
instead, the documentation is placed in a subdirectory of ~/.rustup.

With this patch, we call `rustc --print sysroot` to determine the
installation directory of the currently activated Rust toolchain –
either the system Rust installation or a Rust installation managed by
rustup.  We then use `$(sysroot)/share/doc/…` as a default path for the
standard library documentation (with /usr as a fallback if the call to
rustc does not return a valid path).

CHANGELOG.md

@@ -5,6 +5,11 @@ SPDX-License-Identifier: MIT
 
 # Changelog for rusty-man
 
+## Unreleased
+
+- Use `rustc --print sysroot` to determine the Rust installation directory
+  instead of always using `/usr`.
+
 ## v0.1.2 (2020-07-25)
 
 This patch release adds basic logging output and a new `-e`/`--examples` option

README.md

@@ -31,17 +31,17 @@ You don’t have to specificy the full item name:
 $ rusty-man NodeRef
 ```
 
-If the documentation is not placed in `./target/doc` or
-`/usr/share/doc/rust{,-doc}/html`, you have to manually set the source path
-using the `-s`/`--source` option:
+If the documentation is not placed in `./target/doc` or the Rust installation
+directory, you have to manually set the source path using the `-s`/`--source`
+option:
 ```
-$ rusty-man --source ~/.rustup/toolchains/stable-*/share/doc/html io::Error
+$ rusty-man --source my/other/crate/target/doc io::Error
 ```
 
 If there are multiple matches for the keyword, rusty-man will show you a list
 of all matching items:
 ```
-$ rusty-man --source ~/.rustup/toolchains/stable-*/share/doc/rust/html u8
+$ rusty-man --source my/other/crate/target/doc u8
 Found mulitple matches for u8 – select one of:
 
 [ 0 ] core::u8: The 8-bit unsigned integer type.

src/main.rs

@@ -6,8 +6,9 @@
 //! rusty-man opens the documentation for a given keyword.  It performs these steps to find the
 //! documentation for an item:
 //! 1. The sources, currently only local directories, are loaded, see the `load_sources` funnction
-//!    and the `source` module.  Per default, we look for documentation in `/usr/share/doc` and in
-//!    `./target/doc`.
+//!    and the `source` module.  Per default, we look for documentation in the directory
+//!    `share/doc/rust{,-doc}/html` relative to the Rust installation path (`rustc --print sysroot`
+//!    or `usr`) and in `./target/doc`.
 //! 2. We split the keyword `{crate}::{item}` into the crate and the item and try to find the crate
 //!    in one of the sources – see the `find_crate` function.
 //! 3. If we found a crate, we look up the item in the `all.html` file of the crate and load the
@@ -50,8 +51,9 @@ use structopt::StructOpt;
 /// rusty-man reads the HTML documentation generated by rustdoc and displays a documentation item.
 /// Make sure to run `cargo doc` before using rusty-man.  Per default, rusty-man looks up
 /// documentation in the ./target/doc directory relative to the current working directory and in
-/// the system documentation directories (/usr/share/doc/rust{,-doc}/html).  Use the -s/--source
-/// option if you want to read the documentation from a different directory.
+/// the system documentation directories (share/doc/rust{,-doc}/html relative to the Rust sysroot,
+/// see `rustc --print sysroot`, or /usr).  Use the -s/--source option if you want to read the
+/// documentation from a different directory.
 ///
 /// rusty-man tries to find an item that exactly matches the given keyword.  If it doesn’t find an
 /// exact match, it reads the search indexes of all available sources and tries to find a partial
@@ -74,8 +76,9 @@ struct Opt {
 
     /// Do not search the default documentation sources
     ///
-    /// If this option is not set, rusty-man appends `/usr/share/doc/rust{,-doc}/html` and
-    /// `target/doc` to the list of sources if they exist.
+    /// If this option is not set, rusty-man appends `$sysroot/share/doc/rust{,-doc}/html` and
+    /// `target/doc` to the list of sources if they exist.  `$sysroot` is the output of `rustc
+    /// --print sysroot` or `/usr` if that command does not output a valid path.
     #[structopt(long)]
     no_default_sources: bool,
 
@@ -124,12 +127,6 @@ fn main() -> anyhow::Result<()> {
     }
 }
 
-const DEFAULT_SOURCES: &[&str] = &[
-    "/usr/share/doc/rust/html",
-    "/usr/share/doc/rust-doc/html",
-    "./target/doc",
-];
-
 /// Load all sources given as a command-line argument and, if enabled, the default sources.
 fn load_sources(
     sources: &[String],
@@ -138,10 +135,9 @@ fn load_sources(
     let mut vec: Vec<Box<dyn source::Source>> = Vec::new();
 
     if load_default_sources {
-        for s in DEFAULT_SOURCES {
-            let path: &path::Path = s.as_ref();
+        for path in get_default_sources() {
             if path.is_dir() {
-                vec.push(source::get_source(path)?);
+                vec.push(source::get_source(&path)?);
             } else {
                 log::info!(
                     "Ignoring default source '{}' because it does not exist",
@@ -161,6 +157,29 @@ fn load_sources(
     Ok(vec)
 }
 
+fn get_default_sources() -> Vec<path::PathBuf> {
+    let mut default_sources: Vec<path::PathBuf> = Vec::new();
+
+    let sysroot = get_sysroot().unwrap_or_else(|| path::PathBuf::from("/usr"));
+    default_sources.push(sysroot.join("share/doc/rust/html"));
+    default_sources.push(sysroot.join("share/doc/rust-doc/html"));
+
+    default_sources.push(path::PathBuf::from("./target/doc"));
+
+    default_sources
+}
+
+fn get_sysroot() -> Option<path::PathBuf> {
+    std::process::Command::new("rustc")
+        .arg("--print")
+        .arg("sysroot")
+        .output()
+        .ok()
+        .filter(|o| o.status.success())
+        .and_then(|o| String::from_utf8(o.stdout).ok())
+        .map(|s| s.trim().into())
+}
+
 /// Find the documentation for an item with the given name (exact matches only).
 fn find_doc(
     sources: &[Box<dyn source::Source>],