Launch the manifest-specified Julia version

When running `julia` with no extra arguments, and no explicit version,
it is best to match the manifest version.

This is done by implemented a limited form of the Julia executable's
argument parsing and load path interpreting to determine the appropriate
project to inspect, and then some light ad-hoc parsing of the manifest.

We can then search the installed versions for a matching minor version,
and run that.

src/bin/julialauncher.rs

@@ -12,6 +12,8 @@ use nix::{
     unistd::{fork, ForkResult},
 };
 use normpath::PathExt;
+use semver::Version;
+use std::io::BufRead;
 #[cfg(not(windows))]
 use std::os::unix::process::CommandExt;
 #[cfg(windows)]
@@ -300,6 +302,182 @@ fn get_override_channel(
     }
 }
 
+fn get_project(args: &Vec<String>) -> Option<PathBuf> {
+    let mut project_arg: Option<String> = None;
+    for (_, arg) in args.iter().enumerate() {
+        if arg.starts_with("--project=") {
+            project_arg = Some(arg["--project=".len()..].to_string());
+            // Note: You'd think this might work, but it's not actually supported.
+            // } else if arg == "--project" && i + 1 < args.len() {
+            //     project_arg = Some(args[i + 1].clone());
+        } else if arg == "--" {
+            break;
+        }
+    }
+    let project = if project_arg.is_some() {
+        project_arg.unwrap()
+    } else {
+        match std::env::var("JULIA_PROJECT") {
+            Ok(val) => val,
+            _ => return None,
+        }
+    };
+    if project == "@" {
+        return None;
+    } else if project == "@." || project == "" {
+        let mut path = PathBuf::from(std::env::current_dir().unwrap());
+        while !path.join("Project.toml").exists() && !path.join("JuliaProject.toml").exists() {
+            if !path.pop() {
+                break;
+            }
+        }
+        return Some(path);
+    } else if project == "@script" {
+        let mut program_file: Option<String> = None;
+        let no_arg_short_switches = vec!['v', 'h', 'i', 'q'];
+        let no_arg_long_switches = vec![
+            "--version",
+            "--help",
+            "--help-hidden",
+            "--interactive",
+            "--quiet",
+            // Hidden options
+            "--lisp",
+            "--image-codegen",
+            "--rr-detach",
+            "--strip-metadata",
+            "--strip-ir",
+            "--permalloc-pkgimg",
+            "--heap-size-hint",
+            "--trim",
+        ];
+        // `args` represents [switches...] -- [programfile] [programargs...]
+        // We want to find the first non-switch argument or the first argument after `--`
+        let mut skip_next = false;
+        for (i, arg) in args.iter().skip(1).enumerate() {
+            if skip_next {
+                skip_next = false;
+            } else if arg == "--" {
+                program_file = args.get(i + 1).cloned();
+                break;
+            } else if arg.starts_with("--") {
+                if !no_arg_long_switches.contains(&arg.as_str()) && !arg.contains('=') {
+                    skip_next = true;
+                }
+            } else if arg.starts_with("-") {
+                let arg: Vec<char> = arg.chars().skip(1).collect();
+                if arg.iter().all(|&c| no_arg_short_switches.contains(&c)) {
+                    continue;
+                }
+                for (j, &c) in arg.iter().enumerate() {
+                    if no_arg_short_switches.contains(&c) {
+                        continue;
+                    } else if j < arg.len() - 1 {
+                        break;
+                    } else {
+                        // `j == arg.len() - 1`
+                        skip_next = true;
+                    }
+                }
+            } else {
+                program_file = Some(arg.clone());
+                break;
+            }
+        }
+        if let Some(program_file) = program_file {
+            let mut path = PathBuf::from(program_file);
+            path.pop();
+            while !path.join("Project.toml").exists() && !path.join("JuliaProject.toml").exists() {
+                if !path.pop() {
+                    break;
+                }
+            }
+            return Some(path);
+        } else {
+            return None;
+        }
+    } else if project.starts_with('@') {
+        let depot = match std::env::var("JULIA_DEPOT_PATH") {
+            Ok(val) => match val.split(':').next() {
+                Some(p) => PathBuf::from(p),
+                None => dirs::home_dir().unwrap().join(".julia"),
+            },
+            _ => dirs::home_dir().unwrap().join(".julia"),
+        };
+        let path = depot.join("environments").join(&project[1..]);
+        if path.exists() {
+            return Some(path);
+        } else {
+            return None;
+        }
+    } else {
+        return Some(PathBuf::from(project));
+    }
+}
+
+fn julia_version_from_manifest(path: PathBuf) -> Option<Version> {
+    let manifest = if path.join("Manifest.toml").exists() {
+        path.join("Manifest.toml")
+    } else if path.join("JuliaManifest.toml").exists() {
+        path.join("JuliaManifest.toml")
+    } else {
+        return None;
+    };
+    let file = std::fs::File::open(manifest).ok()?;
+    let reader = std::io::BufReader::new(file);
+    // This is a somewhat bootleg way to parse the manifest,
+    // but since we know the format it should be fine.
+    for line in reader.lines() {
+        let line = line.ok()?;
+        if line.starts_with("julia_version = ") {
+            return Version::parse(line["julia_version = ".len()..].trim_matches('"')).ok();
+        }
+    }
+    return None;
+}
+
+fn get_julia_path_for_version(
+    config_data: &JuliaupConfig,
+    juliaupconfig_path: &Path,
+    version: &Version,
+) -> Result<PathBuf> {
+    let mut best_match: Option<(&String, Version)> = None;
+    for (installed_version_str, path) in &config_data.installed_versions {
+        if let Ok(installed_semver) = Version::parse(installed_version_str) {
+            if installed_semver.major != version.major || installed_semver.minor != version.minor {
+                continue;
+            }
+            if let Some((_, ref best_version)) = best_match {
+                if installed_semver > *best_version {
+                    best_match = Some((&path.path, installed_semver));
+                }
+            } else {
+                best_match = Some((&path.path, installed_semver));
+            }
+        }
+    }
+    if let Some((path, _)) = best_match {
+        let absolute_path = juliaupconfig_path
+            .parent()
+            .unwrap()
+            .join(path)
+            .join("bin")
+            .join(format!("julia{}", std::env::consts::EXE_SUFFIX))
+            .normalize()
+            .with_context(|| {
+                format!(
+                    "Failed to normalize path for Julia binary, starting from `{}`.",
+                    juliaupconfig_path.display()
+                )
+            })?;
+        return Ok(absolute_path.into_path_buf());
+    } else {
+        return Err(anyhow!(
+            "No installed version of Julia matches the requested version."
+        ));
+    }
+}
+
 fn run_app() -> Result<i32> {
     if std::io::stdout().is_terminal() {
         // Set console title
@@ -329,6 +507,16 @@ fn run_app() -> Result<i32> {
         }
     }
 
