Provide a more actionable error message when a toolchain is not selected

Builds on top of previous commit. Fixes #2915.

Author: K900

src/errors.rs

@@ -8,6 +8,7 @@ use std::path::PathBuf;
 use thiserror::Error as ThisError;
 use url::Url;
 
+use crate::currentprocess::process;
 use crate::dist::manifest::{Component, Manifest};
 
 const TOOLSTATE_MSG: &str =
@@ -88,7 +89,11 @@ pub enum RustupError {
     ToolchainNotInstallable(String),
     #[error("toolchain '{0}' is not installed")]
     ToolchainNotInstalled(String),
-    #[error("no override and no default toolchain set")]
+    #[error(
+        "rustup could not choose a version of {} to run, because one wasn't specified explicitly, and no default is configured.\n{}",
+        process().name().unwrap_or_else(|| "Rust".into()),
+        "help: run 'rustup default stable' to download the latest stable release of Rust and set it as your default toolchain."
+    )]
     ToolchainNotSelected,
     #[error("toolchain '{}' does not contain component {}{}{}", .name, .component, if let Some(suggestion) = .suggestion {
         format!("; did you mean '{}'?", suggestion)

tests/cli-exact.rs

@@ -599,7 +599,9 @@ fn default_none() {
             config,
             &["rustc", "--version"],
             "",
-            "error: no override and no default toolchain set\n",
+            "error: rustup could not choose a version of rustc to run, because one wasn't specified explicitly, and no default is configured.
+help: run 'rustup default stable' to download the latest stable release of Rust and set it as your default toolchain.
+",
         );
     })
 }

tests/cli-rustup.rs

@@ -2179,7 +2179,7 @@ fn rust_toolchain_toml() {
         expect_err(
             config,
             &["rustc", "--version"],
-            "no override and no default toolchain set",
+            "rustup could not choose a version of rustc to run",
         );
 
         let cwd = config.current_dir();

tests/cli-v1.rs

@@ -22,7 +22,7 @@ fn rustc_no_default_toolchain() {
         expect_err(
             config,
             &["rustc"],
-            "no override and no default toolchain set",
+            "rustup could not choose a version of rustc to run",
         );
     });
 }
@@ -305,7 +305,7 @@ fn remove_override_no_default() {
             expect_err(
                 config,
                 &["rustc"],
-                "no override and no default toolchain set",
+                "rustup could not choose a version of rustc to run",
             );
         });
     });

tests/cli-v2.rs

@@ -30,7 +30,7 @@ fn rustc_no_default_toolchain() {
         expect_err(
             config,
             &["rustc"],
-            "no override and no default toolchain set",
+            "rustup could not choose a version of rustc to run",
         );
     });
 }
@@ -477,7 +477,7 @@ fn remove_override_no_default() {
             expect_err(
                 config,
                 &["rustc"],
-                "no override and no default toolchain set",
+                "rustup could not choose a version of rustc to run",
             );
         });
     });