+    let manifest_derived_julia_path = if channel_from_cmd_line.is_none() {
+        get_project(&args)
+            .and_then(julia_version_from_manifest)
+            .and_then(|ver| {
+                get_julia_path_for_version(&config_file.data, &paths.juliaupconfig, &ver).ok()
+            })
+    } else {
+        None
+    };
+
     let (julia_channel_to_use, juliaup_channel_source) =
         if let Some(channel) = channel_from_cmd_line {
             (channel, JuliaupChannelSource::CmdLine)
@@ -344,19 +532,23 @@ fn run_app() -> Result<i32> {
             ));
         };
 
-    let (julia_path, julia_args) = get_julia_path_from_channel(
-        &versiondb_data,
-        &config_file.data,
-        &julia_channel_to_use,
-        &paths.juliaupconfig,
-        juliaup_channel_source,
-    )
-    .with_context(|| {
-        format!(
-            "The Julia launcher failed to determine the command for the `{}` channel.",
-            julia_channel_to_use
+    let (julia_path, julia_args) = if let Some(path) = manifest_derived_julia_path {
+        (path, Vec::new())
+    } else {
+        get_julia_path_from_channel(
+            &versiondb_data,
+            &config_file.data,
+            &julia_channel_to_use,
+            &paths.juliaupconfig,
+            juliaup_channel_source,
         )
-    })?;
+        .with_context(|| {
+            format!(
+                "The Julia launcher failed to determine the command for the `{}` channel.",
+                julia_channel_to_use
+            )
+        })?
+    };
 
     let mut new_args: Vec<String> = Vec::new();